cgroup: factor out css_set_move_task()

[linux-2.6-block.git] / kernel / cgroup.c

/*
 *  Generic process-grouping system.
 *
 *  Based originally on the cpuset system, extracted by Paul Menage
 *  Copyright (C) 2006 Google, Inc
 *
 *  Notifications support
 *  Copyright (C) 2009 Nokia Corporation
 *  Author: Kirill A. Shutemov
 *
 *  Copyright notices from the original cpuset code:
 *  --------------------------------------------------
 *  Copyright (C) 2003 BULL SA.
 *  Copyright (C) 2004-2006 Silicon Graphics, Inc.
 *
 *  Portions derived from Patrick Mochel's sysfs code.
 *  sysfs is Copyright (c) 2001-3 Patrick Mochel
 *
 *  2003-10-10 Written by Simon Derr.
 *  2003-10-22 Updates by Stephen Hemminger.
 *  2004 May-July Rework by Paul Jackson.
 *  ---------------------------------------------------
 *
 *  This file is subject to the terms and conditions of the GNU General Public
 *  License.  See the file COPYING in the main directory of the Linux
 *  distribution for more details.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/cgroup.h>
#include <linux/cred.h>
#include <linux/ctype.h>
#include <linux/errno.h>
#include <linux/init_task.h>
#include <linux/kernel.h>
#include <linux/list.h>
#include <linux/magic.h>
#include <linux/mm.h>
#include <linux/mutex.h>
#include <linux/mount.h>
#include <linux/pagemap.h>
#include <linux/proc_fs.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/rwsem.h>
#include <linux/percpu-rwsem.h>
#include <linux/string.h>
#include <linux/sort.h>
#include <linux/kmod.h>
#include <linux/delayacct.h>
#include <linux/cgroupstats.h>
#include <linux/hashtable.h>
#include <linux/pid_namespace.h>
#include <linux/idr.h>
#include <linux/vmalloc.h> /* TODO: replace with more sophisticated array */
#include <linux/kthread.h>
#include <linux/delay.h>

#include <linux/atomic.h>

/*
 * pidlists linger the following amount before being destroyed.  The goal
 * is avoiding frequent destruction in the middle of consecutive read calls
 * Expiring in the middle is a performance problem not a correctness one.
 * 1 sec should be enough.
 */
#define CGROUP_PIDLIST_DESTROY_DELAY    HZ

#define CGROUP_FILE_NAME_MAX            (MAX_CGROUP_TYPE_NAMELEN +      \
                                         MAX_CFTYPE_NAME + 2)

/*
 * cgroup_mutex is the master lock.  Any modification to cgroup or its
 * hierarchy must be performed while holding it.
 *
 * css_set_rwsem protects task->cgroups pointer, the list of css_set
 * objects, and the chain of tasks off each css_set.
 *
 * These locks are exported if CONFIG_PROVE_RCU so that accessors in
 * cgroup.h can use them for lockdep annotations.
 */
#ifdef CONFIG_PROVE_RCU
DEFINE_MUTEX(cgroup_mutex);
DECLARE_RWSEM(css_set_rwsem);
EXPORT_SYMBOL_GPL(cgroup_mutex);
EXPORT_SYMBOL_GPL(css_set_rwsem);
#else
static DEFINE_MUTEX(cgroup_mutex);
static DECLARE_RWSEM(css_set_rwsem);
#endif

/*
 * Protects cgroup_idr and css_idr so that IDs can be released without
 * grabbing cgroup_mutex.
 */
static DEFINE_SPINLOCK(cgroup_idr_lock);

/*
 * Protects cgroup_subsys->release_agent_path.  Modifying it also requires
 * cgroup_mutex.  Reading requires either cgroup_mutex or this spinlock.
 */
static DEFINE_SPINLOCK(release_agent_path_lock);

struct percpu_rw_semaphore cgroup_threadgroup_rwsem;

#define cgroup_assert_mutex_or_rcu_locked()                             \
        RCU_LOCKDEP_WARN(!rcu_read_lock_held() &&                       \
                           !lockdep_is_held(&cgroup_mutex),             \
                           "cgroup_mutex or RCU read lock required");

/*
 * cgroup destruction makes heavy use of work items and there can be a lot
 * of concurrent destructions.  Use a separate workqueue so that cgroup
 * destruction work items don't end up filling up max_active of system_wq
 * which may lead to deadlock.
 */
static struct workqueue_struct *cgroup_destroy_wq;

/*
 * pidlist destructions need to be flushed on cgroup destruction.  Use a
 * separate workqueue as flush domain.
 */
static struct workqueue_struct *cgroup_pidlist_destroy_wq;

/* generate an array of cgroup subsystem pointers */
#define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys,
static struct cgroup_subsys *cgroup_subsys[] = {
#include <linux/cgroup_subsys.h>
};
#undef SUBSYS

/* array of cgroup subsystem names */
#define SUBSYS(_x) [_x ## _cgrp_id] = #_x,
static const char *cgroup_subsys_name[] = {
#include <linux/cgroup_subsys.h>
};
#undef SUBSYS

/* array of static_keys for cgroup_subsys_enabled() and cgroup_subsys_on_dfl() */
#define SUBSYS(_x)                                                              \
        DEFINE_STATIC_KEY_TRUE(_x ## _cgrp_subsys_enabled_key);                 \
        DEFINE_STATIC_KEY_TRUE(_x ## _cgrp_subsys_on_dfl_key);                  \
        EXPORT_SYMBOL_GPL(_x ## _cgrp_subsys_enabled_key);                      \
        EXPORT_SYMBOL_GPL(_x ## _cgrp_subsys_on_dfl_key);
#include <linux/cgroup_subsys.h>
#undef SUBSYS

#define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys_enabled_key,
static struct static_key_true *cgroup_subsys_enabled_key[] = {
#include <linux/cgroup_subsys.h>
};
#undef SUBSYS

#define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys_on_dfl_key,
static struct static_key_true *cgroup_subsys_on_dfl_key[] = {
#include <linux/cgroup_subsys.h>
};
#undef SUBSYS

/*
 * The default hierarchy, reserved for the subsystems that are otherwise
 * unattached - it never has more than a single cgroup, and all tasks are
 * part of that cgroup.
 */
struct cgroup_root cgrp_dfl_root;
EXPORT_SYMBOL_GPL(cgrp_dfl_root);

/*
 * The default hierarchy always exists but is hidden until mounted for the
 * first time.  This is for backward compatibility.
 */
static bool cgrp_dfl_root_visible;

/*
 * Set by the boot param of the same name and makes subsystems with NULL
 * ->dfl_files to use ->legacy_files on the default hierarchy.
 */
static bool cgroup_legacy_files_on_dfl;

/* some controllers are not supported in the default hierarchy */
static unsigned long cgrp_dfl_root_inhibit_ss_mask;

/* The list of hierarchy roots */

static LIST_HEAD(cgroup_roots);
static int cgroup_root_count;

/* hierarchy ID allocation and mapping, protected by cgroup_mutex */
static DEFINE_IDR(cgroup_hierarchy_idr);

/*
 * Assign a monotonically increasing serial number to csses.  It guarantees
 * cgroups with bigger numbers are newer than those with smaller numbers.
 * Also, as csses are always appended to the parent's ->children list, it
 * guarantees that sibling csses are always sorted in the ascending serial
 * number order on the list.  Protected by cgroup_mutex.
 */
static u64 css_serial_nr_next = 1;

/*
 * These bitmask flags indicate whether tasks in the fork and exit paths have
 * fork/exit handlers to call. This avoids us having to do extra work in the
 * fork/exit path to check which subsystems have fork/exit callbacks.
 */
static unsigned long have_fork_callback __read_mostly;
static unsigned long have_exit_callback __read_mostly;

/* Ditto for the can_fork callback. */
static unsigned long have_canfork_callback __read_mostly;

static struct cftype cgroup_dfl_base_files[];
static struct cftype cgroup_legacy_base_files[];

static int rebind_subsystems(struct cgroup_root *dst_root,
                             unsigned long ss_mask);
static int cgroup_destroy_locked(struct cgroup *cgrp);
static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss,
                      bool visible);
static void css_release(struct percpu_ref *ref);
static void kill_css(struct cgroup_subsys_state *css);
static int cgroup_addrm_files(struct cgroup_subsys_state *css,
                              struct cgroup *cgrp, struct cftype cfts[],
                              bool is_add);

/**
 * cgroup_ssid_enabled - cgroup subsys enabled test by subsys ID
 * @ssid: subsys ID of interest
 *
 * cgroup_subsys_enabled() can only be used with literal subsys names which
 * is fine for individual subsystems but unsuitable for cgroup core.  This
 * is slower static_key_enabled() based test indexed by @ssid.
 */
static bool cgroup_ssid_enabled(int ssid)
{
        return static_key_enabled(cgroup_subsys_enabled_key[ssid]);
}

/**
 * cgroup_on_dfl - test whether a cgroup is on the default hierarchy
 * @cgrp: the cgroup of interest
 *
 * The default hierarchy is the v2 interface of cgroup and this function
 * can be used to test whether a cgroup is on the default hierarchy for
 * cases where a subsystem should behave differnetly depending on the
 * interface version.
 *
 * The set of behaviors which change on the default hierarchy are still
 * being determined and the mount option is prefixed with __DEVEL__.
 *
 * List of changed behaviors:
 *
 * - Mount options "noprefix", "xattr", "clone_children", "release_agent"
 *   and "name" are disallowed.
 *
 * - When mounting an existing superblock, mount options should match.
 *
 * - Remount is disallowed.
 *
 * - rename(2) is disallowed.
 *
 * - "tasks" is removed.  Everything should be at process granularity.  Use
 *   "cgroup.procs" instead.
 *
 * - "cgroup.procs" is not sorted.  pids will be unique unless they got
 *   recycled inbetween reads.
 *
 * - "release_agent" and "notify_on_release" are removed.  Replacement
 *   notification mechanism will be implemented.
 *
 * - "cgroup.clone_children" is removed.
 *
 * - "cgroup.subtree_populated" is available.  Its value is 0 if the cgroup
 *   and its descendants contain no task; otherwise, 1.  The file also
 *   generates kernfs notification which can be monitored through poll and
 *   [di]notify when the value of the file changes.
 *
 * - cpuset: tasks will be kept in empty cpusets when hotplug happens and
 *   take masks of ancestors with non-empty cpus/mems, instead of being
 *   moved to an ancestor.
 *
 * - cpuset: a task can be moved into an empty cpuset, and again it takes
 *   masks of ancestors.
 *
 * - memcg: use_hierarchy is on by default and the cgroup file for the flag
 *   is not created.
 *
 * - blkcg: blk-throttle becomes properly hierarchical.
 *
 * - debug: disallowed on the default hierarchy.
 */
static bool cgroup_on_dfl(const struct cgroup *cgrp)
{
        return cgrp->root == &cgrp_dfl_root;
}

/* IDR wrappers which synchronize using cgroup_idr_lock */
static int cgroup_idr_alloc(struct idr *idr, void *ptr, int start, int end,
                            gfp_t gfp_mask)
{
        int ret;

        idr_preload(gfp_mask);
        spin_lock_bh(&cgroup_idr_lock);
        ret = idr_alloc(idr, ptr, start, end, gfp_mask & ~__GFP_WAIT);
        spin_unlock_bh(&cgroup_idr_lock);
        idr_preload_end();
        return ret;
}

static void *cgroup_idr_replace(struct idr *idr, void *ptr, int id)
{
        void *ret;

        spin_lock_bh(&cgroup_idr_lock);
        ret = idr_replace(idr, ptr, id);
        spin_unlock_bh(&cgroup_idr_lock);
        return ret;
}

static void cgroup_idr_remove(struct idr *idr, int id)
{
        spin_lock_bh(&cgroup_idr_lock);
        idr_remove(idr, id);
        spin_unlock_bh(&cgroup_idr_lock);
}

static struct cgroup *cgroup_parent(struct cgroup *cgrp)
{
        struct cgroup_subsys_state *parent_css = cgrp->self.parent;

        if (parent_css)
                return container_of(parent_css, struct cgroup, self);
        return NULL;
}

/**
 * cgroup_css - obtain a cgroup's css for the specified subsystem
 * @cgrp: the cgroup of interest
 * @ss: the subsystem of interest (%NULL returns @cgrp->self)
 *
 * Return @cgrp's css (cgroup_subsys_state) associated with @ss.  This
 * function must be called either under cgroup_mutex or rcu_read_lock() and
 * the caller is responsible for pinning the returned css if it wants to
 * keep accessing it outside the said locks.  This function may return
 * %NULL if @cgrp doesn't have @subsys_id enabled.
 */
static struct cgroup_subsys_state *cgroup_css(struct cgroup *cgrp,
                                              struct cgroup_subsys *ss)
{
        if (ss)
                return rcu_dereference_check(cgrp->subsys[ss->id],
                                        lockdep_is_held(&cgroup_mutex));
        else
                return &cgrp->self;
}

/**
 * cgroup_e_css - obtain a cgroup's effective css for the specified subsystem
 * @cgrp: the cgroup of interest
 * @ss: the subsystem of interest (%NULL returns @cgrp->self)
 *
 * Similar to cgroup_css() but returns the effective css, which is defined
 * as the matching css of the nearest ancestor including self which has @ss
 * enabled.  If @ss is associated with the hierarchy @cgrp is on, this
 * function is guaranteed to return non-NULL css.
 */
static struct cgroup_subsys_state *cgroup_e_css(struct cgroup *cgrp,
                                                struct cgroup_subsys *ss)
{
        lockdep_assert_held(&cgroup_mutex);

        if (!ss)
                return &cgrp->self;

        if (!(cgrp->root->subsys_mask & (1 << ss->id)))
                return NULL;

        /*
         * This function is used while updating css associations and thus
         * can't test the csses directly.  Use ->child_subsys_mask.
         */
        while (cgroup_parent(cgrp) &&
               !(cgroup_parent(cgrp)->child_subsys_mask & (1 << ss->id)))
                cgrp = cgroup_parent(cgrp);

        return cgroup_css(cgrp, ss);
}

/**
 * cgroup_get_e_css - get a cgroup's effective css for the specified subsystem
 * @cgrp: the cgroup of interest
 * @ss: the subsystem of interest
 *
 * Find and get the effective css of @cgrp for @ss.  The effective css is
 * defined as the matching css of the nearest ancestor including self which
 * has @ss enabled.  If @ss is not mounted on the hierarchy @cgrp is on,
 * the root css is returned, so this function always returns a valid css.
 * The returned css must be put using css_put().
 */
struct cgroup_subsys_state *cgroup_get_e_css(struct cgroup *cgrp,
                                             struct cgroup_subsys *ss)
{
        struct cgroup_subsys_state *css;

        rcu_read_lock();

        do {
                css = cgroup_css(cgrp, ss);

                if (css && css_tryget_online(css))
                        goto out_unlock;
                cgrp = cgroup_parent(cgrp);
        } while (cgrp);

        css = init_css_set.subsys[ss->id];
        css_get(css);
out_unlock:
        rcu_read_unlock();
        return css;
}

/* convenient tests for these bits */
static inline bool cgroup_is_dead(const struct cgroup *cgrp)
{
        return !(cgrp->self.flags & CSS_ONLINE);
}

static void cgroup_get(struct cgroup *cgrp)
{
        WARN_ON_ONCE(cgroup_is_dead(cgrp));
        css_get(&cgrp->self);
}

static bool cgroup_tryget(struct cgroup *cgrp)
{
        return css_tryget(&cgrp->self);
}

static void cgroup_put(struct cgroup *cgrp)
{
        css_put(&cgrp->self);
}

struct cgroup_subsys_state *of_css(struct kernfs_open_file *of)
{
        struct cgroup *cgrp = of->kn->parent->priv;
        struct cftype *cft = of_cft(of);

        /*
         * This is open and unprotected implementation of cgroup_css().
         * seq_css() is only called from a kernfs file operation which has
         * an active reference on the file.  Because all the subsystem
         * files are drained before a css is disassociated with a cgroup,
         * the matching css from the cgroup's subsys table is guaranteed to
         * be and stay valid until the enclosing operation is complete.
         */
        if (cft->ss)
                return rcu_dereference_raw(cgrp->subsys[cft->ss->id]);
        else
                return &cgrp->self;
}
EXPORT_SYMBOL_GPL(of_css);

/**
 * cgroup_is_descendant - test ancestry
 * @cgrp: the cgroup to be tested
 * @ancestor: possible ancestor of @cgrp
 *
 * Test whether @cgrp is a descendant of @ancestor.  It also returns %true
 * if @cgrp == @ancestor.  This function is safe to call as long as @cgrp
 * and @ancestor are accessible.
 */
bool cgroup_is_descendant(struct cgroup *cgrp, struct cgroup *ancestor)
{
        while (cgrp) {
                if (cgrp == ancestor)
                        return true;
                cgrp = cgroup_parent(cgrp);
        }
        return false;
}

static int notify_on_release(const struct cgroup *cgrp)
{
        return test_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
}

/**
 * for_each_css - iterate all css's of a cgroup
 * @css: the iteration cursor
 * @ssid: the index of the subsystem, CGROUP_SUBSYS_COUNT after reaching the end
 * @cgrp: the target cgroup to iterate css's of
 *
 * Should be called under cgroup_[tree_]mutex.
 */
#define for_each_css(css, ssid, cgrp)                                   \
        for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT; (ssid)++)        \
                if (!((css) = rcu_dereference_check(                    \
                                (cgrp)->subsys[(ssid)],                 \
                                lockdep_is_held(&cgroup_mutex)))) { }   \
                else

/**
 * for_each_e_css - iterate all effective css's of a cgroup
 * @css: the iteration cursor
 * @ssid: the index of the subsystem, CGROUP_SUBSYS_COUNT after reaching the end
 * @cgrp: the target cgroup to iterate css's of
 *
 * Should be called under cgroup_[tree_]mutex.
 */
#define for_each_e_css(css, ssid, cgrp)                                 \
        for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT; (ssid)++)        \
                if (!((css) = cgroup_e_css(cgrp, cgroup_subsys[(ssid)]))) \
                        ;                                               \
                else

/**
 * for_each_subsys - iterate all enabled cgroup subsystems
 * @ss: the iteration cursor
 * @ssid: the index of @ss, CGROUP_SUBSYS_COUNT after reaching the end
 */
#define for_each_subsys(ss, ssid)                                       \
        for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT &&                \
             (((ss) = cgroup_subsys[ssid]) || true); (ssid)++)

/**
 * for_each_subsys_which - filter for_each_subsys with a bitmask
 * @ss: the iteration cursor
 * @ssid: the index of @ss, CGROUP_SUBSYS_COUNT after reaching the end
 * @ss_maskp: a pointer to the bitmask
 *
 * The block will only run for cases where the ssid-th bit (1 << ssid) of
 * mask is set to 1.
 */
#define for_each_subsys_which(ss, ssid, ss_maskp)                       \
        if (!CGROUP_SUBSYS_COUNT) /* to avoid spurious gcc warning */   \
                (ssid) = 0;                                             \
        else                                                            \
                for_each_set_bit(ssid, ss_maskp, CGROUP_SUBSYS_COUNT)   \
                        if (((ss) = cgroup_subsys[ssid]) && false)      \
                                break;                                  \
                        else

/* iterate across the hierarchies */
#define for_each_root(root)                                             \
        list_for_each_entry((root), &cgroup_roots, root_list)

/* iterate over child cgrps, lock should be held throughout iteration */
#define cgroup_for_each_live_child(child, cgrp)                         \
        list_for_each_entry((child), &(cgrp)->self.children, self.sibling) \
                if (({ lockdep_assert_held(&cgroup_mutex);              \
                       cgroup_is_dead(child); }))                       \
                        ;                                               \
                else

static void cgroup_release_agent(struct work_struct *work);
static void check_for_release(struct cgroup *cgrp);

/*
 * A cgroup can be associated with multiple css_sets as different tasks may
 * belong to different cgroups on different hierarchies.  In the other
 * direction, a css_set is naturally associated with multiple cgroups.
 * This M:N relationship is represented by the following link structure
 * which exists for each association and allows traversing the associations
 * from both sides.
 */
struct cgrp_cset_link {
        /* the cgroup and css_set this link associates */
        struct cgroup           *cgrp;
        struct css_set          *cset;

        /* list of cgrp_cset_links anchored at cgrp->cset_links */
        struct list_head        cset_link;

        /* list of cgrp_cset_links anchored at css_set->cgrp_links */
        struct list_head        cgrp_link;
};

/*
 * The default css_set - used by init and its children prior to any
 * hierarchies being mounted. It contains a pointer to the root state
 * for each subsystem. Also used to anchor the list of css_sets. Not
 * reference-counted, to improve performance when child cgroups
 * haven't been created.
 */
struct css_set init_css_set = {
        .refcount               = ATOMIC_INIT(1),
        .cgrp_links             = LIST_HEAD_INIT(init_css_set.cgrp_links),
        .tasks                  = LIST_HEAD_INIT(init_css_set.tasks),
        .mg_tasks               = LIST_HEAD_INIT(init_css_set.mg_tasks),
        .mg_preload_node        = LIST_HEAD_INIT(init_css_set.mg_preload_node),
        .mg_node                = LIST_HEAD_INIT(init_css_set.mg_node),
};

static int css_set_count        = 1;    /* 1 for init_css_set */

/**
 * css_set_populated - does a css_set contain any tasks?
 * @cset: target css_set
 */
static bool css_set_populated(struct css_set *cset)
{
        lockdep_assert_held(&css_set_rwsem);

        return !list_empty(&cset->tasks) || !list_empty(&cset->mg_tasks);
}

/**
 * cgroup_update_populated - updated populated count of a cgroup
 * @cgrp: the target cgroup
 * @populated: inc or dec populated count
 *
 * One of the css_sets associated with @cgrp is either getting its first
 * task or losing the last.  Update @cgrp->populated_cnt accordingly.  The
 * count is propagated towards root so that a given cgroup's populated_cnt
 * is zero iff the cgroup and all its descendants don't contain any tasks.
 *
 * @cgrp's interface file "cgroup.populated" is zero if
 * @cgrp->populated_cnt is zero and 1 otherwise.  When @cgrp->populated_cnt
 * changes from or to zero, userland is notified that the content of the
 * interface file has changed.  This can be used to detect when @cgrp and
 * its descendants become populated or empty.
 */
static void cgroup_update_populated(struct cgroup *cgrp, bool populated)
{
        lockdep_assert_held(&css_set_rwsem);

        do {
                bool trigger;

                if (populated)
                        trigger = !cgrp->populated_cnt++;
                else
                        trigger = !--cgrp->populated_cnt;

                if (!trigger)
                        break;

                check_for_release(cgrp);
                cgroup_file_notify(&cgrp->events_file);

                cgrp = cgroup_parent(cgrp);
        } while (cgrp);
}

/**
 * css_set_update_populated - update populated state of a css_set
 * @cset: target css_set
 * @populated: whether @cset is populated or depopulated
 *
 * @cset is either getting the first task or losing the last.  Update the
 * ->populated_cnt of all associated cgroups accordingly.
 */
static void css_set_update_populated(struct css_set *cset, bool populated)
{
        struct cgrp_cset_link *link;

        lockdep_assert_held(&css_set_rwsem);

        list_for_each_entry(link, &cset->cgrp_links, cgrp_link)
                cgroup_update_populated(link->cgrp, populated);
}

/**
 * css_set_move_task - move a task from one css_set to another
 * @task: task being moved
 * @from_cset: css_set @task currently belongs to (may be NULL)
 * @to_cset: new css_set @task is being moved to (may be NULL)
 * @use_mg_tasks: move to @to_cset->mg_tasks instead of ->tasks
 *
 * Move @task from @from_cset to @to_cset.  If @task didn't belong to any
 * css_set, @from_cset can be NULL.  If @task is being disassociated
 * instead of moved, @to_cset can be NULL.
 *
 * This function automatically handles populated_cnt updates but the caller
 * is responsible for managing @from_cset and @to_cset's reference counts.
 */
static void css_set_move_task(struct task_struct *task,
                              struct css_set *from_cset, struct css_set *to_cset,
                              bool use_mg_tasks)
{
        lockdep_assert_held(&css_set_rwsem);

        if (from_cset) {
                WARN_ON_ONCE(list_empty(&task->cg_list));
                list_del_init(&task->cg_list);
                if (!css_set_populated(from_cset))
                        css_set_update_populated(from_cset, false);
        } else {
                WARN_ON_ONCE(!list_empty(&task->cg_list));
        }

        if (to_cset) {
                /*
                 * We are synchronized through cgroup_threadgroup_rwsem
                 * against PF_EXITING setting such that we can't race
                 * against cgroup_exit() changing the css_set to
                 * init_css_set and dropping the old one.
                 */
                WARN_ON_ONCE(task->flags & PF_EXITING);

                if (!css_set_populated(to_cset))
                        css_set_update_populated(to_cset, true);
                rcu_assign_pointer(task->cgroups, to_cset);
                list_add_tail(&task->cg_list, use_mg_tasks ? &to_cset->mg_tasks :
                                                             &to_cset->tasks);
        }
}

/*
 * hash table for cgroup groups. This improves the performance to find
 * an existing css_set. This hash doesn't (currently) take into
 * account cgroups in empty hierarchies.
 */
#define CSS_SET_HASH_BITS       7
static DEFINE_HASHTABLE(css_set_table, CSS_SET_HASH_BITS);

static unsigned long css_set_hash(struct cgroup_subsys_state *css[])
{
        unsigned long key = 0UL;
        struct cgroup_subsys *ss;
        int i;

        for_each_subsys(ss, i)
                key += (unsigned long)css[i];
        key = (key >> 16) ^ key;

        return key;
}

static void put_css_set_locked(struct css_set *cset)
{
        struct cgrp_cset_link *link, *tmp_link;
        struct cgroup_subsys *ss;
        int ssid;

        lockdep_assert_held(&css_set_rwsem);

        if (!atomic_dec_and_test(&cset->refcount))
                return;

        /* This css_set is dead. unlink it and release cgroup refcounts */
        for_each_subsys(ss, ssid)
                list_del(&cset->e_cset_node[ssid]);
        hash_del(&cset->hlist);
        css_set_count--;

        list_for_each_entry_safe(link, tmp_link, &cset->cgrp_links, cgrp_link) {
                list_del(&link->cset_link);
                list_del(&link->cgrp_link);
                if (cgroup_parent(link->cgrp))
                        cgroup_put(link->cgrp);
                kfree(link);
        }

        kfree_rcu(cset, rcu_head);
}

static void put_css_set(struct css_set *cset)
{
        /*
         * Ensure that the refcount doesn't hit zero while any readers
         * can see it. Similar to atomic_dec_and_lock(), but for an
         * rwlock
         */
        if (atomic_add_unless(&cset->refcount, -1, 1))
                return;

        down_write(&css_set_rwsem);
        put_css_set_locked(cset);
        up_write(&css_set_rwsem);
}

/*
 * refcounted get/put for css_set objects
 */
static inline void get_css_set(struct css_set *cset)
{
        atomic_inc(&cset->refcount);
}

/**
 * compare_css_sets - helper function for find_existing_css_set().
 * @cset: candidate css_set being tested
 * @old_cset: existing css_set for a task
 * @new_cgrp: cgroup that's being entered by the task
 * @template: desired set of css pointers in css_set (pre-calculated)
 *
 * Returns true if "cset" matches "old_cset" except for the hierarchy
 * which "new_cgrp" belongs to, for which it should match "new_cgrp".
 */
static bool compare_css_sets(struct css_set *cset,
                             struct css_set *old_cset,
                             struct cgroup *new_cgrp,
                             struct cgroup_subsys_state *template[])
{
        struct list_head *l1, *l2;

        /*
         * On the default hierarchy, there can be csets which are
         * associated with the same set of cgroups but different csses.
         * Let's first ensure that csses match.
         */
        if (memcmp(template, cset->subsys, sizeof(cset->subsys)))
                return false;

        /*
         * Compare cgroup pointers in order to distinguish between
         * different cgroups in hierarchies.  As different cgroups may
         * share the same effective css, this comparison is always
         * necessary.
         */
        l1 = &cset->cgrp_links;
        l2 = &old_cset->cgrp_links;
        while (1) {
                struct cgrp_cset_link *link1, *link2;
                struct cgroup *cgrp1, *cgrp2;

                l1 = l1->next;
                l2 = l2->next;
                /* See if we reached the end - both lists are equal length. */
                if (l1 == &cset->cgrp_links) {
                        BUG_ON(l2 != &old_cset->cgrp_links);
                        break;
                } else {
                        BUG_ON(l2 == &old_cset->cgrp_links);
                }
                /* Locate the cgroups associated with these links. */
                link1 = list_entry(l1, struct cgrp_cset_link, cgrp_link);
                link2 = list_entry(l2, struct cgrp_cset_link, cgrp_link);
                cgrp1 = link1->cgrp;
                cgrp2 = link2->cgrp;
                /* Hierarchies should be linked in the same order. */
                BUG_ON(cgrp1->root != cgrp2->root);

                /*
                 * If this hierarchy is the hierarchy of the cgroup
                 * that's changing, then we need to check that this
                 * css_set points to the new cgroup; if it's any other
                 * hierarchy, then this css_set should point to the
                 * same cgroup as the old css_set.
                 */
                if (cgrp1->root == new_cgrp->root) {
                        if (cgrp1 != new_cgrp)
                                return false;
                } else {
                        if (cgrp1 != cgrp2)
                                return false;
                }
        }
        return true;
}

/**
 * find_existing_css_set - init css array and find the matching css_set
 * @old_cset: the css_set that we're using before the cgroup transition
 * @cgrp: the cgroup that we're moving into
 * @template: out param for the new set of csses, should be clear on entry
 */
static struct css_set *find_existing_css_set(struct css_set *old_cset,
                                        struct cgroup *cgrp,
                                        struct cgroup_subsys_state *template[])
{
        struct cgroup_root *root = cgrp->root;
        struct cgroup_subsys *ss;
        struct css_set *cset;
        unsigned long key;
        int i;

        /*
         * Build the set of subsystem state objects that we want to see in the
         * new css_set. while subsystems can change globally, the entries here
         * won't change, so no need for locking.
         */
        for_each_subsys(ss, i) {
                if (root->subsys_mask & (1UL << i)) {
                        /*
                         * @ss is in this hierarchy, so we want the
                         * effective css from @cgrp.
                         */
                        template[i] = cgroup_e_css(cgrp, ss);
                } else {
                        /*
                         * @ss is not in this hierarchy, so we don't want
                         * to change the css.
                         */
                        template[i] = old_cset->subsys[i];
                }
        }

        key = css_set_hash(template);
        hash_for_each_possible(css_set_table, cset, hlist, key) {
                if (!compare_css_sets(cset, old_cset, cgrp, template))
                        continue;

                /* This css_set matches what we need */
                return cset;
        }

        /* No existing cgroup group matched */
        return NULL;
}

static void free_cgrp_cset_links(struct list_head *links_to_free)
{
        struct cgrp_cset_link *link, *tmp_link;

        list_for_each_entry_safe(link, tmp_link, links_to_free, cset_link) {
                list_del(&link->cset_link);
                kfree(link);
        }
}

/**
 * allocate_cgrp_cset_links - allocate cgrp_cset_links
 * @count: the number of links to allocate
 * @tmp_links: list_head the allocated links are put on
 *
 * Allocate @count cgrp_cset_link structures and chain them on @tmp_links
 * through ->cset_link.  Returns 0 on success or -errno.
 */
static int allocate_cgrp_cset_links(int count, struct list_head *tmp_links)
{
        struct cgrp_cset_link *link;
        int i;

        INIT_LIST_HEAD(tmp_links);

        for (i = 0; i < count; i++) {
                link = kzalloc(sizeof(*link), GFP_KERNEL);
                if (!link) {
                        free_cgrp_cset_links(tmp_links);
                        return -ENOMEM;
                }
                list_add(&link->cset_link, tmp_links);
        }
        return 0;
}

/**
 * link_css_set - a helper function to link a css_set to a cgroup
 * @tmp_links: cgrp_cset_link objects allocated by allocate_cgrp_cset_links()
 * @cset: the css_set to be linked
 * @cgrp: the destination cgroup
 */
static void link_css_set(struct list_head *tmp_links, struct css_set *cset,
                         struct cgroup *cgrp)
{
        struct cgrp_cset_link *link;

        BUG_ON(list_empty(tmp_links));

        if (cgroup_on_dfl(cgrp))
                cset->dfl_cgrp = cgrp;

        link = list_first_entry(tmp_links, struct cgrp_cset_link, cset_link);
        link->cset = cset;
        link->cgrp = cgrp;

        /*
         * Always add links to the tail of the lists so that the lists are
         * in choronological order.
         */
        list_move_tail(&link->cset_link, &cgrp->cset_links);
        list_add_tail(&link->cgrp_link, &cset->cgrp_links);

        if (cgroup_parent(cgrp))
                cgroup_get(cgrp);
}

/**
 * find_css_set - return a new css_set with one cgroup updated
 * @old_cset: the baseline css_set
 * @cgrp: the cgroup to be updated
 *
 * Return a new css_set that's equivalent to @old_cset, but with @cgrp
 * substituted into the appropriate hierarchy.
 */
static struct css_set *find_css_set(struct css_set *old_cset,
                                    struct cgroup *cgrp)
{
        struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT] = { };
        struct css_set *cset;
        struct list_head tmp_links;
        struct cgrp_cset_link *link;
        struct cgroup_subsys *ss;
        unsigned long key;
        int ssid;

        lockdep_assert_held(&cgroup_mutex);

        /* First see if we already have a cgroup group that matches
         * the desired set */
        down_read(&css_set_rwsem);
        cset = find_existing_css_set(old_cset, cgrp, template);
        if (cset)
                get_css_set(cset);
        up_read(&css_set_rwsem);

        if (cset)
                return cset;

        cset = kzalloc(sizeof(*cset), GFP_KERNEL);
        if (!cset)
                return NULL;

        /* Allocate all the cgrp_cset_link objects that we'll need */
        if (allocate_cgrp_cset_links(cgroup_root_count, &tmp_links) < 0) {
                kfree(cset);
                return NULL;
        }

        atomic_set(&cset->refcount, 1);
        INIT_LIST_HEAD(&cset->cgrp_links);
        INIT_LIST_HEAD(&cset->tasks);
        INIT_LIST_HEAD(&cset->mg_tasks);
        INIT_LIST_HEAD(&cset->mg_preload_node);
        INIT_LIST_HEAD(&cset->mg_node);
        INIT_HLIST_NODE(&cset->hlist);

        /* Copy the set of subsystem state objects generated in
         * find_existing_css_set() */
        memcpy(cset->subsys, template, sizeof(cset->subsys));

        down_write(&css_set_rwsem);
        /* Add reference counts and links from the new css_set. */
        list_for_each_entry(link, &old_cset->cgrp_links, cgrp_link) {
                struct cgroup *c = link->cgrp;

                if (c->root == cgrp->root)
                        c = cgrp;
                link_css_set(&tmp_links, cset, c);
        }

        BUG_ON(!list_empty(&tmp_links));

        css_set_count++;

        /* Add @cset to the hash table */
        key = css_set_hash(cset->subsys);
        hash_add(css_set_table, &cset->hlist, key);

        for_each_subsys(ss, ssid)
                list_add_tail(&cset->e_cset_node[ssid],
                              &cset->subsys[ssid]->cgroup->e_csets[ssid]);

        up_write(&css_set_rwsem);

        return cset;
}

static struct cgroup_root *cgroup_root_from_kf(struct kernfs_root *kf_root)
{
        struct cgroup *root_cgrp = kf_root->kn->priv;

        return root_cgrp->root;
}

static int cgroup_init_root_id(struct cgroup_root *root)
{
        int id;

        lockdep_assert_held(&cgroup_mutex);

        id = idr_alloc_cyclic(&cgroup_hierarchy_idr, root, 0, 0, GFP_KERNEL);
        if (id < 0)
                return id;

        root->hierarchy_id = id;
        return 0;
}

static void cgroup_exit_root_id(struct cgroup_root *root)
{
        lockdep_assert_held(&cgroup_mutex);

        if (root->hierarchy_id) {
                idr_remove(&cgroup_hierarchy_idr, root->hierarchy_id);
                root->hierarchy_id = 0;
        }
}

static void cgroup_free_root(struct cgroup_root *root)
{
        if (root) {
                /* hierarchy ID should already have been released */
                WARN_ON_ONCE(root->hierarchy_id);

                idr_destroy(&root->cgroup_idr);
                kfree(root);
        }
}

static void cgroup_destroy_root(struct cgroup_root *root)
{
        struct cgroup *cgrp = &root->cgrp;
        struct cgrp_cset_link *link, *tmp_link;

        mutex_lock(&cgroup_mutex);

        BUG_ON(atomic_read(&root->nr_cgrps));
        BUG_ON(!list_empty(&cgrp->self.children));

        /* Rebind all subsystems back to the default hierarchy */
        rebind_subsystems(&cgrp_dfl_root, root->subsys_mask);

        /*
         * Release all the links from cset_links to this hierarchy's
         * root cgroup
         */
        down_write(&css_set_rwsem);

        list_for_each_entry_safe(link, tmp_link, &cgrp->cset_links, cset_link) {
                list_del(&link->cset_link);
                list_del(&link->cgrp_link);
                kfree(link);
        }
        up_write(&css_set_rwsem);

        if (!list_empty(&root->root_list)) {
                list_del(&root->root_list);
                cgroup_root_count--;
        }

        cgroup_exit_root_id(root);

        mutex_unlock(&cgroup_mutex);

        kernfs_destroy_root(root->kf_root);
        cgroup_free_root(root);
}

/* look up cgroup associated with given css_set on the specified hierarchy */
static struct cgroup *cset_cgroup_from_root(struct css_set *cset,
                                            struct cgroup_root *root)
{
        struct cgroup *res = NULL;

        lockdep_assert_held(&cgroup_mutex);
        lockdep_assert_held(&css_set_rwsem);

        if (cset == &init_css_set) {
                res = &root->cgrp;
        } else {
                struct cgrp_cset_link *link;

                list_for_each_entry(link, &cset->cgrp_links, cgrp_link) {
                        struct cgroup *c = link->cgrp;

                        if (c->root == root) {
                                res = c;
                                break;
                        }
                }
        }

        BUG_ON(!res);
        return res;
}

/*
 * Return the cgroup for "task" from the given hierarchy. Must be
 * called with cgroup_mutex and css_set_rwsem held.
 */
static struct cgroup *task_cgroup_from_root(struct task_struct *task,
                                            struct cgroup_root *root)
{
        /*
         * No need to lock the task - since we hold cgroup_mutex the
         * task can't change groups, so the only thing that can happen
         * is that it exits and its css is set back to init_css_set.
         */
        return cset_cgroup_from_root(task_css_set(task), root);
}

/*
 * A task must hold cgroup_mutex to modify cgroups.
 *
 * Any task can increment and decrement the count field without lock.
 * So in general, code holding cgroup_mutex can't rely on the count
 * field not changing.  However, if the count goes to zero, then only
 * cgroup_attach_task() can increment it again.  Because a count of zero
 * means that no tasks are currently attached, therefore there is no
 * way a task attached to that cgroup can fork (the other way to
 * increment the count).  So code holding cgroup_mutex can safely
 * assume that if the count is zero, it will stay zero. Similarly, if
 * a task holds cgroup_mutex on a cgroup with zero count, it
 * knows that the cgroup won't be removed, as cgroup_rmdir()
 * needs that mutex.
 *
 * A cgroup can only be deleted if both its 'count' of using tasks
 * is zero, and its list of 'children' cgroups is empty.  Since all
 * tasks in the system use _some_ cgroup, and since there is always at
 * least one task in the system (init, pid == 1), therefore, root cgroup
 * always has either children cgroups and/or using tasks.  So we don't
 * need a special hack to ensure that root cgroup cannot be deleted.
 *
 * P.S.  One more locking exception.  RCU is used to guard the
 * update of a tasks cgroup pointer by cgroup_attach_task()
 */

static struct kernfs_syscall_ops cgroup_kf_syscall_ops;
static const struct file_operations proc_cgroupstats_operations;

static char *cgroup_file_name(struct cgroup *cgrp, const struct cftype *cft,
                              char *buf)
{
        struct cgroup_subsys *ss = cft->ss;

        if (cft->ss && !(cft->flags & CFTYPE_NO_PREFIX) &&
            !(cgrp->root->flags & CGRP_ROOT_NOPREFIX))
                snprintf(buf, CGROUP_FILE_NAME_MAX, "%s.%s",
                         cgroup_on_dfl(cgrp) ? ss->name : ss->legacy_name,
                         cft->name);
        else
                strncpy(buf, cft->name, CGROUP_FILE_NAME_MAX);
        return buf;
}

/**
 * cgroup_file_mode - deduce file mode of a control file
 * @cft: the control file in question
 *
 * S_IRUGO for read, S_IWUSR for write.
 */
static umode_t cgroup_file_mode(const struct cftype *cft)
{
        umode_t mode = 0;

        if (cft->read_u64 || cft->read_s64 || cft->seq_show)
                mode |= S_IRUGO;

        if (cft->write_u64 || cft->write_s64 || cft->write) {
                if (cft->flags & CFTYPE_WORLD_WRITABLE)
                        mode |= S_IWUGO;
                else
                        mode |= S_IWUSR;
        }

        return mode;
}

/**
 * cgroup_calc_child_subsys_mask - calculate child_subsys_mask
 * @cgrp: the target cgroup
 * @subtree_control: the new subtree_control mask to consider
 *
 * On the default hierarchy, a subsystem may request other subsystems to be
 * enabled together through its ->depends_on mask.  In such cases, more
 * subsystems than specified in "cgroup.subtree_control" may be enabled.
 *
 * This function calculates which subsystems need to be enabled if
 * @subtree_control is to be applied to @cgrp.  The returned mask is always
 * a superset of @subtree_control and follows the usual hierarchy rules.
 */
static unsigned long cgroup_calc_child_subsys_mask(struct cgroup *cgrp,
                                                  unsigned long subtree_control)
{
        struct cgroup *parent = cgroup_parent(cgrp);
        unsigned long cur_ss_mask = subtree_control;
        struct cgroup_subsys *ss;
        int ssid;

        lockdep_assert_held(&cgroup_mutex);

        if (!cgroup_on_dfl(cgrp))
                return cur_ss_mask;

        while (true) {
                unsigned long new_ss_mask = cur_ss_mask;

                for_each_subsys_which(ss, ssid, &cur_ss_mask)
                        new_ss_mask |= ss->depends_on;

                /*
                 * Mask out subsystems which aren't available.  This can
                 * happen only if some depended-upon subsystems were bound
                 * to non-default hierarchies.
                 */
                if (parent)
                        new_ss_mask &= parent->child_subsys_mask;
                else
                        new_ss_mask &= cgrp->root->subsys_mask;

                if (new_ss_mask == cur_ss_mask)
                        break;
                cur_ss_mask = new_ss_mask;
        }

        return cur_ss_mask;
}

/**
 * cgroup_refresh_child_subsys_mask - update child_subsys_mask
 * @cgrp: the target cgroup
 *
 * Update @cgrp->child_subsys_mask according to the current
 * @cgrp->subtree_control using cgroup_calc_child_subsys_mask().
 */
static void cgroup_refresh_child_subsys_mask(struct cgroup *cgrp)
{
        cgrp->child_subsys_mask =
                cgroup_calc_child_subsys_mask(cgrp, cgrp->subtree_control);
}

/**
 * cgroup_kn_unlock - unlocking helper for cgroup kernfs methods
 * @kn: the kernfs_node being serviced
 *
 * This helper undoes cgroup_kn_lock_live() and should be invoked before
 * the method finishes if locking succeeded.  Note that once this function
 * returns the cgroup returned by cgroup_kn_lock_live() may become
 * inaccessible any time.  If the caller intends to continue to access the
 * cgroup, it should pin it before invoking this function.
 */
static void cgroup_kn_unlock(struct kernfs_node *kn)
{
        struct cgroup *cgrp;

        if (kernfs_type(kn) == KERNFS_DIR)
                cgrp = kn->priv;
        else
                cgrp = kn->parent->priv;

        mutex_unlock(&cgroup_mutex);

        kernfs_unbreak_active_protection(kn);
        cgroup_put(cgrp);
}

/**
 * cgroup_kn_lock_live - locking helper for cgroup kernfs methods
 * @kn: the kernfs_node being serviced
 *
 * This helper is to be used by a cgroup kernfs method currently servicing
 * @kn.  It breaks the active protection, performs cgroup locking and
 * verifies that the associated cgroup is alive.  Returns the cgroup if
 * alive; otherwise, %NULL.  A successful return should be undone by a
 * matching cgroup_kn_unlock() invocation.
 *
 * Any cgroup kernfs method implementation which requires locking the
 * associated cgroup should use this helper.  It avoids nesting cgroup
 * locking under kernfs active protection and allows all kernfs operations
 * including self-removal.
 */
static struct cgroup *cgroup_kn_lock_live(struct kernfs_node *kn)
{
        struct cgroup *cgrp;

        if (kernfs_type(kn) == KERNFS_DIR)
                cgrp = kn->priv;
        else
                cgrp = kn->parent->priv;

        /*
         * We're gonna grab cgroup_mutex which nests outside kernfs
         * active_ref.  cgroup liveliness check alone provides enough
         * protection against removal.  Ensure @cgrp stays accessible and
         * break the active_ref protection.
         */
        if (!cgroup_tryget(cgrp))
                return NULL;
        kernfs_break_active_protection(kn);

        mutex_lock(&cgroup_mutex);

        if (!cgroup_is_dead(cgrp))
                return cgrp;

        cgroup_kn_unlock(kn);
        return NULL;
}

static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
{
        char name[CGROUP_FILE_NAME_MAX];

        lockdep_assert_held(&cgroup_mutex);
        kernfs_remove_by_name(cgrp->kn, cgroup_file_name(cgrp, cft, name));
}

/**
 * css_clear_dir - remove subsys files in a cgroup directory
 * @css: taget css
 * @cgrp_override: specify if target cgroup is different from css->cgroup
 */
static void css_clear_dir(struct cgroup_subsys_state *css,
                          struct cgroup *cgrp_override)
{
        struct cgroup *cgrp = cgrp_override ?: css->cgroup;
        struct cftype *cfts;

        list_for_each_entry(cfts, &css->ss->cfts, node)
                cgroup_addrm_files(css, cgrp, cfts, false);
}

/**
 * css_populate_dir - create subsys files in a cgroup directory
 * @css: target css
 * @cgrp_overried: specify if target cgroup is different from css->cgroup
 *
 * On failure, no file is added.
 */
static int css_populate_dir(struct cgroup_subsys_state *css,
                            struct cgroup *cgrp_override)
{
        struct cgroup *cgrp = cgrp_override ?: css->cgroup;
        struct cftype *cfts, *failed_cfts;
        int ret;

        if (!css->ss) {
                if (cgroup_on_dfl(cgrp))
                        cfts = cgroup_dfl_base_files;
                else
                        cfts = cgroup_legacy_base_files;

                return cgroup_addrm_files(&cgrp->self, cgrp, cfts, true);
        }

        list_for_each_entry(cfts, &css->ss->cfts, node) {
                ret = cgroup_addrm_files(css, cgrp, cfts, true);
                if (ret < 0) {
                        failed_cfts = cfts;
                        goto err;
                }
        }
        return 0;
err:
        list_for_each_entry(cfts, &css->ss->cfts, node) {
                if (cfts == failed_cfts)
                        break;
                cgroup_addrm_files(css, cgrp, cfts, false);
        }
        return ret;
}

static int rebind_subsystems(struct cgroup_root *dst_root,
                             unsigned long ss_mask)
{
        struct cgroup *dcgrp = &dst_root->cgrp;
        struct cgroup_subsys *ss;
        unsigned long tmp_ss_mask;
        int ssid, i, ret;

        lockdep_assert_held(&cgroup_mutex);

        for_each_subsys_which(ss, ssid, &ss_mask) {
                /* if @ss has non-root csses attached to it, can't move */
                if (css_next_child(NULL, cgroup_css(&ss->root->cgrp, ss)))
                        return -EBUSY;

                /* can't move between two non-dummy roots either */
                if (ss->root != &cgrp_dfl_root && dst_root != &cgrp_dfl_root)
                        return -EBUSY;
        }

        /* skip creating root files on dfl_root for inhibited subsystems */
        tmp_ss_mask = ss_mask;
        if (dst_root == &cgrp_dfl_root)
                tmp_ss_mask &= ~cgrp_dfl_root_inhibit_ss_mask;

        for_each_subsys_which(ss, ssid, &tmp_ss_mask) {
                struct cgroup *scgrp = &ss->root->cgrp;
                int tssid;

                ret = css_populate_dir(cgroup_css(scgrp, ss), dcgrp);
                if (!ret)
                        continue;

                /*
                 * Rebinding back to the default root is not allowed to
                 * fail.  Using both default and non-default roots should
                 * be rare.  Moving subsystems back and forth even more so.
                 * Just warn about it and continue.
                 */
                if (dst_root == &cgrp_dfl_root) {
                        if (cgrp_dfl_root_visible) {
                                pr_warn("failed to create files (%d) while rebinding 0x%lx to default root\n",
                                        ret, ss_mask);
                                pr_warn("you may retry by moving them to a different hierarchy and unbinding\n");
                        }
                        continue;
                }

                for_each_subsys_which(ss, tssid, &tmp_ss_mask) {
                        if (tssid == ssid)
                                break;
                        css_clear_dir(cgroup_css(scgrp, ss), dcgrp);
                }
                return ret;
        }

        /*
         * Nothing can fail from this point on.  Remove files for the
         * removed subsystems and rebind each subsystem.
         */
        for_each_subsys_which(ss, ssid, &ss_mask) {
                struct cgroup_root *src_root = ss->root;
                struct cgroup *scgrp = &src_root->cgrp;
                struct cgroup_subsys_state *css = cgroup_css(scgrp, ss);
                struct css_set *cset;

                WARN_ON(!css || cgroup_css(dcgrp, ss));

                css_clear_dir(css, NULL);

                RCU_INIT_POINTER(scgrp->subsys[ssid], NULL);
                rcu_assign_pointer(dcgrp->subsys[ssid], css);
                ss->root = dst_root;
                css->cgroup = dcgrp;

                down_write(&css_set_rwsem);
                hash_for_each(css_set_table, i, cset, hlist)
                        list_move_tail(&cset->e_cset_node[ss->id],
                                       &dcgrp->e_csets[ss->id]);
                up_write(&css_set_rwsem);

                src_root->subsys_mask &= ~(1 << ssid);
                scgrp->subtree_control &= ~(1 << ssid);
                cgroup_refresh_child_subsys_mask(scgrp);

                /* default hierarchy doesn't enable controllers by default */
                dst_root->subsys_mask |= 1 << ssid;
                if (dst_root == &cgrp_dfl_root) {
                        static_branch_enable(cgroup_subsys_on_dfl_key[ssid]);
                } else {
                        dcgrp->subtree_control |= 1 << ssid;
                        cgroup_refresh_child_subsys_mask(dcgrp);
                        static_branch_disable(cgroup_subsys_on_dfl_key[ssid]);
                }

                if (ss->bind)
                        ss->bind(css);
        }

        kernfs_activate(dcgrp->kn);
        return 0;
}

static int cgroup_show_options(struct seq_file *seq,
                               struct kernfs_root *kf_root)
{
        struct cgroup_root *root = cgroup_root_from_kf(kf_root);
        struct cgroup_subsys *ss;
        int ssid;

        if (root != &cgrp_dfl_root)
                for_each_subsys(ss, ssid)
                        if (root->subsys_mask & (1 << ssid))
                                seq_show_option(seq, ss->legacy_name, NULL);
        if (root->flags & CGRP_ROOT_NOPREFIX)
                seq_puts(seq, ",noprefix");
        if (root->flags & CGRP_ROOT_XATTR)
                seq_puts(seq, ",xattr");

        spin_lock(&release_agent_path_lock);
        if (strlen(root->release_agent_path))
                seq_show_option(seq, "release_agent",
                                root->release_agent_path);
        spin_unlock(&release_agent_path_lock);

        if (test_bit(CGRP_CPUSET_CLONE_CHILDREN, &root->cgrp.flags))
                seq_puts(seq, ",clone_children");
        if (strlen(root->name))
                seq_show_option(seq, "name", root->name);
        return 0;
}

struct cgroup_sb_opts {
        unsigned long subsys_mask;
        unsigned int flags;
        char *release_agent;
        bool cpuset_clone_children;
        char *name;
        /* User explicitly requested empty subsystem */
        bool none;
};

static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
{
        char *token, *o = data;
        bool all_ss = false, one_ss = false;
        unsigned long mask = -1UL;
        struct cgroup_subsys *ss;
        int nr_opts = 0;
        int i;

#ifdef CONFIG_CPUSETS
        mask = ~(1U << cpuset_cgrp_id);
#endif

        memset(opts, 0, sizeof(*opts));

        while ((token = strsep(&o, ",")) != NULL) {
                nr_opts++;

                if (!*token)
                        return -EINVAL;
                if (!strcmp(token, "none")) {
                        /* Explicitly have no subsystems */
                        opts->none = true;
                        continue;
                }
                if (!strcmp(token, "all")) {
                        /* Mutually exclusive option 'all' + subsystem name */
                        if (one_ss)
                                return -EINVAL;
                        all_ss = true;
                        continue;
                }
                if (!strcmp(token, "__DEVEL__sane_behavior")) {
                        opts->flags |= CGRP_ROOT_SANE_BEHAVIOR;
                        continue;
                }
                if (!strcmp(token, "noprefix")) {
                        opts->flags |= CGRP_ROOT_NOPREFIX;
                        continue;
                }
                if (!strcmp(token, "clone_children")) {
                        opts->cpuset_clone_children = true;
                        continue;
                }
                if (!strcmp(token, "xattr")) {
                        opts->flags |= CGRP_ROOT_XATTR;
                        continue;
                }
                if (!strncmp(token, "release_agent=", 14)) {
                        /* Specifying two release agents is forbidden */
                        if (opts->release_agent)
                                return -EINVAL;
                        opts->release_agent =
                                kstrndup(token + 14, PATH_MAX - 1, GFP_KERNEL);
                        if (!opts->release_agent)
                                return -ENOMEM;
                        continue;
                }
                if (!strncmp(token, "name=", 5)) {
                        const char *name = token + 5;
                        /* Can't specify an empty name */
                        if (!strlen(name))
                                return -EINVAL;
                        /* Must match [\w.-]+ */
                        for (i = 0; i < strlen(name); i++) {
                                char c = name[i];
                                if (isalnum(c))
                                        continue;
                                if ((c == '.') || (c == '-') || (c == '_'))
                                        continue;
                                return -EINVAL;
                        }
                        /* Specifying two names is forbidden */
                        if (opts->name)
                                return -EINVAL;
                        opts->name = kstrndup(name,
                                              MAX_CGROUP_ROOT_NAMELEN - 1,
                                              GFP_KERNEL);
                        if (!opts->name)
                                return -ENOMEM;

                        continue;
                }

                for_each_subsys(ss, i) {
                        if (strcmp(token, ss->legacy_name))
                                continue;
                        if (!cgroup_ssid_enabled(i))
                                continue;

                        /* Mutually exclusive option 'all' + subsystem name */
                        if (all_ss)
                                return -EINVAL;
                        opts->subsys_mask |= (1 << i);
                        one_ss = true;

                        break;
                }
                if (i == CGROUP_SUBSYS_COUNT)
                        return -ENOENT;
        }

        if (opts->flags & CGRP_ROOT_SANE_BEHAVIOR) {
                pr_warn("sane_behavior: this is still under development and its behaviors will change, proceed at your own risk\n");
                if (nr_opts != 1) {
                        pr_err("sane_behavior: no other mount options allowed\n");
                        return -EINVAL;
                }
                return 0;
        }

        /*
         * If the 'all' option was specified select all the subsystems,
         * otherwise if 'none', 'name=' and a subsystem name options were
         * not specified, let's default to 'all'
         */
        if (all_ss || (!one_ss && !opts->none && !opts->name))
                for_each_subsys(ss, i)
                        if (cgroup_ssid_enabled(i))
                                opts->subsys_mask |= (1 << i);

        /*
         * We either have to specify by name or by subsystems. (So all
         * empty hierarchies must have a name).
         */
        if (!opts->subsys_mask && !opts->name)
                return -EINVAL;

        /*
         * Option noprefix was introduced just for backward compatibility
         * with the old cpuset, so we allow noprefix only if mounting just
         * the cpuset subsystem.
         */
        if ((opts->flags & CGRP_ROOT_NOPREFIX) && (opts->subsys_mask & mask))
                return -EINVAL;

        /* Can't specify "none" and some subsystems */
        if (opts->subsys_mask && opts->none)
                return -EINVAL;

        return 0;
}

static int cgroup_remount(struct kernfs_root *kf_root, int *flags, char *data)
{
        int ret = 0;
        struct cgroup_root *root = cgroup_root_from_kf(kf_root);
        struct cgroup_sb_opts opts;
        unsigned long added_mask, removed_mask;

        if (root == &cgrp_dfl_root) {
                pr_err("remount is not allowed\n");
                return -EINVAL;
        }

        mutex_lock(&cgroup_mutex);

        /* See what subsystems are wanted */
        ret = parse_cgroupfs_options(data, &opts);
        if (ret)
                goto out_unlock;

        if (opts.subsys_mask != root->subsys_mask || opts.release_agent)
                pr_warn("option changes via remount are deprecated (pid=%d comm=%s)\n",
                        task_tgid_nr(current), current->comm);

        added_mask = opts.subsys_mask & ~root->subsys_mask;
        removed_mask = root->subsys_mask & ~opts.subsys_mask;

        /* Don't allow flags or name to change at remount */
        if ((opts.flags ^ root->flags) ||
            (opts.name && strcmp(opts.name, root->name))) {
                pr_err("option or name mismatch, new: 0x%x \"%s\", old: 0x%x \"%s\"\n",
                       opts.flags, opts.name ?: "", root->flags, root->name);
                ret = -EINVAL;
                goto out_unlock;
        }

        /* remounting is not allowed for populated hierarchies */
        if (!list_empty(&root->cgrp.self.children)) {
                ret = -EBUSY;
                goto out_unlock;
        }

        ret = rebind_subsystems(root, added_mask);
        if (ret)
                goto out_unlock;

        rebind_subsystems(&cgrp_dfl_root, removed_mask);

        if (opts.release_agent) {
                spin_lock(&release_agent_path_lock);
                strcpy(root->release_agent_path, opts.release_agent);
                spin_unlock(&release_agent_path_lock);
        }
 out_unlock:
        kfree(opts.release_agent);
        kfree(opts.name);
        mutex_unlock(&cgroup_mutex);
        return ret;
}

/*
 * To reduce the fork() overhead for systems that are not actually using
 * their cgroups capability, we don't maintain the lists running through
 * each css_set to its tasks until we see the list actually used - in other
 * words after the first mount.
 */
static bool use_task_css_set_links __read_mostly;

static void cgroup_enable_task_cg_lists(void)
{
        struct task_struct *p, *g;

        down_write(&css_set_rwsem);

        if (use_task_css_set_links)
                goto out_unlock;

        use_task_css_set_links = true;

        /*
         * We need tasklist_lock because RCU is not safe against
         * while_each_thread(). Besides, a forking task that has passed
         * cgroup_post_fork() without seeing use_task_css_set_links = 1
         * is not guaranteed to have its child immediately visible in the
         * tasklist if we walk through it with RCU.
         */
        read_lock(&tasklist_lock);
        do_each_thread(g, p) {
                WARN_ON_ONCE(!list_empty(&p->cg_list) ||
                             task_css_set(p) != &init_css_set);

                /*
                 * We should check if the process is exiting, otherwise
                 * it will race with cgroup_exit() in that the list
                 * entry won't be deleted though the process has exited.
                 * Do it while holding siglock so that we don't end up
                 * racing against cgroup_exit().
                 */
                spin_lock_irq(&p->sighand->siglock);
                if (!(p->flags & PF_EXITING)) {
                        struct css_set *cset = task_css_set(p);

                        if (!css_set_populated(cset))
                                css_set_update_populated(cset, true);
                        list_add_tail(&p->cg_list, &cset->tasks);
                        get_css_set(cset);
                }
                spin_unlock_irq(&p->sighand->siglock);
        } while_each_thread(g, p);
        read_unlock(&tasklist_lock);
out_unlock:
        up_write(&css_set_rwsem);
}

static void init_cgroup_housekeeping(struct cgroup *cgrp)
{
        struct cgroup_subsys *ss;
        int ssid;

        INIT_LIST_HEAD(&cgrp->self.sibling);
        INIT_LIST_HEAD(&cgrp->self.children);
        INIT_LIST_HEAD(&cgrp->self.files);
        INIT_LIST_HEAD(&cgrp->cset_links);
        INIT_LIST_HEAD(&cgrp->pidlists);
        mutex_init(&cgrp->pidlist_mutex);
        cgrp->self.cgroup = cgrp;
        cgrp->self.flags |= CSS_ONLINE;

        for_each_subsys(ss, ssid)
                INIT_LIST_HEAD(&cgrp->e_csets[ssid]);

        init_waitqueue_head(&cgrp->offline_waitq);
        INIT_WORK(&cgrp->release_agent_work, cgroup_release_agent);
}

static void init_cgroup_root(struct cgroup_root *root,
                             struct cgroup_sb_opts *opts)
{
        struct cgroup *cgrp = &root->cgrp;

        INIT_LIST_HEAD(&root->root_list);
        atomic_set(&root->nr_cgrps, 1);
        cgrp->root = root;
        init_cgroup_housekeeping(cgrp);
        idr_init(&root->cgroup_idr);

        root->flags = opts->flags;
        if (opts->release_agent)
                strcpy(root->release_agent_path, opts->release_agent);
        if (opts->name)
                strcpy(root->name, opts->name);
        if (opts->cpuset_clone_children)
                set_bit(CGRP_CPUSET_CLONE_CHILDREN, &root->cgrp.flags);
}

static int cgroup_setup_root(struct cgroup_root *root, unsigned long ss_mask)
{
        LIST_HEAD(tmp_links);
        struct cgroup *root_cgrp = &root->cgrp;
        struct css_set *cset;
        int i, ret;

        lockdep_assert_held(&cgroup_mutex);

        ret = cgroup_idr_alloc(&root->cgroup_idr, root_cgrp, 1, 2, GFP_KERNEL);
        if (ret < 0)
                goto out;
        root_cgrp->id = ret;

        ret = percpu_ref_init(&root_cgrp->self.refcnt, css_release, 0,
                              GFP_KERNEL);
        if (ret)
                goto out;

        /*
         * We're accessing css_set_count without locking css_set_rwsem here,
         * but that's OK - it can only be increased by someone holding
         * cgroup_lock, and that's us. The worst that can happen is that we
         * have some link structures left over
         */
        ret = allocate_cgrp_cset_links(css_set_count, &tmp_links);
        if (ret)
                goto cancel_ref;

        ret = cgroup_init_root_id(root);
        if (ret)
                goto cancel_ref;

        root->kf_root = kernfs_create_root(&cgroup_kf_syscall_ops,
                                           KERNFS_ROOT_CREATE_DEACTIVATED,
                                           root_cgrp);
        if (IS_ERR(root->kf_root)) {
                ret = PTR_ERR(root->kf_root);
                goto exit_root_id;
        }
        root_cgrp->kn = root->kf_root->kn;

        ret = css_populate_dir(&root_cgrp->self, NULL);
        if (ret)
                goto destroy_root;

        ret = rebind_subsystems(root, ss_mask);
        if (ret)
                goto destroy_root;

        /*
         * There must be no failure case after here, since rebinding takes
         * care of subsystems' refcounts, which are explicitly dropped in
         * the failure exit path.
         */
        list_add(&root->root_list, &cgroup_roots);
        cgroup_root_count++;

        /*
         * Link the root cgroup in this hierarchy into all the css_set
         * objects.
         */
        down_write(&css_set_rwsem);
        hash_for_each(css_set_table, i, cset, hlist) {
                link_css_set(&tmp_links, cset, root_cgrp);
                if (css_set_populated(cset))
                        cgroup_update_populated(root_cgrp, true);
        }
        up_write(&css_set_rwsem);

        BUG_ON(!list_empty(&root_cgrp->self.children));
        BUG_ON(atomic_read(&root->nr_cgrps) != 1);

        kernfs_activate(root_cgrp->kn);
        ret = 0;
        goto out;

destroy_root:
        kernfs_destroy_root(root->kf_root);
        root->kf_root = NULL;
exit_root_id:
        cgroup_exit_root_id(root);
cancel_ref:
        percpu_ref_exit(&root_cgrp->self.refcnt);
out:
        free_cgrp_cset_links(&tmp_links);
        return ret;
}

static struct dentry *cgroup_mount(struct file_system_type *fs_type,
                         int flags, const char *unused_dev_name,
                         void *data)
{
        struct super_block *pinned_sb = NULL;
        struct cgroup_subsys *ss;
        struct cgroup_root *root;
        struct cgroup_sb_opts opts;
        struct dentry *dentry;
        int ret;
        int i;
        bool new_sb;

        /*
         * The first time anyone tries to mount a cgroup, enable the list
         * linking each css_set to its tasks and fix up all existing tasks.
         */
        if (!use_task_css_set_links)
                cgroup_enable_task_cg_lists();

        mutex_lock(&cgroup_mutex);

        /* First find the desired set of subsystems */
        ret = parse_cgroupfs_options(data, &opts);
        if (ret)
                goto out_unlock;

        /* look for a matching existing root */
        if (opts.flags & CGRP_ROOT_SANE_BEHAVIOR) {
                cgrp_dfl_root_visible = true;
                root = &cgrp_dfl_root;
                cgroup_get(&root->cgrp);
                ret = 0;
                goto out_unlock;
        }

        /*
         * Destruction of cgroup root is asynchronous, so subsystems may
         * still be dying after the previous unmount.  Let's drain the
         * dying subsystems.  We just need to ensure that the ones
         * unmounted previously finish dying and don't care about new ones
         * starting.  Testing ref liveliness is good enough.
         */
        for_each_subsys(ss, i) {
                if (!(opts.subsys_mask & (1 << i)) ||
                    ss->root == &cgrp_dfl_root)
                        continue;

                if (!percpu_ref_tryget_live(&ss->root->cgrp.self.refcnt)) {
                        mutex_unlock(&cgroup_mutex);
                        msleep(10);
                        ret = restart_syscall();
                        goto out_free;
                }
                cgroup_put(&ss->root->cgrp);
        }

        for_each_root(root) {
                bool name_match = false;

                if (root == &cgrp_dfl_root)
                        continue;

                /*
                 * If we asked for a name then it must match.  Also, if
                 * name matches but sybsys_mask doesn't, we should fail.
                 * Remember whether name matched.
                 */
                if (opts.name) {
                        if (strcmp(opts.name, root->name))
                                continue;
                        name_match = true;
                }

                /*
                 * If we asked for subsystems (or explicitly for no
                 * subsystems) then they must match.
                 */
                if ((opts.subsys_mask || opts.none) &&
                    (opts.subsys_mask != root->subsys_mask)) {
                        if (!name_match)
                                continue;
                        ret = -EBUSY;
                        goto out_unlock;
                }

                if (root->flags ^ opts.flags)
                        pr_warn("new mount options do not match the existing superblock, will be ignored\n");

                /*
                 * We want to reuse @root whose lifetime is governed by its
                 * ->cgrp.  Let's check whether @root is alive and keep it
                 * that way.  As cgroup_kill_sb() can happen anytime, we
                 * want to block it by pinning the sb so that @root doesn't
                 * get killed before mount is complete.
                 *
                 * With the sb pinned, tryget_live can reliably indicate
                 * whether @root can be reused.  If it's being killed,
                 * drain it.  We can use wait_queue for the wait but this
                 * path is super cold.  Let's just sleep a bit and retry.
                 */
                pinned_sb = kernfs_pin_sb(root->kf_root, NULL);
                if (IS_ERR(pinned_sb) ||
                    !percpu_ref_tryget_live(&root->cgrp.self.refcnt)) {
                        mutex_unlock(&cgroup_mutex);
                        if (!IS_ERR_OR_NULL(pinned_sb))
                                deactivate_super(pinned_sb);
                        msleep(10);
                        ret = restart_syscall();
                        goto out_free;
                }

                ret = 0;
                goto out_unlock;
        }

        /*
         * No such thing, create a new one.  name= matching without subsys
         * specification is allowed for already existing hierarchies but we
         * can't create new one without subsys specification.
         */
        if (!opts.subsys_mask && !opts.none) {
                ret = -EINVAL;
                goto out_unlock;
        }

        root = kzalloc(sizeof(*root), GFP_KERNEL);
        if (!root) {
                ret = -ENOMEM;
                goto out_unlock;
        }

        init_cgroup_root(root, &opts);

        ret = cgroup_setup_root(root, opts.subsys_mask);
        if (ret)
                cgroup_free_root(root);

out_unlock:
        mutex_unlock(&cgroup_mutex);
out_free:
        kfree(opts.release_agent);
        kfree(opts.name);

        if (ret)
                return ERR_PTR(ret);

        dentry = kernfs_mount(fs_type, flags, root->kf_root,
                                CGROUP_SUPER_MAGIC, &new_sb);
        if (IS_ERR(dentry) || !new_sb)
                cgroup_put(&root->cgrp);

        /*
         * If @pinned_sb, we're reusing an existing root and holding an
         * extra ref on its sb.  Mount is complete.  Put the extra ref.
         */
        if (pinned_sb) {
                WARN_ON(new_sb);
                deactivate_super(pinned_sb);
        }

        return dentry;
}

static void cgroup_kill_sb(struct super_block *sb)
{
        struct kernfs_root *kf_root = kernfs_root_from_sb(sb);
        struct cgroup_root *root = cgroup_root_from_kf(kf_root);

        /*
         * If @root doesn't have any mounts or children, start killing it.
         * This prevents new mounts by disabling percpu_ref_tryget_live().
         * cgroup_mount() may wait for @root's release.
         *
         * And don't kill the default root.
         */
        if (!list_empty(&root->cgrp.self.children) ||
            root == &cgrp_dfl_root)
                cgroup_put(&root->cgrp);
        else
                percpu_ref_kill(&root->cgrp.self.refcnt);

        kernfs_kill_sb(sb);
}

static struct file_system_type cgroup_fs_type = {
        .name = "cgroup",
        .mount = cgroup_mount,
        .kill_sb = cgroup_kill_sb,
};

/**
 * task_cgroup_path - cgroup path of a task in the first cgroup hierarchy
 * @task: target task
 * @buf: the buffer to write the path into
 * @buflen: the length of the buffer
 *
 * Determine @task's cgroup on the first (the one with the lowest non-zero
 * hierarchy_id) cgroup hierarchy and copy its path into @buf.  This
 * function grabs cgroup_mutex and shouldn't be used inside locks used by
 * cgroup controller callbacks.
 *
 * Return value is the same as kernfs_path().
 */
char *task_cgroup_path(struct task_struct *task, char *buf, size_t buflen)
{
        struct cgroup_root *root;
        struct cgroup *cgrp;
        int hierarchy_id = 1;
        char *path = NULL;

        mutex_lock(&cgroup_mutex);
        down_read(&css_set_rwsem);

        root = idr_get_next(&cgroup_hierarchy_idr, &hierarchy_id);

        if (root) {
                cgrp = task_cgroup_from_root(task, root);
                path = cgroup_path(cgrp, buf, buflen);
        } else {
                /* if no hierarchy exists, everyone is in "/" */
                if (strlcpy(buf, "/", buflen) < buflen)
                        path = buf;
        }

        up_read(&css_set_rwsem);
        mutex_unlock(&cgroup_mutex);
        return path;
}
EXPORT_SYMBOL_GPL(task_cgroup_path);

/* used to track tasks and other necessary states during migration */
struct cgroup_taskset {
        /* the src and dst cset list running through cset->mg_node */
        struct list_head        src_csets;
        struct list_head        dst_csets;

        /*
         * Fields for cgroup_taskset_*() iteration.
         *
         * Before migration is committed, the target migration tasks are on
         * ->mg_tasks of the csets on ->src_csets.  After, on ->mg_tasks of
         * the csets on ->dst_csets.  ->csets point to either ->src_csets
         * or ->dst_csets depending on whether migration is committed.
         *
         * ->cur_csets and ->cur_task point to the current task position
         * during iteration.
         */
        struct list_head        *csets;
        struct css_set          *cur_cset;
        struct task_struct      *cur_task;
};

#define CGROUP_TASKSET_INIT(tset)       (struct cgroup_taskset){        \
        .src_csets              = LIST_HEAD_INIT(tset.src_csets),       \
        .dst_csets              = LIST_HEAD_INIT(tset.dst_csets),       \
        .csets                  = &tset.src_csets,                      \
}

/**
 * cgroup_taskset_add - try to add a migration target task to a taskset
 * @task: target task
 * @tset: target taskset
 *
 * Add @task, which is a migration target, to @tset.  This function becomes
 * noop if @task doesn't need to be migrated.  @task's css_set should have
 * been added as a migration source and @task->cg_list will be moved from
 * the css_set's tasks list to mg_tasks one.
 */
static void cgroup_taskset_add(struct task_struct *task,
                               struct cgroup_taskset *tset)
{
        struct css_set *cset;

        lockdep_assert_held(&css_set_rwsem);

        /* @task either already exited or can't exit until the end */
        if (task->flags & PF_EXITING)
                return;

        /* leave @task alone if post_fork() hasn't linked it yet */
        if (list_empty(&task->cg_list))
                return;

        cset = task_css_set(task);
        if (!cset->mg_src_cgrp)
                return;

        list_move_tail(&task->cg_list, &cset->mg_tasks);
        if (list_empty(&cset->mg_node))
                list_add_tail(&cset->mg_node, &tset->src_csets);
        if (list_empty(&cset->mg_dst_cset->mg_node))
                list_move_tail(&cset->mg_dst_cset->mg_node,
                               &tset->dst_csets);
}

/**
 * cgroup_taskset_first - reset taskset and return the first task
 * @tset: taskset of interest
 *
 * @tset iteration is initialized and the first task is returned.
 */
struct task_struct *cgroup_taskset_first(struct cgroup_taskset *tset)
{
        tset->cur_cset = list_first_entry(tset->csets, struct css_set, mg_node);
        tset->cur_task = NULL;

        return cgroup_taskset_next(tset);
}

/**
 * cgroup_taskset_next - iterate to the next task in taskset
 * @tset: taskset of interest
 *
 * Return the next task in @tset.  Iteration must have been initialized
 * with cgroup_taskset_first().
 */
struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset)
{
        struct css_set *cset = tset->cur_cset;
        struct task_struct *task = tset->cur_task;

        while (&cset->mg_node != tset->csets) {
                if (!task)
                        task = list_first_entry(&cset->mg_tasks,
                                                struct task_struct, cg_list);
                else
                        task = list_next_entry(task, cg_list);

                if (&task->cg_list != &cset->mg_tasks) {
                        tset->cur_cset = cset;
                        tset->cur_task = task;
                        return task;
                }

                cset = list_next_entry(cset, mg_node);
                task = NULL;
        }

        return NULL;
}

/**
 * cgroup_taskset_migrate - migrate a taskset to a cgroup
 * @tset: taget taskset
 * @dst_cgrp: destination cgroup
 *
 * Migrate tasks in @tset to @dst_cgrp.  This function fails iff one of the
 * ->can_attach callbacks fails and guarantees that either all or none of
 * the tasks in @tset are migrated.  @tset is consumed regardless of
 * success.
 */
static int cgroup_taskset_migrate(struct cgroup_taskset *tset,
                                  struct cgroup *dst_cgrp)
{
        struct cgroup_subsys_state *css, *failed_css = NULL;
        struct task_struct *task, *tmp_task;
        struct css_set *cset, *tmp_cset;
        int i, ret;

        /* methods shouldn't be called if no task is actually migrating */
        if (list_empty(&tset->src_csets))
                return 0;

        /* check that we can legitimately attach to the cgroup */
        for_each_e_css(css, i, dst_cgrp) {
                if (css->ss->can_attach) {
                        ret = css->ss->can_attach(css, tset);
                        if (ret) {
                                failed_css = css;
                                goto out_cancel_attach;
                        }
                }
        }

        /*
         * Now that we're guaranteed success, proceed to move all tasks to
         * the new cgroup.  There are no failure cases after here, so this
         * is the commit point.
         */
        down_write(&css_set_rwsem);
        list_for_each_entry(cset, &tset->src_csets, mg_node) {
                list_for_each_entry_safe(task, tmp_task, &cset->mg_tasks, cg_list) {
                        struct css_set *from_cset = task_css_set(task);
                        struct css_set *to_cset = cset->mg_dst_cset;

                        get_css_set(to_cset);
                        css_set_move_task(task, from_cset, to_cset, true);
                        put_css_set_locked(from_cset);
                }
        }
        up_write(&css_set_rwsem);

        /*
         * Migration is committed, all target tasks are now on dst_csets.
         * Nothing is sensitive to fork() after this point.  Notify
         * controllers that migration is complete.
         */
        tset->csets = &tset->dst_csets;

        for_each_e_css(css, i, dst_cgrp)
                if (css->ss->attach)
                        css->ss->attach(css, tset);

        ret = 0;
        goto out_release_tset;

out_cancel_attach:
        for_each_e_css(css, i, dst_cgrp) {
                if (css == failed_css)
                        break;
                if (css->ss->cancel_attach)
                        css->ss->cancel_attach(css, tset);
        }
out_release_tset:
        down_write(&css_set_rwsem);
        list_splice_init(&tset->dst_csets, &tset->src_csets);
        list_for_each_entry_safe(cset, tmp_cset, &tset->src_csets, mg_node) {
                list_splice_tail_init(&cset->mg_tasks, &cset->tasks);
                list_del_init(&cset->mg_node);
        }
        up_write(&css_set_rwsem);
        return ret;
}

/**
 * cgroup_migrate_finish - cleanup after attach
 * @preloaded_csets: list of preloaded css_sets
 *
 * Undo cgroup_migrate_add_src() and cgroup_migrate_prepare_dst().  See
 * those functions for details.
 */
static void cgroup_migrate_finish(struct list_head *preloaded_csets)
{
        struct css_set *cset, *tmp_cset;

        lockdep_assert_held(&cgroup_mutex);

        down_write(&css_set_rwsem);
        list_for_each_entry_safe(cset, tmp_cset, preloaded_csets, mg_preload_node) {
                cset->mg_src_cgrp = NULL;
                cset->mg_dst_cset = NULL;
                list_del_init(&cset->mg_preload_node);
                put_css_set_locked(cset);
        }
        up_write(&css_set_rwsem);
}

/**
 * cgroup_migrate_add_src - add a migration source css_set
 * @src_cset: the source css_set to add
 * @dst_cgrp: the destination cgroup
 * @preloaded_csets: list of preloaded css_sets
 *
 * Tasks belonging to @src_cset are about to be migrated to @dst_cgrp.  Pin
 * @src_cset and add it to @preloaded_csets, which should later be cleaned
 * up by cgroup_migrate_finish().
 *
 * This function may be called without holding cgroup_threadgroup_rwsem
 * even if the target is a process.  Threads may be created and destroyed
 * but as long as cgroup_mutex is not dropped, no new css_set can be put
 * into play and the preloaded css_sets are guaranteed to cover all
 * migrations.
 */
static void cgroup_migrate_add_src(struct css_set *src_cset,
                                   struct cgroup *dst_cgrp,
                                   struct list_head *preloaded_csets)
{
        struct cgroup *src_cgrp;

        lockdep_assert_held(&cgroup_mutex);
        lockdep_assert_held(&css_set_rwsem);

        src_cgrp = cset_cgroup_from_root(src_cset, dst_cgrp->root);

        if (!list_empty(&src_cset->mg_preload_node))
                return;

        WARN_ON(src_cset->mg_src_cgrp);
        WARN_ON(!list_empty(&src_cset->mg_tasks));
        WARN_ON(!list_empty(&src_cset->mg_node));

        src_cset->mg_src_cgrp = src_cgrp;
        get_css_set(src_cset);
        list_add(&src_cset->mg_preload_node, preloaded_csets);
}

/**
 * cgroup_migrate_prepare_dst - prepare destination css_sets for migration
 * @dst_cgrp: the destination cgroup (may be %NULL)
 * @preloaded_csets: list of preloaded source css_sets
 *
 * Tasks are about to be moved to @dst_cgrp and all the source css_sets
 * have been preloaded to @preloaded_csets.  This function looks up and
 * pins all destination css_sets, links each to its source, and append them
 * to @preloaded_csets.  If @dst_cgrp is %NULL, the destination of each
 * source css_set is assumed to be its cgroup on the default hierarchy.
 *
 * This function must be called after cgroup_migrate_add_src() has been
 * called on each migration source css_set.  After migration is performed
 * using cgroup_migrate(), cgroup_migrate_finish() must be called on
 * @preloaded_csets.
 */
static int cgroup_migrate_prepare_dst(struct cgroup *dst_cgrp,
                                      struct list_head *preloaded_csets)
{
        LIST_HEAD(csets);
        struct css_set *src_cset, *tmp_cset;

        lockdep_assert_held(&cgroup_mutex);

        /*
         * Except for the root, child_subsys_mask must be zero for a cgroup
         * with tasks so that child cgroups don't compete against tasks.
         */
        if (dst_cgrp && cgroup_on_dfl(dst_cgrp) && cgroup_parent(dst_cgrp) &&
            dst_cgrp->child_subsys_mask)
                return -EBUSY;

        /* look up the dst cset for each src cset and link it to src */
        list_for_each_entry_safe(src_cset, tmp_cset, preloaded_csets, mg_preload_node) {
                struct css_set *dst_cset;

                dst_cset = find_css_set(src_cset,
                                        dst_cgrp ?: src_cset->dfl_cgrp);
                if (!dst_cset)
                        goto err;

                WARN_ON_ONCE(src_cset->mg_dst_cset || dst_cset->mg_dst_cset);

                /*
                 * If src cset equals dst, it's noop.  Drop the src.
                 * cgroup_migrate() will skip the cset too.  Note that we
                 * can't handle src == dst as some nodes are used by both.
                 */
                if (src_cset == dst_cset) {
                        src_cset->mg_src_cgrp = NULL;
                        list_del_init(&src_cset->mg_preload_node);
                        put_css_set(src_cset);
                        put_css_set(dst_cset);
                        continue;
                }

                src_cset->mg_dst_cset = dst_cset;

                if (list_empty(&dst_cset->mg_preload_node))
                        list_add(&dst_cset->mg_preload_node, &csets);
                else
                        put_css_set(dst_cset);
        }

        list_splice_tail(&csets, preloaded_csets);
        return 0;
err:
        cgroup_migrate_finish(&csets);
        return -ENOMEM;
}

/**
 * cgroup_migrate - migrate a process or task to a cgroup
 * @leader: the leader of the process or the task to migrate
 * @threadgroup: whether @leader points to the whole process or a single task
 * @cgrp: the destination cgroup
 *
 * Migrate a process or task denoted by @leader to @cgrp.  If migrating a
 * process, the caller must be holding cgroup_threadgroup_rwsem.  The
 * caller is also responsible for invoking cgroup_migrate_add_src() and
 * cgroup_migrate_prepare_dst() on the targets before invoking this
 * function and following up with cgroup_migrate_finish().
 *
 * As long as a controller's ->can_attach() doesn't fail, this function is
 * guaranteed to succeed.  This means that, excluding ->can_attach()
 * failure, when migrating multiple targets, the success or failure can be
 * decided for all targets by invoking group_migrate_prepare_dst() before
 * actually starting migrating.
 */
static int cgroup_migrate(struct task_struct *leader, bool threadgroup,
                          struct cgroup *cgrp)
{
        struct cgroup_taskset tset = CGROUP_TASKSET_INIT(tset);
        struct task_struct *task;

        /*
         * Prevent freeing of tasks while we take a snapshot. Tasks that are
         * already PF_EXITING could be freed from underneath us unless we
         * take an rcu_read_lock.
         */
        down_write(&css_set_rwsem);
        rcu_read_lock();
        task = leader;
        do {
                cgroup_taskset_add(task, &tset);
                if (!threadgroup)
                        break;
        } while_each_thread(leader, task);
        rcu_read_unlock();
        up_write(&css_set_rwsem);

        return cgroup_taskset_migrate(&tset, cgrp);
}

/**
 * cgroup_attach_task - attach a task or a whole threadgroup to a cgroup
 * @dst_cgrp: the cgroup to attach to
 * @leader: the task or the leader of the threadgroup to be attached
 * @threadgroup: attach the whole threadgroup?
 *
 * Call holding cgroup_mutex and cgroup_threadgroup_rwsem.
 */
static int cgroup_attach_task(struct cgroup *dst_cgrp,
                              struct task_struct *leader, bool threadgroup)
{
        LIST_HEAD(preloaded_csets);
        struct task_struct *task;
        int ret;

        /* look up all src csets */
        down_read(&css_set_rwsem);
        rcu_read_lock();
        task = leader;
        do {
                cgroup_migrate_add_src(task_css_set(task), dst_cgrp,
                                       &preloaded_csets);
                if (!threadgroup)
                        break;
        } while_each_thread(leader, task);
        rcu_read_unlock();
        up_read(&css_set_rwsem);

        /* prepare dst csets and commit */
        ret = cgroup_migrate_prepare_dst(dst_cgrp, &preloaded_csets);
        if (!ret)
                ret = cgroup_migrate(leader, threadgroup, dst_cgrp);

        cgroup_migrate_finish(&preloaded_csets);
        return ret;
}

static int cgroup_procs_write_permission(struct task_struct *task,
                                         struct cgroup *dst_cgrp,
                                         struct kernfs_open_file *of)
{
        const struct cred *cred = current_cred();
        const struct cred *tcred = get_task_cred(task);
        int ret = 0;

        /*
         * even if we're attaching all tasks in the thread group, we only
         * need to check permissions on one of them.
         */
        if (!uid_eq(cred->euid, GLOBAL_ROOT_UID) &&
            !uid_eq(cred->euid, tcred->uid) &&
            !uid_eq(cred->euid, tcred->suid))
                ret = -EACCES;

        if (!ret && cgroup_on_dfl(dst_cgrp)) {
                struct super_block *sb = of->file->f_path.dentry->d_sb;
                struct cgroup *cgrp;
                struct inode *inode;

                down_read(&css_set_rwsem);
                cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
                up_read(&css_set_rwsem);

                while (!cgroup_is_descendant(dst_cgrp, cgrp))
                        cgrp = cgroup_parent(cgrp);

                ret = -ENOMEM;
                inode = kernfs_get_inode(sb, cgrp->procs_file.kn);
                if (inode) {
                        ret = inode_permission(inode, MAY_WRITE);
                        iput(inode);
                }
        }

        put_cred(tcred);
        return ret;
}

/*
 * Find the task_struct of the task to attach by vpid and pass it along to the
 * function to attach either it or all tasks in its threadgroup. Will lock
 * cgroup_mutex and threadgroup.
 */
static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
                                    size_t nbytes, loff_t off, bool threadgroup)
{
        struct task_struct *tsk;
        struct cgroup *cgrp;
        pid_t pid;
        int ret;

        if (kstrtoint(strstrip(buf), 0, &pid) || pid < 0)
                return -EINVAL;

        cgrp = cgroup_kn_lock_live(of->kn);
        if (!cgrp)
                return -ENODEV;

        percpu_down_write(&cgroup_threadgroup_rwsem);
        rcu_read_lock();
        if (pid) {
                tsk = find_task_by_vpid(pid);
                if (!tsk) {
                        ret = -ESRCH;
                        goto out_unlock_rcu;
                }
        } else {
                tsk = current;
        }

        if (threadgroup)
                tsk = tsk->group_leader;

        /*
         * Workqueue threads may acquire PF_NO_SETAFFINITY and become
         * trapped in a cpuset, or RT worker may be born in a cgroup
         * with no rt_runtime allocated.  Just say no.
         */
        if (tsk == kthreadd_task || (tsk->flags & PF_NO_SETAFFINITY)) {
                ret = -EINVAL;
                goto out_unlock_rcu;
        }

        get_task_struct(tsk);
        rcu_read_unlock();

        ret = cgroup_procs_write_permission(tsk, cgrp, of);
        if (!ret)
                ret = cgroup_attach_task(cgrp, tsk, threadgroup);

        put_task_struct(tsk);
        goto out_unlock_threadgroup;

out_unlock_rcu:
        rcu_read_unlock();
out_unlock_threadgroup:
        percpu_up_write(&cgroup_threadgroup_rwsem);
        cgroup_kn_unlock(of->kn);
        return ret ?: nbytes;
}

/**
 * cgroup_attach_task_all - attach task 'tsk' to all cgroups of task 'from'
 * @from: attach to all cgroups of a given task
 * @tsk: the task to be attached
 */
int cgroup_attach_task_all(struct task_struct *from, struct task_struct *tsk)
{
        struct cgroup_root *root;
        int retval = 0;

        mutex_lock(&cgroup_mutex);
        for_each_root(root) {
                struct cgroup *from_cgrp;

                if (root == &cgrp_dfl_root)
                        continue;

                down_read(&css_set_rwsem);
                from_cgrp = task_cgroup_from_root(from, root);
                up_read(&css_set_rwsem);

                retval = cgroup_attach_task(from_cgrp, tsk, false);
                if (retval)
                        break;
        }
        mutex_unlock(&cgroup_mutex);

        return retval;
}
EXPORT_SYMBOL_GPL(cgroup_attach_task_all);

static ssize_t cgroup_tasks_write(struct kernfs_open_file *of,
                                  char *buf, size_t nbytes, loff_t off)
{
        return __cgroup_procs_write(of, buf, nbytes, off, false);
}

static ssize_t cgroup_procs_write(struct kernfs_open_file *of,
                                  char *buf, size_t nbytes, loff_t off)
{
        return __cgroup_procs_write(of, buf, nbytes, off, true);
}

static ssize_t cgroup_release_agent_write(struct kernfs_open_file *of,
                                          char *buf, size_t nbytes, loff_t off)
{
        struct cgroup *cgrp;

        BUILD_BUG_ON(sizeof(cgrp->root->release_agent_path) < PATH_MAX);

        cgrp = cgroup_kn_lock_live(of->kn);
        if (!cgrp)
                return -ENODEV;
        spin_lock(&release_agent_path_lock);
        strlcpy(cgrp->root->release_agent_path, strstrip(buf),
                sizeof(cgrp->root->release_agent_path));
        spin_unlock(&release_agent_path_lock);
        cgroup_kn_unlock(of->kn);
        return nbytes;
}

static int cgroup_release_agent_show(struct seq_file *seq, void *v)
{
        struct cgroup *cgrp = seq_css(seq)->cgroup;

        spin_lock(&release_agent_path_lock);
        seq_puts(seq, cgrp->root->release_agent_path);
        spin_unlock(&release_agent_path_lock);
        seq_putc(seq, '\n');
        return 0;
}

static int cgroup_sane_behavior_show(struct seq_file *seq, void *v)
{
        seq_puts(seq, "0\n");
        return 0;
}

static void cgroup_print_ss_mask(struct seq_file *seq, unsigned long ss_mask)
{
        struct cgroup_subsys *ss;
        bool printed = false;
        int ssid;

        for_each_subsys_which(ss, ssid, &ss_mask) {
                if (printed)
                        seq_putc(seq, ' ');
                seq_printf(seq, "%s", ss->name);
                printed = true;
        }
        if (printed)
                seq_putc(seq, '\n');
}

/* show controllers which are currently attached to the default hierarchy */
static int cgroup_root_controllers_show(struct seq_file *seq, void *v)
{
        struct cgroup *cgrp = seq_css(seq)->cgroup;

        cgroup_print_ss_mask(seq, cgrp->root->subsys_mask &
                             ~cgrp_dfl_root_inhibit_ss_mask);
        return 0;
}

/* show controllers which are enabled from the parent */
static int cgroup_controllers_show(struct seq_file *seq, void *v)
{
        struct cgroup *cgrp = seq_css(seq)->cgroup;

        cgroup_print_ss_mask(seq, cgroup_parent(cgrp)->subtree_control);
        return 0;
}

/* show controllers which are enabled for a given cgroup's children */
static int cgroup_subtree_control_show(struct seq_file *seq, void *v)
{
        struct cgroup *cgrp = seq_css(seq)->cgroup;

        cgroup_print_ss_mask(seq, cgrp->subtree_control);
        return 0;
}

/**
 * cgroup_update_dfl_csses - update css assoc of a subtree in default hierarchy
 * @cgrp: root of the subtree to update csses for
 *
 * @cgrp's child_subsys_mask has changed and its subtree's (self excluded)
 * css associations need to be updated accordingly.  This function looks up
 * all css_sets which are attached to the subtree, creates the matching
 * updated css_sets and migrates the tasks to the new ones.
 */
static int cgroup_update_dfl_csses(struct cgroup *cgrp)
{
        LIST_HEAD(preloaded_csets);
        struct cgroup_taskset tset = CGROUP_TASKSET_INIT(tset);
        struct cgroup_subsys_state *css;
        struct css_set *src_cset;
        int ret;

        lockdep_assert_held(&cgroup_mutex);

        percpu_down_write(&cgroup_threadgroup_rwsem);

        /* look up all csses currently attached to @cgrp's subtree */
        down_read(&css_set_rwsem);
        css_for_each_descendant_pre(css, cgroup_css(cgrp, NULL)) {
                struct cgrp_cset_link *link;

                /* self is not affected by child_subsys_mask change */
                if (css->cgroup == cgrp)
                        continue;

                list_for_each_entry(link, &css->cgroup->cset_links, cset_link)
                        cgroup_migrate_add_src(link->cset, cgrp,
                                               &preloaded_csets);
        }
        up_read(&css_set_rwsem);

        /* NULL dst indicates self on default hierarchy */
        ret = cgroup_migrate_prepare_dst(NULL, &preloaded_csets);
        if (ret)
                goto out_finish;

        down_write(&css_set_rwsem);
        list_for_each_entry(src_cset, &preloaded_csets, mg_preload_node) {
                struct task_struct *task, *ntask;

                /* src_csets precede dst_csets, break on the first dst_cset */
                if (!src_cset->mg_src_cgrp)
                        break;

                /* all tasks in src_csets need to be migrated */
                list_for_each_entry_safe(task, ntask, &src_cset->tasks, cg_list)
                        cgroup_taskset_add(task, &tset);
        }
        up_write(&css_set_rwsem);

        ret = cgroup_taskset_migrate(&tset, cgrp);
out_finish:
        cgroup_migrate_finish(&preloaded_csets);
        percpu_up_write(&cgroup_threadgroup_rwsem);
        return ret;
}

/* change the enabled child controllers for a cgroup in the default hierarchy */
static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
                                            char *buf, size_t nbytes,
                                            loff_t off)
{
        unsigned long enable = 0, disable = 0;
        unsigned long css_enable, css_disable, old_sc, new_sc, old_ss, new_ss;
        struct cgroup *cgrp, *child;
        struct cgroup_subsys *ss;
        char *tok;
        int ssid, ret;

        /*
         * Parse input - space separated list of subsystem names prefixed
         * with either + or -.
         */
        buf = strstrip(buf);
        while ((tok = strsep(&buf, " "))) {
                unsigned long tmp_ss_mask = ~cgrp_dfl_root_inhibit_ss_mask;

                if (tok[0] == '\0')
                        continue;
                for_each_subsys_which(ss, ssid, &tmp_ss_mask) {
                        if (!cgroup_ssid_enabled(ssid) ||
                            strcmp(tok + 1, ss->name))
                                continue;

                        if (*tok == '+') {
                                enable |= 1 << ssid;
                                disable &= ~(1 << ssid);
                        } else if (*tok == '-') {
                                disable |= 1 << ssid;
                                enable &= ~(1 << ssid);
                        } else {
                                return -EINVAL;
                        }
                        break;
                }
                if (ssid == CGROUP_SUBSYS_COUNT)
                        return -EINVAL;
        }

        cgrp = cgroup_kn_lock_live(of->kn);
        if (!cgrp)
                return -ENODEV;

        for_each_subsys(ss, ssid) {
                if (enable & (1 << ssid)) {
                        if (cgrp->subtree_control & (1 << ssid)) {
                                enable &= ~(1 << ssid);
                                continue;
                        }

                        /* unavailable or not enabled on the parent? */
                        if (!(cgrp_dfl_root.subsys_mask & (1 << ssid)) ||
                            (cgroup_parent(cgrp) &&
                             !(cgroup_parent(cgrp)->subtree_control & (1 << ssid)))) {
                                ret = -ENOENT;
                                goto out_unlock;
                        }
                } else if (disable & (1 << ssid)) {
                        if (!(cgrp->subtree_control & (1 << ssid))) {
                                disable &= ~(1 << ssid);
                                continue;
                        }

                        /* a child has it enabled? */
                        cgroup_for_each_live_child(child, cgrp) {
                                if (child->subtree_control & (1 << ssid)) {
                                        ret = -EBUSY;
                                        goto out_unlock;
                                }
                        }
                }
        }

        if (!enable && !disable) {
                ret = 0;
                goto out_unlock;
        }

        /*
         * Except for the root, subtree_control must be zero for a cgroup
         * with tasks so that child cgroups don't compete against tasks.
         */
        if (enable && cgroup_parent(cgrp) && !list_empty(&cgrp->cset_links)) {
                ret = -EBUSY;
                goto out_unlock;
        }

        /*
         * Update subsys masks and calculate what needs to be done.  More
         * subsystems than specified may need to be enabled or disabled
         * depending on subsystem dependencies.
         */
        old_sc = cgrp->subtree_control;
        old_ss = cgrp->child_subsys_mask;
        new_sc = (old_sc | enable) & ~disable;
        new_ss = cgroup_calc_child_subsys_mask(cgrp, new_sc);

        css_enable = ~old_ss & new_ss;
        css_disable = old_ss & ~new_ss;
        enable |= css_enable;
        disable |= css_disable;

        /*
         * Because css offlining is asynchronous, userland might try to
         * re-enable the same controller while the previous instance is
         * still around.  In such cases, wait till it's gone using
         * offline_waitq.
         */
        for_each_subsys_which(ss, ssid, &css_enable) {
                cgroup_for_each_live_child(child, cgrp) {
                        DEFINE_WAIT(wait);

                        if (!cgroup_css(child, ss))
                                continue;

                        cgroup_get(child);
                        prepare_to_wait(&child->offline_waitq, &wait,
                                        TASK_UNINTERRUPTIBLE);
                        cgroup_kn_unlock(of->kn);
                        schedule();
                        finish_wait(&child->offline_waitq, &wait);
                        cgroup_put(child);

                        return restart_syscall();
                }
        }

        cgrp->subtree_control = new_sc;
        cgrp->child_subsys_mask = new_ss;

        /*
         * Create new csses or make the existing ones visible.  A css is
         * created invisible if it's being implicitly enabled through
         * dependency.  An invisible css is made visible when the userland
         * explicitly enables it.
         */
        for_each_subsys(ss, ssid) {
                if (!(enable & (1 << ssid)))
                        continue;

                cgroup_for_each_live_child(child, cgrp) {
                        if (css_enable & (1 << ssid))
                                ret = create_css(child, ss,
                                        cgrp->subtree_control & (1 << ssid));
                        else
                                ret = css_populate_dir(cgroup_css(child, ss),
                                                       NULL);
                        if (ret)
                                goto err_undo_css;
                }
        }

        /*
         * At this point, cgroup_e_css() results reflect the new csses
         * making the following cgroup_update_dfl_csses() properly update
         * css associations of all tasks in the subtree.
         */
        ret = cgroup_update_dfl_csses(cgrp);
        if (ret)
                goto err_undo_css;

        /*
         * All tasks are migrated out of disabled csses.  Kill or hide
         * them.  A css is hidden when the userland requests it to be
         * disabled while other subsystems are still depending on it.  The
         * css must not actively control resources and be in the vanilla
         * state if it's made visible again later.  Controllers which may
         * be depended upon should provide ->css_reset() for this purpose.
         */
        for_each_subsys(ss, ssid) {
                if (!(disable & (1 << ssid)))
                        continue;

                cgroup_for_each_live_child(child, cgrp) {
                        struct cgroup_subsys_state *css = cgroup_css(child, ss);

                        if (css_disable & (1 << ssid)) {
                                kill_css(css);
                        } else {
                                css_clear_dir(css, NULL);
                                if (ss->css_reset)
                                        ss->css_reset(css);
                        }
                }
        }

        /*
         * The effective csses of all the descendants (excluding @cgrp) may
         * have changed.  Subsystems can optionally subscribe to this event
         * by implementing ->css_e_css_changed() which is invoked if any of
         * the effective csses seen from the css's cgroup may have changed.
         */
        for_each_subsys(ss, ssid) {
                struct cgroup_subsys_state *this_css = cgroup_css(cgrp, ss);
                struct cgroup_subsys_state *css;

                if (!ss->css_e_css_changed || !this_css)
                        continue;

                css_for_each_descendant_pre(css, this_css)
                        if (css != this_css)
                                ss->css_e_css_changed(css);
        }

        kernfs_activate(cgrp->kn);
        ret = 0;
out_unlock:
        cgroup_kn_unlock(of->kn);
        return ret ?: nbytes;

err_undo_css:
        cgrp->subtree_control = old_sc;
        cgrp->child_subsys_mask = old_ss;

        for_each_subsys(ss, ssid) {
                if (!(enable & (1 << ssid)))
                        continue;

                cgroup_for_each_live_child(child, cgrp) {
                        struct cgroup_subsys_state *css = cgroup_css(child, ss);

                        if (!css)
                                continue;

                        if (css_enable & (1 << ssid))
                                kill_css(css);
                        else
                                css_clear_dir(css, NULL);
                }
        }
        goto out_unlock;
}

static int cgroup_events_show(struct seq_file *seq, void *v)
{
        seq_printf(seq, "populated %d\n",
                   cgroup_is_populated(seq_css(seq)->cgroup));
        return 0;
}

static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
                                 size_t nbytes, loff_t off)
{
        struct cgroup *cgrp = of->kn->parent->priv;
        struct cftype *cft = of->kn->priv;
        struct cgroup_subsys_state *css;
        int ret;

        if (cft->write)
                return cft->write(of, buf, nbytes, off);

        /*
         * kernfs guarantees that a file isn't deleted with operations in
         * flight, which means that the matching css is and stays alive and
         * doesn't need to be pinned.  The RCU locking is not necessary
         * either.  It's just for the convenience of using cgroup_css().
         */
        rcu_read_lock();
        css = cgroup_css(cgrp, cft->ss);
        rcu_read_unlock();

        if (cft->write_u64) {
                unsigned long long v;
                ret = kstrtoull(buf, 0, &v);
                if (!ret)
                        ret = cft->write_u64(css, cft, v);
        } else if (cft->write_s64) {
                long long v;
                ret = kstrtoll(buf, 0, &v);
                if (!ret)
                        ret = cft->write_s64(css, cft, v);
        } else {
                ret = -EINVAL;
        }

        return ret ?: nbytes;
}

static void *cgroup_seqfile_start(struct seq_file *seq, loff_t *ppos)
{
        return seq_cft(seq)->seq_start(seq, ppos);
}

static void *cgroup_seqfile_next(struct seq_file *seq, void *v, loff_t *ppos)
{
        return seq_cft(seq)->seq_next(seq, v, ppos);
}

static void cgroup_seqfile_stop(struct seq_file *seq, void *v)
{
        seq_cft(seq)->seq_stop(seq, v);
}

static int cgroup_seqfile_show(struct seq_file *m, void *arg)
{
        struct cftype *cft = seq_cft(m);
        struct cgroup_subsys_state *css = seq_css(m);

        if (cft->seq_show)
                return cft->seq_show(m, arg);

        if (cft->read_u64)
                seq_printf(m, "%llu\n", cft->read_u64(css, cft));
        else if (cft->read_s64)
                seq_printf(m, "%lld\n", cft->read_s64(css, cft));
        else
                return -EINVAL;
        return 0;
}

static struct kernfs_ops cgroup_kf_single_ops = {
        .atomic_write_len       = PAGE_SIZE,
        .write                  = cgroup_file_write,
        .seq_show               = cgroup_seqfile_show,
};

static struct kernfs_ops cgroup_kf_ops = {
        .atomic_write_len       = PAGE_SIZE,
        .write                  = cgroup_file_write,
        .seq_start              = cgroup_seqfile_start,
        .seq_next               = cgroup_seqfile_next,
        .seq_stop               = cgroup_seqfile_stop,
        .seq_show               = cgroup_seqfile_show,
};

/*
 * cgroup_rename - Only allow simple rename of directories in place.
 */
static int cgroup_rename(struct kernfs_node *kn, struct kernfs_node *new_parent,
                         const char *new_name_str)
{
        struct cgroup *cgrp = kn->priv;
        int ret;

        if (kernfs_type(kn) != KERNFS_DIR)
                return -ENOTDIR;
        if (kn->parent != new_parent)
                return -EIO;

        /*
         * This isn't a proper migration and its usefulness is very
         * limited.  Disallow on the default hierarchy.
         */
        if (cgroup_on_dfl(cgrp))
                return -EPERM;

        /*
         * We're gonna grab cgroup_mutex which nests outside kernfs
         * active_ref.  kernfs_rename() doesn't require active_ref
         * protection.  Break them before grabbing cgroup_mutex.
         */
        kernfs_break_active_protection(new_parent);
        kernfs_break_active_protection(kn);

        mutex_lock(&cgroup_mutex);

        ret = kernfs_rename(kn, new_parent, new_name_str);

        mutex_unlock(&cgroup_mutex);

        kernfs_unbreak_active_protection(kn);
        kernfs_unbreak_active_protection(new_parent);
        return ret;
}

/* set uid and gid of cgroup dirs and files to that of the creator */
static int cgroup_kn_set_ugid(struct kernfs_node *kn)
{
        struct iattr iattr = { .ia_valid = ATTR_UID | ATTR_GID,
                               .ia_uid = current_fsuid(),
                               .ia_gid = current_fsgid(), };

        if (uid_eq(iattr.ia_uid, GLOBAL_ROOT_UID) &&
            gid_eq(iattr.ia_gid, GLOBAL_ROOT_GID))
                return 0;

        return kernfs_setattr(kn, &iattr);
}

static int cgroup_add_file(struct cgroup_subsys_state *css, struct cgroup *cgrp,
                           struct cftype *cft)
{
        char name[CGROUP_FILE_NAME_MAX];
        struct kernfs_node *kn;
        struct lock_class_key *key = NULL;
        int ret;

#ifdef CONFIG_DEBUG_LOCK_ALLOC
        key = &cft->lockdep_key;
#endif
        kn = __kernfs_create_file(cgrp->kn, cgroup_file_name(cgrp, cft, name),
                                  cgroup_file_mode(cft), 0, cft->kf_ops, cft,
                                  NULL, key);
        if (IS_ERR(kn))
                return PTR_ERR(kn);

        ret = cgroup_kn_set_ugid(kn);
        if (ret) {
                kernfs_remove(kn);
                return ret;
        }

        if (cft->file_offset) {
                struct cgroup_file *cfile = (void *)css + cft->file_offset;

                kernfs_get(kn);
                cfile->kn = kn;
                list_add(&cfile->node, &css->files);
        }

        return 0;
}

/**
 * cgroup_addrm_files - add or remove files to a cgroup directory
 * @css: the target css
 * @cgrp: the target cgroup (usually css->cgroup)
 * @cfts: array of cftypes to be added
 * @is_add: whether to add or remove
 *
 * Depending on @is_add, add or remove files defined by @cfts on @cgrp.
 * For removals, this function never fails.
 */
static int cgroup_addrm_files(struct cgroup_subsys_state *css,
                              struct cgroup *cgrp, struct cftype cfts[],
                              bool is_add)
{
        struct cftype *cft, *cft_end = NULL;
        int ret;

        lockdep_assert_held(&cgroup_mutex);

restart:
        for (cft = cfts; cft != cft_end && cft->name[0] != '\0'; cft++) {
                /* does cft->flags tell us to skip this file on @cgrp? */
                if ((cft->flags & __CFTYPE_ONLY_ON_DFL) && !cgroup_on_dfl(cgrp))
                        continue;
                if ((cft->flags & __CFTYPE_NOT_ON_DFL) && cgroup_on_dfl(cgrp))
                        continue;
                if ((cft->flags & CFTYPE_NOT_ON_ROOT) && !cgroup_parent(cgrp))
                        continue;
                if ((cft->flags & CFTYPE_ONLY_ON_ROOT) && cgroup_parent(cgrp))
                        continue;

                if (is_add) {
                        ret = cgroup_add_file(css, cgrp, cft);
                        if (ret) {
                                pr_warn("%s: failed to add %s, err=%d\n",
                                        __func__, cft->name, ret);
                                cft_end = cft;
                                is_add = false;
                                goto restart;
                        }
                } else {
                        cgroup_rm_file(cgrp, cft);
                }
        }
        return 0;
}

static int cgroup_apply_cftypes(struct cftype *cfts, bool is_add)
{
        LIST_HEAD(pending);
        struct cgroup_subsys *ss = cfts[0].ss;
        struct cgroup *root = &ss->root->cgrp;
        struct cgroup_subsys_state *css;
        int ret = 0;

        lockdep_assert_held(&cgroup_mutex);

        /* add/rm files for all cgroups created before */
        css_for_each_descendant_pre(css, cgroup_css(root, ss)) {
                struct cgroup *cgrp = css->cgroup;

                if (cgroup_is_dead(cgrp))
                        continue;

                ret = cgroup_addrm_files(css, cgrp, cfts, is_add);
                if (ret)
                        break;
        }

        if (is_add && !ret)
                kernfs_activate(root->kn);
        return ret;
}

static void cgroup_exit_cftypes(struct cftype *cfts)
{
        struct cftype *cft;

        for (cft = cfts; cft->name[0] != '\0'; cft++) {
                /* free copy for custom atomic_write_len, see init_cftypes() */
                if (cft->max_write_len && cft->max_write_len != PAGE_SIZE)
                        kfree(cft->kf_ops);
                cft->kf_ops = NULL;
                cft->ss = NULL;

                /* revert flags set by cgroup core while adding @cfts */
                cft->flags &= ~(__CFTYPE_ONLY_ON_DFL | __CFTYPE_NOT_ON_DFL);
        }
}

static int cgroup_init_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
{
        struct cftype *cft;

        for (cft = cfts; cft->name[0] != '\0'; cft++) {
                struct kernfs_ops *kf_ops;

                WARN_ON(cft->ss || cft->kf_ops);

                if (cft->seq_start)
                        kf_ops = &cgroup_kf_ops;
                else
                        kf_ops = &cgroup_kf_single_ops;

                /*
                 * Ugh... if @cft wants a custom max_write_len, we need to
                 * make a copy of kf_ops to set its atomic_write_len.
                 */
                if (cft->max_write_len && cft->max_write_len != PAGE_SIZE) {
                        kf_ops = kmemdup(kf_ops, sizeof(*kf_ops), GFP_KERNEL);
                        if (!kf_ops) {
                                cgroup_exit_cftypes(cfts);
                                return -ENOMEM;
                        }
                        kf_ops->atomic_write_len = cft->max_write_len;
                }

                cft->kf_ops = kf_ops;
                cft->ss = ss;
        }

        return 0;
}

static int cgroup_rm_cftypes_locked(struct cftype *cfts)
{
        lockdep_assert_held(&cgroup_mutex);

        if (!cfts || !cfts[0].ss)
                return -ENOENT;

        list_del(&cfts->node);
        cgroup_apply_cftypes(cfts, false);
        cgroup_exit_cftypes(cfts);
        return 0;
}

/**
 * cgroup_rm_cftypes - remove an array of cftypes from a subsystem
 * @cfts: zero-length name terminated array of cftypes
 *
 * Unregister @cfts.  Files described by @cfts are removed from all
 * existing cgroups and all future cgroups won't have them either.  This
 * function can be called anytime whether @cfts' subsys is attached or not.
 *
 * Returns 0 on successful unregistration, -ENOENT if @cfts is not
 * registered.
 */
int cgroup_rm_cftypes(struct cftype *cfts)
{
        int ret;

        mutex_lock(&cgroup_mutex);
        ret = cgroup_rm_cftypes_locked(cfts);
        mutex_unlock(&cgroup_mutex);
        return ret;
}

/**
 * cgroup_add_cftypes - add an array of cftypes to a subsystem
 * @ss: target cgroup subsystem
 * @cfts: zero-length name terminated array of cftypes
 *
 * Register @cfts to @ss.  Files described by @cfts are created for all
 * existing cgroups to which @ss is attached and all future cgroups will
 * have them too.  This function can be called anytime whether @ss is
 * attached or not.
 *
 * Returns 0 on successful registration, -errno on failure.  Note that this
 * function currently returns 0 as long as @cfts registration is successful
 * even if some file creation attempts on existing cgroups fail.
 */
static int cgroup_add_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
{
        int ret;

        if (!cgroup_ssid_enabled(ss->id))
                return 0;

        if (!cfts || cfts[0].name[0] == '\0')
                return 0;

        ret = cgroup_init_cftypes(ss, cfts);
        if (ret)
                return ret;

        mutex_lock(&cgroup_mutex);

        list_add_tail(&cfts->node, &ss->cfts);
        ret = cgroup_apply_cftypes(cfts, true);
        if (ret)
                cgroup_rm_cftypes_locked(cfts);

        mutex_unlock(&cgroup_mutex);
        return ret;
}

/**
 * cgroup_add_dfl_cftypes - add an array of cftypes for default hierarchy
 * @ss: target cgroup subsystem
 * @cfts: zero-length name terminated array of cftypes
 *
 * Similar to cgroup_add_cftypes() but the added files are only used for
 * the default hierarchy.
 */
int cgroup_add_dfl_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
{
        struct cftype *cft;

        for (cft = cfts; cft && cft->name[0] != '\0'; cft++)
                cft->flags |= __CFTYPE_ONLY_ON_DFL;
        return cgroup_add_cftypes(ss, cfts);
}

/**
 * cgroup_add_legacy_cftypes - add an array of cftypes for legacy hierarchies
 * @ss: target cgroup subsystem
 * @cfts: zero-length name terminated array of cftypes
 *
 * Similar to cgroup_add_cftypes() but the added files are only used for
 * the legacy hierarchies.
 */
int cgroup_add_legacy_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
{
        struct cftype *cft;

        /*
         * If legacy_flies_on_dfl, we want to show the legacy files on the
         * dfl hierarchy but iff the target subsystem hasn't been updated
         * for the dfl hierarchy yet.
         */
        if (!cgroup_legacy_files_on_dfl ||
            ss->dfl_cftypes != ss->legacy_cftypes) {
                for (cft = cfts; cft && cft->name[0] != '\0'; cft++)
                        cft->flags |= __CFTYPE_NOT_ON_DFL;
        }

        return cgroup_add_cftypes(ss, cfts);
}

/**
 * cgroup_task_count - count the number of tasks in a cgroup.
 * @cgrp: the cgroup in question
 *
 * Return the number of tasks in the cgroup.
 */
static int cgroup_task_count(const struct cgroup *cgrp)
{
        int count = 0;
        struct cgrp_cset_link *link;

        down_read(&css_set_rwsem);
        list_for_each_entry(link, &cgrp->cset_links, cset_link)
                count += atomic_read(&link->cset->refcount);
        up_read(&css_set_rwsem);
        return count;
}

/**
 * css_next_child - find the next child of a given css
 * @pos: the current position (%NULL to initiate traversal)
 * @parent: css whose children to walk
 *
 * This function returns the next child of @parent and should be called
 * under either cgroup_mutex or RCU read lock.  The only requirement is
 * that @parent and @pos are accessible.  The next sibling is guaranteed to
 * be returned regardless of their states.
 *
 * If a subsystem synchronizes ->css_online() and the start of iteration, a
 * css which finished ->css_online() is guaranteed to be visible in the
 * future iterations and will stay visible until the last reference is put.
 * A css which hasn't finished ->css_online() or already finished
 * ->css_offline() may show up during traversal.  It's each subsystem's
 * responsibility to synchronize against on/offlining.
 */
struct cgroup_subsys_state *css_next_child(struct cgroup_subsys_state *pos,
                                           struct cgroup_subsys_state *parent)
{
        struct cgroup_subsys_state *next;

        cgroup_assert_mutex_or_rcu_locked();

        /*
         * @pos could already have been unlinked from the sibling list.
         * Once a cgroup is removed, its ->sibling.next is no longer
         * updated when its next sibling changes.  CSS_RELEASED is set when
         * @pos is taken off list, at which time its next pointer is valid,
         * and, as releases are serialized, the one pointed to by the next
         * pointer is guaranteed to not have started release yet.  This
         * implies that if we observe !CSS_RELEASED on @pos in this RCU
         * critical section, the one pointed to by its next pointer is
         * guaranteed to not have finished its RCU grace period even if we
         * have dropped rcu_read_lock() inbetween iterations.
         *
         * If @pos has CSS_RELEASED set, its next pointer can't be
         * dereferenced; however, as each css is given a monotonically
         * increasing unique serial number and always appended to the
         * sibling list, the next one can be found by walking the parent's
         * children until the first css with higher serial number than
         * @pos's.  While this path can be slower, it happens iff iteration
         * races against release and the race window is very small.
         */
        if (!pos) {
                next = list_entry_rcu(parent->children.next, struct cgroup_subsys_state, sibling);
        } else if (likely(!(pos->flags & CSS_RELEASED))) {
                next = list_entry_rcu(pos->sibling.next, struct cgroup_subsys_state, sibling);
        } else {
                list_for_each_entry_rcu(next, &parent->children, sibling)
                        if (next->serial_nr > pos->serial_nr)
                                break;
        }

        /*
         * @next, if not pointing to the head, can be dereferenced and is
         * the next sibling.
         */
        if (&next->sibling != &parent->children)
                return next;
        return NULL;
}

/**
 * css_next_descendant_pre - find the next descendant for pre-order walk
 * @pos: the current position (%NULL to initiate traversal)
 * @root: css whose descendants to walk
 *
 * To be used by css_for_each_descendant_pre().  Find the next descendant
 * to visit for pre-order traversal of @root's descendants.  @root is
 * included in the iteration and the first node to be visited.
 *
 * While this function requires cgroup_mutex or RCU read locking, it
 * doesn't require the whole traversal to be contained in a single critical
 * section.  This function will return the correct next descendant as long
 * as both @pos and @root are accessible and @pos is a descendant of @root.
 *
 * If a subsystem synchronizes ->css_online() and the start of iteration, a
 * css which finished ->css_online() is guaranteed to be visible in the
 * future iterations and will stay visible until the last reference is put.
 * A css which hasn't finished ->css_online() or already finished
 * ->css_offline() may show up during traversal.  It's each subsystem's
 * responsibility to synchronize against on/offlining.
 */
struct cgroup_subsys_state *
css_next_descendant_pre(struct cgroup_subsys_state *pos,
                        struct cgroup_subsys_state *root)
{
        struct cgroup_subsys_state *next;

        cgroup_assert_mutex_or_rcu_locked();

        /* if first iteration, visit @root */
        if (!pos)
                return root;

        /* visit the first child if exists */
        next = css_next_child(NULL, pos);
        if (next)
                return next;

        /* no child, visit my or the closest ancestor's next sibling */
        while (pos != root) {
                next = css_next_child(pos, pos->parent);
                if (next)
                        return next;
                pos = pos->parent;
        }

        return NULL;
}

/**
 * css_rightmost_descendant - return the rightmost descendant of a css
 * @pos: css of interest
 *
 * Return the rightmost descendant of @pos.  If there's no descendant, @pos
 * is returned.  This can be used during pre-order traversal to skip
 * subtree of @pos.
 *
 * While this function requires cgroup_mutex or RCU read locking, it
 * doesn't require the whole traversal to be contained in a single critical
 * section.  This function will return the correct rightmost descendant as
 * long as @pos is accessible.
 */
struct cgroup_subsys_state *
css_rightmost_descendant(struct cgroup_subsys_state *pos)
{
        struct cgroup_subsys_state *last, *tmp;

        cgroup_assert_mutex_or_rcu_locked();

        do {
                last = pos;
                /* ->prev isn't RCU safe, walk ->next till the end */
                pos = NULL;
                css_for_each_child(tmp, last)
                        pos = tmp;
        } while (pos);

        return last;
}

static struct cgroup_subsys_state *
css_leftmost_descendant(struct cgroup_subsys_state *pos)
{
        struct cgroup_subsys_state *last;

        do {
                last = pos;
                pos = css_next_child(NULL, pos);
        } while (pos);

        return last;
}

/**
 * css_next_descendant_post - find the next descendant for post-order walk
 * @pos: the current position (%NULL to initiate traversal)
 * @root: css whose descendants to walk
 *
 * To be used by css_for_each_descendant_post().  Find the next descendant
 * to visit for post-order traversal of @root's descendants.  @root is
 * included in the iteration and the last node to be visited.
 *
 * While this function requires cgroup_mutex or RCU read locking, it
 * doesn't require the whole traversal to be contained in a single critical
 * section.  This function will return the correct next descendant as long
 * as both @pos and @cgroup are accessible and @pos is a descendant of
 * @cgroup.
 *
 * If a subsystem synchronizes ->css_online() and the start of iteration, a
 * css which finished ->css_online() is guaranteed to be visible in the
 * future iterations and will stay visible until the last reference is put.
 * A css which hasn't finished ->css_online() or already finished
 * ->css_offline() may show up during traversal.  It's each subsystem's
 * responsibility to synchronize against on/offlining.
 */
struct cgroup_subsys_state *
css_next_descendant_post(struct cgroup_subsys_state *pos,
                         struct cgroup_subsys_state *root)
{
        struct cgroup_subsys_state *next;

        cgroup_assert_mutex_or_rcu_locked();

        /* if first iteration, visit leftmost descendant which may be @root */
        if (!pos)
                return css_leftmost_descendant(root);

        /* if we visited @root, we're done */
        if (pos == root)
                return NULL;

        /* if there's an unvisited sibling, visit its leftmost descendant */
        next = css_next_child(pos, pos->parent);
        if (next)
                return css_leftmost_descendant(next);

        /* no sibling left, visit parent */
        return pos->parent;
}

/**
 * css_has_online_children - does a css have online children
 * @css: the target css
 *
 * Returns %true if @css has any online children; otherwise, %false.  This
 * function can be called from any context but the caller is responsible
 * for synchronizing against on/offlining as necessary.
 */
bool css_has_online_children(struct cgroup_subsys_state *css)
{
        struct cgroup_subsys_state *child;
        bool ret = false;

        rcu_read_lock();
        css_for_each_child(child, css) {
                if (child->flags & CSS_ONLINE) {
                        ret = true;
                        break;
                }
        }
        rcu_read_unlock();
        return ret;
}

/**
 * css_advance_task_iter - advance a task itererator to the next css_set
 * @it: the iterator to advance
 *
 * Advance @it to the next css_set to walk.
 */
static void css_advance_task_iter(struct css_task_iter *it)
{
        struct list_head *l = it->cset_pos;
        struct cgrp_cset_link *link;
        struct css_set *cset;

        /* Advance to the next non-empty css_set */
        do {
                l = l->next;
                if (l == it->cset_head) {
                        it->cset_pos = NULL;
                        return;
                }

                if (it->ss) {
                        cset = container_of(l, struct css_set,
                                            e_cset_node[it->ss->id]);
                } else {
                        link = list_entry(l, struct cgrp_cset_link, cset_link);
                        cset = link->cset;
                }
        } while (!css_set_populated(cset));

        it->cset_pos = l;

        if (!list_empty(&cset->tasks))
                it->task_pos = cset->tasks.next;
        else
                it->task_pos = cset->mg_tasks.next;

        it->tasks_head = &cset->tasks;
        it->mg_tasks_head = &cset->mg_tasks;
}

/**
 * css_task_iter_start - initiate task iteration
 * @css: the css to walk tasks of
 * @it: the task iterator to use
 *
 * Initiate iteration through the tasks of @css.  The caller can call
 * css_task_iter_next() to walk through the tasks until the function
 * returns NULL.  On completion of iteration, css_task_iter_end() must be
 * called.
 *
 * Note that this function acquires a lock which is released when the
 * iteration finishes.  The caller can't sleep while iteration is in
 * progress.
 */
void css_task_iter_start(struct cgroup_subsys_state *css,
                         struct css_task_iter *it)
        __acquires(css_set_rwsem)
{
        /* no one should try to iterate before mounting cgroups */
        WARN_ON_ONCE(!use_task_css_set_links);

        down_read(&css_set_rwsem);

        it->ss = css->ss;

        if (it->ss)
                it->cset_pos = &css->cgroup->e_csets[css->ss->id];
        else
                it->cset_pos = &css->cgroup->cset_links;

        it->cset_head = it->cset_pos;

        css_advance_task_iter(it);
}

/**
 * css_task_iter_next - return the next task for the iterator
 * @it: the task iterator being iterated
 *
 * The "next" function for task iteration.  @it should have been
 * initialized via css_task_iter_start().  Returns NULL when the iteration
 * reaches the end.
 */
struct task_struct *css_task_iter_next(struct css_task_iter *it)
{
        struct task_struct *res;
        struct list_head *l = it->task_pos;

        /* If the iterator cg is NULL, we have no tasks */
        if (!it->cset_pos)
                return NULL;
        res = list_entry(l, struct task_struct, cg_list);

        /*
         * Advance iterator to find next entry.  cset->tasks is consumed
         * first and then ->mg_tasks.  After ->mg_tasks, we move onto the
         * next cset.
         */
        l = l->next;

        if (l == it->tasks_head)
                l = it->mg_tasks_head->next;

        if (l == it->mg_tasks_head)
                css_advance_task_iter(it);
        else
                it->task_pos = l;

        return res;
}

/**
 * css_task_iter_end - finish task iteration
 * @it: the task iterator to finish
 *
 * Finish task iteration started by css_task_iter_start().
 */
void css_task_iter_end(struct css_task_iter *it)
        __releases(css_set_rwsem)
{
        up_read(&css_set_rwsem);
}

/**
 * cgroup_trasnsfer_tasks - move tasks from one cgroup to another
 * @to: cgroup to which the tasks will be moved
 * @from: cgroup in which the tasks currently reside
 *
 * Locking rules between cgroup_post_fork() and the migration path
 * guarantee that, if a task is forking while being migrated, the new child
 * is guaranteed to be either visible in the source cgroup after the
 * parent's migration is complete or put into the target cgroup.  No task
 * can slip out of migration through forking.
 */
int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
{
        LIST_HEAD(preloaded_csets);
        struct cgrp_cset_link *link;
        struct css_task_iter it;
        struct task_struct *task;
        int ret;

        mutex_lock(&cgroup_mutex);

        /* all tasks in @from are being moved, all csets are source */
        down_read(&css_set_rwsem);
        list_for_each_entry(link, &from->cset_links, cset_link)
                cgroup_migrate_add_src(link->cset, to, &preloaded_csets);
        up_read(&css_set_rwsem);

        ret = cgroup_migrate_prepare_dst(to, &preloaded_csets);
        if (ret)
                goto out_err;

        /*
         * Migrate tasks one-by-one until @form is empty.  This fails iff
         * ->can_attach() fails.
         */
        do {
                css_task_iter_start(&from->self, &it);
                task = css_task_iter_next(&it);
                if (task)
                        get_task_struct(task);
                css_task_iter_end(&it);

                if (task) {
                        ret = cgroup_migrate(task, false, to);
                        put_task_struct(task);
                }
        } while (task && !ret);
out_err:
        cgroup_migrate_finish(&preloaded_csets);
        mutex_unlock(&cgroup_mutex);
        return ret;
}

/*
 * Stuff for reading the 'tasks'/'procs' files.
 *
 * Reading this file can return large amounts of data if a cgroup has
 * *lots* of attached tasks. So it may need several calls to read(),
 * but we cannot guarantee that the information we produce is correct
 * unless we produce it entirely atomically.
 *
 */

/* which pidlist file are we talking about? */
enum cgroup_filetype {
        CGROUP_FILE_PROCS,
        CGROUP_FILE_TASKS,
};

/*
 * A pidlist is a list of pids that virtually represents the contents of one
 * of the cgroup files ("procs" or "tasks"). We keep a list of such pidlists,
 * a pair (one each for procs, tasks) for each pid namespace that's relevant
 * to the cgroup.
 */
struct cgroup_pidlist {
        /*
         * used to find which pidlist is wanted. doesn't change as long as
         * this particular list stays in the list.
        */
        struct { enum cgroup_filetype type; struct pid_namespace *ns; } key;
        /* array of xids */
        pid_t *list;
        /* how many elements the above list has */
        int length;
        /* each of these stored in a list by its cgroup */
        struct list_head links;
        /* pointer to the cgroup we belong to, for list removal purposes */
        struct cgroup *owner;
        /* for delayed destruction */
        struct delayed_work destroy_dwork;
};

/*
 * The following two functions "fix" the issue where there are more pids
 * than kmalloc will give memory for; in such cases, we use vmalloc/vfree.
 * TODO: replace with a kernel-wide solution to this problem
 */
#define PIDLIST_TOO_LARGE(c) ((c) * sizeof(pid_t) > (PAGE_SIZE * 2))
static void *pidlist_allocate(int count)
{
        if (PIDLIST_TOO_LARGE(count))
                return vmalloc(count * sizeof(pid_t));
        else
                return kmalloc(count * sizeof(pid_t), GFP_KERNEL);
}

static void pidlist_free(void *p)
{
        kvfree(p);
}

/*
 * Used to destroy all pidlists lingering waiting for destroy timer.  None
 * should be left afterwards.
 */
static void cgroup_pidlist_destroy_all(struct cgroup *cgrp)
{
        struct cgroup_pidlist *l, *tmp_l;

        mutex_lock(&cgrp->pidlist_mutex);
        list_for_each_entry_safe(l, tmp_l, &cgrp->pidlists, links)
                mod_delayed_work(cgroup_pidlist_destroy_wq, &l->destroy_dwork, 0);
        mutex_unlock(&cgrp->pidlist_mutex);

        flush_workqueue(cgroup_pidlist_destroy_wq);
        BUG_ON(!list_empty(&cgrp->pidlists));
}

static void cgroup_pidlist_destroy_work_fn(struct work_struct *work)
{
        struct delayed_work *dwork = to_delayed_work(work);
        struct cgroup_pidlist *l = container_of(dwork, struct cgroup_pidlist,
                                                destroy_dwork);
        struct cgroup_pidlist *tofree = NULL;

        mutex_lock(&l->owner->pidlist_mutex);

        /*
         * Destroy iff we didn't get queued again.  The state won't change
         * as destroy_dwork can only be queued while locked.
         */
        if (!delayed_work_pending(dwork)) {
                list_del(&l->links);
                pidlist_free(l->list);
                put_pid_ns(l->key.ns);
                tofree = l;
        }

        mutex_unlock(&l->owner->pidlist_mutex);
        kfree(tofree);
}

/*
 * pidlist_uniq - given a kmalloc()ed list, strip out all duplicate entries
 * Returns the number of unique elements.
 */
static int pidlist_uniq(pid_t *list, int length)
{
        int src, dest = 1;

        /*
         * we presume the 0th element is unique, so i starts at 1. trivial
         * edge cases first; no work needs to be done for either
         */
        if (length == 0 || length == 1)
                return length;
        /* src and dest walk down the list; dest counts unique elements */
        for (src = 1; src < length; src++) {
                /* find next unique element */
                while (list[src] == list[src-1]) {
                        src++;
                        if (src == length)
                                goto after;
                }
                /* dest always points to where the next unique element goes */
                list[dest] = list[src];
                dest++;
        }
after:
        return dest;
}

/*
 * The two pid files - task and cgroup.procs - guaranteed that the result
 * is sorted, which forced this whole pidlist fiasco.  As pid order is
 * different per namespace, each namespace needs differently sorted list,
 * making it impossible to use, for example, single rbtree of member tasks
 * sorted by task pointer.  As pidlists can be fairly large, allocating one
 * per open file is dangerous, so cgroup had to implement shared pool of
 * pidlists keyed by cgroup and namespace.
 *
 * All this extra complexity was caused by the original implementation
 * committing to an entirely unnecessary property.  In the long term, we
 * want to do away with it.  Explicitly scramble sort order if on the
 * default hierarchy so that no such expectation exists in the new
 * interface.
 *
 * Scrambling is done by swapping every two consecutive bits, which is
 * non-identity one-to-one mapping which disturbs sort order sufficiently.
 */
static pid_t pid_fry(pid_t pid)
{
        unsigned a = pid & 0x55555555;
        unsigned b = pid & 0xAAAAAAAA;

        return (a << 1) | (b >> 1);
}

static pid_t cgroup_pid_fry(struct cgroup *cgrp, pid_t pid)
{
        if (cgroup_on_dfl(cgrp))
                return pid_fry(pid);
        else
                return pid;
}

static int cmppid(const void *a, const void *b)
{
        return *(pid_t *)a - *(pid_t *)b;
}

static int fried_cmppid(const void *a, const void *b)
{
        return pid_fry(*(pid_t *)a) - pid_fry(*(pid_t *)b);
}

static struct cgroup_pidlist *cgroup_pidlist_find(struct cgroup *cgrp,
                                                  enum cgroup_filetype type)
{
        struct cgroup_pidlist *l;
        /* don't need task_nsproxy() if we're looking at ourself */
        struct pid_namespace *ns = task_active_pid_ns(current);

        lockdep_assert_held(&cgrp->pidlist_mutex);

        list_for_each_entry(l, &cgrp->pidlists, links)
                if (l->key.type == type && l->key.ns == ns)
                        return l;
        return NULL;
}

/*
 * find the appropriate pidlist for our purpose (given procs vs tasks)
 * returns with the lock on that pidlist already held, and takes care
 * of the use count, or returns NULL with no locks held if we're out of
 * memory.
 */
static struct cgroup_pidlist *cgroup_pidlist_find_create(struct cgroup *cgrp,
                                                enum cgroup_filetype type)
{
        struct cgroup_pidlist *l;

        lockdep_assert_held(&cgrp->pidlist_mutex);

        l = cgroup_pidlist_find(cgrp, type);
        if (l)
                return l;

        /* entry not found; create a new one */
        l = kzalloc(sizeof(struct cgroup_pidlist), GFP_KERNEL);
        if (!l)
                return l;

        INIT_DELAYED_WORK(&l->destroy_dwork, cgroup_pidlist_destroy_work_fn);
        l->key.type = type;
        /* don't need task_nsproxy() if we're looking at ourself */
        l->key.ns = get_pid_ns(task_active_pid_ns(current));
        l->owner = cgrp;
        list_add(&l->links, &cgrp->pidlists);
        return l;
}

/*
 * Load a cgroup's pidarray with either procs' tgids or tasks' pids
 */
static int pidlist_array_load(struct cgroup *cgrp, enum cgroup_filetype type,
                              struct cgroup_pidlist **lp)
{
        pid_t *array;
        int length;
        int pid, n = 0; /* used for populating the array */
        struct css_task_iter it;
        struct task_struct *tsk;
        struct cgroup_pidlist *l;

        lockdep_assert_held(&cgrp->pidlist_mutex);

        /*
         * If cgroup gets more users after we read count, we won't have
         * enough space - tough.  This race is indistinguishable to the
         * caller from the case that the additional cgroup users didn't
         * show up until sometime later on.
         */
        length = cgroup_task_count(cgrp);
        array = pidlist_allocate(length);
        if (!array)
                return -ENOMEM;
        /* now, populate the array */
        css_task_iter_start(&cgrp->self, &it);
        while ((tsk = css_task_iter_next(&it))) {
                if (unlikely(n == length))
                        break;
                /* get tgid or pid for procs or tasks file respectively */
                if (type == CGROUP_FILE_PROCS)
                        pid = task_tgid_vnr(tsk);
                else
                        pid = task_pid_vnr(tsk);
                if (pid > 0) /* make sure to only use valid results */
                        array[n++] = pid;
        }
        css_task_iter_end(&it);
        length = n;
        /* now sort & (if procs) strip out duplicates */
        if (cgroup_on_dfl(cgrp))
                sort(array, length, sizeof(pid_t), fried_cmppid, NULL);
        else
                sort(array, length, sizeof(pid_t), cmppid, NULL);
        if (type == CGROUP_FILE_PROCS)
                length = pidlist_uniq(array, length);

        l = cgroup_pidlist_find_create(cgrp, type);
        if (!l) {
                pidlist_free(array);
                return -ENOMEM;
        }

        /* store array, freeing old if necessary */
        pidlist_free(l->list);
        l->list = array;
        l->length = length;
        *lp = l;
        return 0;
}

/**
 * cgroupstats_build - build and fill cgroupstats
 * @stats: cgroupstats to fill information into
 * @dentry: A dentry entry belonging to the cgroup for which stats have
 * been requested.
 *
 * Build and fill cgroupstats so that taskstats can export it to user
 * space.
 */
int cgroupstats_build(struct cgroupstats *stats, struct dentry *dentry)
{
        struct kernfs_node *kn = kernfs_node_from_dentry(dentry);
        struct cgroup *cgrp;
        struct css_task_iter it;
        struct task_struct *tsk;

        /* it should be kernfs_node belonging to cgroupfs and is a directory */
        if (dentry->d_sb->s_type != &cgroup_fs_type || !kn ||
            kernfs_type(kn) != KERNFS_DIR)
                return -EINVAL;

        mutex_lock(&cgroup_mutex);

        /*
         * We aren't being called from kernfs and there's no guarantee on
         * @kn->priv's validity.  For this and css_tryget_online_from_dir(),
         * @kn->priv is RCU safe.  Let's do the RCU dancing.
         */
        rcu_read_lock();
        cgrp = rcu_dereference(kn->priv);
        if (!cgrp || cgroup_is_dead(cgrp)) {
                rcu_read_unlock();
                mutex_unlock(&cgroup_mutex);
                return -ENOENT;
        }
        rcu_read_unlock();

        css_task_iter_start(&cgrp->self, &it);
        while ((tsk = css_task_iter_next(&it))) {
                switch (tsk->state) {
                case TASK_RUNNING:
                        stats->nr_running++;
                        break;
                case TASK_INTERRUPTIBLE:
                        stats->nr_sleeping++;
                        break;
                case TASK_UNINTERRUPTIBLE:
                        stats->nr_uninterruptible++;
                        break;
                case TASK_STOPPED:
                        stats->nr_stopped++;
                        break;
                default:
                        if (delayacct_is_task_waiting_on_io(tsk))
                                stats->nr_io_wait++;
                        break;
                }
        }
        css_task_iter_end(&it);

        mutex_unlock(&cgroup_mutex);
        return 0;
}


/*
 * seq_file methods for the tasks/procs files. The seq_file position is the
 * next pid to display; the seq_file iterator is a pointer to the pid
 * in the cgroup->l->list array.
 */

static void *cgroup_pidlist_start(struct seq_file *s, loff_t *pos)
{
        /*
         * Initially we receive a position value that corresponds to
         * one more than the last pid shown (or 0 on the first call or
         * after a seek to the start). Use a binary-search to find the
         * next pid to display, if any
         */
        struct kernfs_open_file *of = s->private;
        struct cgroup *cgrp = seq_css(s)->cgroup;
        struct cgroup_pidlist *l;
        enum cgroup_filetype type = seq_cft(s)->private;
        int index = 0, pid = *pos;
        int *iter, ret;

        mutex_lock(&cgrp->pidlist_mutex);

        /*
         * !NULL @of->priv indicates that this isn't the first start()
         * after open.  If the matching pidlist is around, we can use that.
         * Look for it.  Note that @of->priv can't be used directly.  It
         * could already have been destroyed.
         */
        if (of->priv)
                of->priv = cgroup_pidlist_find(cgrp, type);

        /*
         * Either this is the first start() after open or the matching
         * pidlist has been destroyed inbetween.  Create a new one.
         */
        if (!of->priv) {
                ret = pidlist_array_load(cgrp, type,
                                         (struct cgroup_pidlist **)&of->priv);
                if (ret)
                        return ERR_PTR(ret);
        }
        l = of->priv;

        if (pid) {
                int end = l->length;

                while (index < end) {
                        int mid = (index + end) / 2;
                        if (cgroup_pid_fry(cgrp, l->list[mid]) == pid) {
                                index = mid;
                                break;
                        } else if (cgroup_pid_fry(cgrp, l->list[mid]) <= pid)
                                index = mid + 1;
                        else
                                end = mid;
                }
        }
        /* If we're off the end of the array, we're done */
        if (index >= l->length)
                return NULL;
        /* Update the abstract position to be the actual pid that we found */
        iter = l->list + index;
        *pos = cgroup_pid_fry(cgrp, *iter);
        return iter;
}

static void cgroup_pidlist_stop(struct seq_file *s, void *v)
{
        struct kernfs_open_file *of = s->private;
        struct cgroup_pidlist *l = of->priv;

        if (l)
                mod_delayed_work(cgroup_pidlist_destroy_wq, &l->destroy_dwork,
                                 CGROUP_PIDLIST_DESTROY_DELAY);
        mutex_unlock(&seq_css(s)->cgroup->pidlist_mutex);
}

static void *cgroup_pidlist_next(struct seq_file *s, void *v, loff_t *pos)
{
        struct kernfs_open_file *of = s->private;
        struct cgroup_pidlist *l = of->priv;
        pid_t *p = v;
        pid_t *end = l->list + l->length;
        /*
         * Advance to the next pid in the array. If this goes off the
         * end, we're done
         */
        p++;
        if (p >= end) {
                return NULL;
        } else {
                *pos = cgroup_pid_fry(seq_css(s)->cgroup, *p);
                return p;
        }
}

static int cgroup_pidlist_show(struct seq_file *s, void *v)
{
        seq_printf(s, "%d\n", *(int *)v);

        return 0;
}

static u64 cgroup_read_notify_on_release(struct cgroup_subsys_state *css,
                                         struct cftype *cft)
{
        return notify_on_release(css->cgroup);
}

static int cgroup_write_notify_on_release(struct cgroup_subsys_state *css,
                                          struct cftype *cft, u64 val)
{
        if (val)
                set_bit(CGRP_NOTIFY_ON_RELEASE, &css->cgroup->flags);
        else
                clear_bit(CGRP_NOTIFY_ON_RELEASE, &css->cgroup->flags);
        return 0;
}

static u64 cgroup_clone_children_read(struct cgroup_subsys_state *css,
                                      struct cftype *cft)
{
        return test_bit(CGRP_CPUSET_CLONE_CHILDREN, &css->cgroup->flags);
}

static int cgroup_clone_children_write(struct cgroup_subsys_state *css,
                                       struct cftype *cft, u64 val)
{
        if (val)
                set_bit(CGRP_CPUSET_CLONE_CHILDREN, &css->cgroup->flags);
        else
                clear_bit(CGRP_CPUSET_CLONE_CHILDREN, &css->cgroup->flags);
        return 0;
}

/* cgroup core interface files for the default hierarchy */
static struct cftype cgroup_dfl_base_files[] = {
        {
                .name = "cgroup.procs",
                .file_offset = offsetof(struct cgroup, procs_file),
                .seq_start = cgroup_pidlist_start,
                .seq_next = cgroup_pidlist_next,
                .seq_stop = cgroup_pidlist_stop,
                .seq_show = cgroup_pidlist_show,
                .private = CGROUP_FILE_PROCS,
                .write = cgroup_procs_write,
        },
        {
                .name = "cgroup.controllers",
                .flags = CFTYPE_ONLY_ON_ROOT,
                .seq_show = cgroup_root_controllers_show,
        },
        {
                .name = "cgroup.controllers",
                .flags = CFTYPE_NOT_ON_ROOT,
                .seq_show = cgroup_controllers_show,
        },
        {
                .name = "cgroup.subtree_control",
                .seq_show = cgroup_subtree_control_show,
                .write = cgroup_subtree_control_write,
        },
        {
                .name = "cgroup.events",
                .flags = CFTYPE_NOT_ON_ROOT,
                .file_offset = offsetof(struct cgroup, events_file),
                .seq_show = cgroup_events_show,
        },
        { }     /* terminate */
};

/* cgroup core interface files for the legacy hierarchies */
static struct cftype cgroup_legacy_base_files[] = {
        {
                .name = "cgroup.procs",
                .seq_start = cgroup_pidlist_start,
                .seq_next = cgroup_pidlist_next,
                .seq_stop = cgroup_pidlist_stop,
                .seq_show = cgroup_pidlist_show,
                .private = CGROUP_FILE_PROCS,
                .write = cgroup_procs_write,
        },
        {
                .name = "cgroup.clone_children",
                .read_u64 = cgroup_clone_children_read,
                .write_u64 = cgroup_clone_children_write,
        },
        {
                .name = "cgroup.sane_behavior",
                .flags = CFTYPE_ONLY_ON_ROOT,
                .seq_show = cgroup_sane_behavior_show,
        },
        {
                .name = "tasks",
                .seq_start = cgroup_pidlist_start,
                .seq_next = cgroup_pidlist_next,
                .seq_stop = cgroup_pidlist_stop,
                .seq_show = cgroup_pidlist_show,
                .private = CGROUP_FILE_TASKS,
                .write = cgroup_tasks_write,
        },
        {
                .name = "notify_on_release",
                .read_u64 = cgroup_read_notify_on_release,
                .write_u64 = cgroup_write_notify_on_release,
        },
        {
                .name = "release_agent",
                .flags = CFTYPE_ONLY_ON_ROOT,
                .seq_show = cgroup_release_agent_show,
                .write = cgroup_release_agent_write,
                .max_write_len = PATH_MAX - 1,
        },
        { }     /* terminate */
};

/*
 * css destruction is four-stage process.
 *
 * 1. Destruction starts.  Killing of the percpu_ref is initiated.
 *    Implemented in kill_css().
 *
 * 2. When the percpu_ref is confirmed to be visible as killed on all CPUs
 *    and thus css_tryget_online() is guaranteed to fail, the css can be
 *    offlined by invoking offline_css().  After offlining, the base ref is
 *    put.  Implemented in css_killed_work_fn().
 *
 * 3. When the percpu_ref reaches zero, the only possible remaining
 *    accessors are inside RCU read sections.  css_release() schedules the
 *    RCU callback.
 *
 * 4. After the grace period, the css can be freed.  Implemented in
 *    css_free_work_fn().
 *
 * It is actually hairier because both step 2 and 4 require process context
 * and thus involve punting to css->destroy_work adding two additional
 * steps to the already complex sequence.
 */
static void css_free_work_fn(struct work_struct *work)
{
        struct cgroup_subsys_state *css =
                container_of(work, struct cgroup_subsys_state, destroy_work);
        struct cgroup_subsys *ss = css->ss;
        struct cgroup *cgrp = css->cgroup;
        struct cgroup_file *cfile;

        percpu_ref_exit(&css->refcnt);

        list_for_each_entry(cfile, &css->files, node)
                kernfs_put(cfile->kn);

        if (ss) {
                /* css free path */
                int id = css->id;

                if (css->parent)
                        css_put(css->parent);

                ss->css_free(css);
                cgroup_idr_remove(&ss->css_idr, id);
                cgroup_put(cgrp);
        } else {
                /* cgroup free path */
                atomic_dec(&cgrp->root->nr_cgrps);
                cgroup_pidlist_destroy_all(cgrp);
                cancel_work_sync(&cgrp->release_agent_work);

                if (cgroup_parent(cgrp)) {
                        /*
                         * We get a ref to the parent, and put the ref when
                         * this cgroup is being freed, so it's guaranteed
                         * that the parent won't be destroyed before its
                         * children.
                         */
                        cgroup_put(cgroup_parent(cgrp));
                        kernfs_put(cgrp->kn);
                        kfree(cgrp);
                } else {
                        /*
                         * This is root cgroup's refcnt reaching zero,
                         * which indicates that the root should be
                         * released.
                         */
                        cgroup_destroy_root(cgrp->root);
                }
        }
}

static void css_free_rcu_fn(struct rcu_head *rcu_head)
{
        struct cgroup_subsys_state *css =
                container_of(rcu_head, struct cgroup_subsys_state, rcu_head);

        INIT_WORK(&css->destroy_work, css_free_work_fn);
        queue_work(cgroup_destroy_wq, &css->destroy_work);
}

static void css_release_work_fn(struct work_struct *work)
{
        struct cgroup_subsys_state *css =
                container_of(work, struct cgroup_subsys_state, destroy_work);
        struct cgroup_subsys *ss = css->ss;
        struct cgroup *cgrp = css->cgroup;

        mutex_lock(&cgroup_mutex);

        css->flags |= CSS_RELEASED;
        list_del_rcu(&css->sibling);

        if (ss) {
                /* css release path */
                cgroup_idr_replace(&ss->css_idr, NULL, css->id);
                if (ss->css_released)
                        ss->css_released(css);
        } else {
                /* cgroup release path */
                cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
                cgrp->id = -1;

                /*
                 * There are two control paths which try to determine
                 * cgroup from dentry without going through kernfs -
                 * cgroupstats_build() and css_tryget_online_from_dir().
                 * Those are supported by RCU protecting clearing of
                 * cgrp->kn->priv backpointer.
                 */
                RCU_INIT_POINTER(*(void __rcu __force **)&cgrp->kn->priv, NULL);
        }

        mutex_unlock(&cgroup_mutex);

        call_rcu(&css->rcu_head, css_free_rcu_fn);
}

static void css_release(struct percpu_ref *ref)
{
        struct cgroup_subsys_state *css =
                container_of(ref, struct cgroup_subsys_state, refcnt);

        INIT_WORK(&css->destroy_work, css_release_work_fn);
        queue_work(cgroup_destroy_wq, &css->destroy_work);
}

static void init_and_link_css(struct cgroup_subsys_state *css,
                              struct cgroup_subsys *ss, struct cgroup *cgrp)
{
        lockdep_assert_held(&cgroup_mutex);

        cgroup_get(cgrp);

        memset(css, 0, sizeof(*css));
        css->cgroup = cgrp;
        css->ss = ss;
        INIT_LIST_HEAD(&css->sibling);
        INIT_LIST_HEAD(&css->children);
        INIT_LIST_HEAD(&css->files);
        css->serial_nr = css_serial_nr_next++;

        if (cgroup_parent(cgrp)) {
                css->parent = cgroup_css(cgroup_parent(cgrp), ss);
                css_get(css->parent);
        }

        BUG_ON(cgroup_css(cgrp, ss));
}

/* invoke ->css_online() on a new CSS and mark it online if successful */
static int online_css(struct cgroup_subsys_state *css)
{
        struct cgroup_subsys *ss = css->ss;
        int ret = 0;

        lockdep_assert_held(&cgroup_mutex);

        if (ss->css_online)
                ret = ss->css_online(css);
        if (!ret) {
                css->flags |= CSS_ONLINE;
                rcu_assign_pointer(css->cgroup->subsys[ss->id], css);
        }
        return ret;
}

/* if the CSS is online, invoke ->css_offline() on it and mark it offline */
static void offline_css(struct cgroup_subsys_state *css)
{
        struct cgroup_subsys *ss = css->ss;

        lockdep_assert_held(&cgroup_mutex);

        if (!(css->flags & CSS_ONLINE))
                return;

        if (ss->css_offline)
                ss->css_offline(css);

        css->flags &= ~CSS_ONLINE;
        RCU_INIT_POINTER(css->cgroup->subsys[ss->id], NULL);

        wake_up_all(&css->cgroup->offline_waitq);
}

/**
 * create_css - create a cgroup_subsys_state
 * @cgrp: the cgroup new css will be associated with
 * @ss: the subsys of new css
 * @visible: whether to create control knobs for the new css or not
 *
 * Create a new css associated with @cgrp - @ss pair.  On success, the new
 * css is online and installed in @cgrp with all interface files created if
 * @visible.  Returns 0 on success, -errno on failure.
 */
static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss,
                      bool visible)
{
        struct cgroup *parent = cgroup_parent(cgrp);
        struct cgroup_subsys_state *parent_css = cgroup_css(parent, ss);
        struct cgroup_subsys_state *css;
        int err;

        lockdep_assert_held(&cgroup_mutex);

        css = ss->css_alloc(parent_css);
        if (IS_ERR(css))
                return PTR_ERR(css);

        init_and_link_css(css, ss, cgrp);

        err = percpu_ref_init(&css->refcnt, css_release, 0, GFP_KERNEL);
        if (err)
                goto err_free_css;

        err = cgroup_idr_alloc(&ss->css_idr, NULL, 2, 0, GFP_KERNEL);
        if (err < 0)
                goto err_free_percpu_ref;
        css->id = err;

        if (visible) {
                err = css_populate_dir(css, NULL);
                if (err)
                        goto err_free_id;
        }

        /* @css is ready to be brought online now, make it visible */
        list_add_tail_rcu(&css->sibling, &parent_css->children);
        cgroup_idr_replace(&ss->css_idr, css, css->id);

        err = online_css(css);
        if (err)
                goto err_list_del;

        if (ss->broken_hierarchy && !ss->warned_broken_hierarchy &&
            cgroup_parent(parent)) {
                pr_warn("%s (%d) created nested cgroup for controller \"%s\" which has incomplete hierarchy support. Nested cgroups may change behavior in the future.\n",
                        current->comm, current->pid, ss->name);
                if (!strcmp(ss->name, "memory"))
                        pr_warn("\"memory\" requires setting use_hierarchy to 1 on the root\n");
                ss->warned_broken_hierarchy = true;
        }

        return 0;

err_list_del:
        list_del_rcu(&css->sibling);
        css_clear_dir(css, NULL);
err_free_id:
        cgroup_idr_remove(&ss->css_idr, css->id);
err_free_percpu_ref:
        percpu_ref_exit(&css->refcnt);
err_free_css:
        call_rcu(&css->rcu_head, css_free_rcu_fn);
        return err;
}

static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
                        umode_t mode)
{
        struct cgroup *parent, *cgrp;
        struct cgroup_root *root;
        struct cgroup_subsys *ss;
        struct kernfs_node *kn;
        int ssid, ret;

        /* Do not accept '\n' to prevent making /proc/<pid>/cgroup unparsable.
         */
        if (strchr(name, '\n'))
                return -EINVAL;

        parent = cgroup_kn_lock_live(parent_kn);
        if (!parent)
                return -ENODEV;
        root = parent->root;

        /* allocate the cgroup and its ID, 0 is reserved for the root */
        cgrp = kzalloc(sizeof(*cgrp), GFP_KERNEL);
        if (!cgrp) {
                ret = -ENOMEM;
                goto out_unlock;
        }

        ret = percpu_ref_init(&cgrp->self.refcnt, css_release, 0, GFP_KERNEL);
        if (ret)
                goto out_free_cgrp;

        /*
         * Temporarily set the pointer to NULL, so idr_find() won't return
         * a half-baked cgroup.
         */
        cgrp->id = cgroup_idr_alloc(&root->cgroup_idr, NULL, 2, 0, GFP_KERNEL);
        if (cgrp->id < 0) {
                ret = -ENOMEM;
                goto out_cancel_ref;
        }

        init_cgroup_housekeeping(cgrp);

        cgrp->self.parent = &parent->self;
        cgrp->root = root;

        if (notify_on_release(parent))
                set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);

        if (test_bit(CGRP_CPUSET_CLONE_CHILDREN, &parent->flags))
                set_bit(CGRP_CPUSET_CLONE_CHILDREN, &cgrp->flags);

        /* create the directory */
        kn = kernfs_create_dir(parent->kn, name, mode, cgrp);
        if (IS_ERR(kn)) {
                ret = PTR_ERR(kn);
                goto out_free_id;
        }
        cgrp->kn = kn;

        /*
         * This extra ref will be put in cgroup_free_fn() and guarantees
         * that @cgrp->kn is always accessible.
         */
        kernfs_get(kn);

        cgrp->self.serial_nr = css_serial_nr_next++;

        /* allocation complete, commit to creation */
        list_add_tail_rcu(&cgrp->self.sibling, &cgroup_parent(cgrp)->self.children);
        atomic_inc(&root->nr_cgrps);
        cgroup_get(parent);

        /*
         * @cgrp is now fully operational.  If something fails after this
         * point, it'll be released via the normal destruction path.
         */
        cgroup_idr_replace(&root->cgroup_idr, cgrp, cgrp->id);

        ret = cgroup_kn_set_ugid(kn);
        if (ret)
                goto out_destroy;

        ret = css_populate_dir(&cgrp->self, NULL);
        if (ret)
                goto out_destroy;

        /* let's create and online css's */
        for_each_subsys(ss, ssid) {
                if (parent->child_subsys_mask & (1 << ssid)) {
                        ret = create_css(cgrp, ss,
                                         parent->subtree_control & (1 << ssid));
                        if (ret)
                                goto out_destroy;
                }
        }

        /*
         * On the default hierarchy, a child doesn't automatically inherit
         * subtree_control from the parent.  Each is configured manually.
         */
        if (!cgroup_on_dfl(cgrp)) {
                cgrp->subtree_control = parent->subtree_control;
                cgroup_refresh_child_subsys_mask(cgrp);
        }

        kernfs_activate(kn);

        ret = 0;
        goto out_unlock;

out_free_id:
        cgroup_idr_remove(&root->cgroup_idr, cgrp->id);
out_cancel_ref:
        percpu_ref_exit(&cgrp->self.refcnt);
out_free_cgrp:
        kfree(cgrp);
out_unlock:
        cgroup_kn_unlock(parent_kn);
        return ret;

out_destroy:
        cgroup_destroy_locked(cgrp);
        goto out_unlock;
}

/*
 * This is called when the refcnt of a css is confirmed to be killed.
 * css_tryget_online() is now guaranteed to fail.  Tell the subsystem to
 * initate destruction and put the css ref from kill_css().
 */
static void css_killed_work_fn(struct work_struct *work)
{
        struct cgroup_subsys_state *css =
                container_of(work, struct cgroup_subsys_state, destroy_work);

        mutex_lock(&cgroup_mutex);
        offline_css(css);
        mutex_unlock(&cgroup_mutex);

        css_put(css);
}

/* css kill confirmation processing requires process context, bounce */
static void css_killed_ref_fn(struct percpu_ref *ref)
{
        struct cgroup_subsys_state *css =
                container_of(ref, struct cgroup_subsys_state, refcnt);

        INIT_WORK(&css->destroy_work, css_killed_work_fn);
        queue_work(cgroup_destroy_wq, &css->destroy_work);
}

/**
 * kill_css - destroy a css
 * @css: css to destroy
 *
 * This function initiates destruction of @css by removing cgroup interface
 * files and putting its base reference.  ->css_offline() will be invoked
 * asynchronously once css_tryget_online() is guaranteed to fail and when
 * the reference count reaches zero, @css will be released.
 */
static void kill_css(struct cgroup_subsys_state *css)
{
        lockdep_assert_held(&cgroup_mutex);

        /*
         * This must happen before css is disassociated with its cgroup.
         * See seq_css() for details.
         */
        css_clear_dir(css, NULL);

        /*
         * Killing would put the base ref, but we need to keep it alive
         * until after ->css_offline().
         */
        css_get(css);

        /*
         * cgroup core guarantees that, by the time ->css_offline() is
         * invoked, no new css reference will be given out via
         * css_tryget_online().  We can't simply call percpu_ref_kill() and
         * proceed to offlining css's because percpu_ref_kill() doesn't
         * guarantee that the ref is seen as killed on all CPUs on return.
         *
         * Use percpu_ref_kill_and_confirm() to get notifications as each
         * css is confirmed to be seen as killed on all CPUs.
         */
        percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn);
}

/**
 * cgroup_destroy_locked - the first stage of cgroup destruction
 * @cgrp: cgroup to be destroyed
 *
 * css's make use of percpu refcnts whose killing latency shouldn't be
 * exposed to userland and are RCU protected.  Also, cgroup core needs to
 * guarantee that css_tryget_online() won't succeed by the time
 * ->css_offline() is invoked.  To satisfy all the requirements,
 * destruction is implemented in the following two steps.
 *
 * s1. Verify @cgrp can be destroyed and mark it dying.  Remove all
 *     userland visible parts and start killing the percpu refcnts of
 *     css's.  Set up so that the next stage will be kicked off once all
 *     the percpu refcnts are confirmed to be killed.
 *
 * s2. Invoke ->css_offline(), mark the cgroup dead and proceed with the
 *     rest of destruction.  Once all cgroup references are gone, the
 *     cgroup is RCU-freed.
 *
 * This function implements s1.  After this step, @cgrp is gone as far as
 * the userland is concerned and a new cgroup with the same name may be
 * created.  As cgroup doesn't care about the names internally, this
 * doesn't cause any problem.
 */
static int cgroup_destroy_locked(struct cgroup *cgrp)
        __releases(&cgroup_mutex) __acquires(&cgroup_mutex)
{
        struct cgroup_subsys_state *css;
        int ssid;

        lockdep_assert_held(&cgroup_mutex);

        /*
         * Only migration can raise populated from zero and we're already
         * holding cgroup_mutex.
         */
        if (cgroup_is_populated(cgrp))
                return -EBUSY;

        /*
         * Make sure there's no live children.  We can't test emptiness of
         * ->self.children as dead children linger on it while being
         * drained; otherwise, "rmdir parent/child parent" may fail.
         */
        if (css_has_online_children(&cgrp->self))
                return -EBUSY;

        /*
         * Mark @cgrp dead.  This prevents further task migration and child
         * creation by disabling cgroup_lock_live_group().
         */
        cgrp->self.flags &= ~CSS_ONLINE;

        /* initiate massacre of all css's */
        for_each_css(css, ssid, cgrp)
                kill_css(css);

        /*
         * Remove @cgrp directory along with the base files.  @cgrp has an
         * extra ref on its kn.
         */
        kernfs_remove(cgrp->kn);

        check_for_release(cgroup_parent(cgrp));

        /* put the base reference */
        percpu_ref_kill(&cgrp->self.refcnt);

        return 0;
};

static int cgroup_rmdir(struct kernfs_node *kn)
{
        struct cgroup *cgrp;
        int ret = 0;

        cgrp = cgroup_kn_lock_live(kn);
        if (!cgrp)
                return 0;

        ret = cgroup_destroy_locked(cgrp);

        cgroup_kn_unlock(kn);
        return ret;
}

static struct kernfs_syscall_ops cgroup_kf_syscall_ops = {
        .remount_fs             = cgroup_remount,
        .show_options           = cgroup_show_options,
        .mkdir                  = cgroup_mkdir,
        .rmdir                  = cgroup_rmdir,
        .rename                 = cgroup_rename,
};

static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
{
        struct cgroup_subsys_state *css;

        printk(KERN_INFO "Initializing cgroup subsys %s\n", ss->name);

        mutex_lock(&cgroup_mutex);

        idr_init(&ss->css_idr);
        INIT_LIST_HEAD(&ss->cfts);

        /* Create the root cgroup state for this subsystem */
        ss->root = &cgrp_dfl_root;
        css = ss->css_alloc(cgroup_css(&cgrp_dfl_root.cgrp, ss));
        /* We don't handle early failures gracefully */
        BUG_ON(IS_ERR(css));
        init_and_link_css(css, ss, &cgrp_dfl_root.cgrp);

        /*
         * Root csses are never destroyed and we can't initialize
         * percpu_ref during early init.  Disable refcnting.
         */
        css->flags |= CSS_NO_REF;

        if (early) {
                /* allocation can't be done safely during early init */
                css->id = 1;
        } else {
                css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2, GFP_KERNEL);
                BUG_ON(css->id < 0);
        }

        /* Update the init_css_set to contain a subsys
         * pointer to this state - since the subsystem is
         * newly registered, all tasks and hence the
         * init_css_set is in the subsystem's root cgroup. */
        init_css_set.subsys[ss->id] = css;

        have_fork_callback |= (bool)ss->fork << ss->id;
        have_exit_callback |= (bool)ss->exit << ss->id;
        have_canfork_callback |= (bool)ss->can_fork << ss->id;

        /* At system boot, before all subsystems have been
         * registered, no tasks have been forked, so we don't
         * need to invoke fork callbacks here. */
        BUG_ON(!list_empty(&init_task.tasks));

        BUG_ON(online_css(css));

        mutex_unlock(&cgroup_mutex);
}

/**
 * cgroup_init_early - cgroup initialization at system boot
 *
 * Initialize cgroups at system boot, and initialize any
 * subsystems that request early init.
 */
int __init cgroup_init_early(void)
{
        static struct cgroup_sb_opts __initdata opts;
        struct cgroup_subsys *ss;
        int i;

        init_cgroup_root(&cgrp_dfl_root, &opts);
        cgrp_dfl_root.cgrp.self.flags |= CSS_NO_REF;

        RCU_INIT_POINTER(init_task.cgroups, &init_css_set);

        for_each_subsys(ss, i) {
                WARN(!ss->css_alloc || !ss->css_free || ss->name || ss->id,
                     "invalid cgroup_subsys %d:%s css_alloc=%p css_free=%p name:id=%d:%s\n",
                     i, cgroup_subsys_name[i], ss->css_alloc, ss->css_free,
                     ss->id, ss->name);
                WARN(strlen(cgroup_subsys_name[i]) > MAX_CGROUP_TYPE_NAMELEN,
                     "cgroup_subsys_name %s too long\n", cgroup_subsys_name[i]);

                ss->id = i;
                ss->name = cgroup_subsys_name[i];
                if (!ss->legacy_name)
                        ss->legacy_name = cgroup_subsys_name[i];

                if (ss->early_init)
                        cgroup_init_subsys(ss, true);
        }
        return 0;
}

static unsigned long cgroup_disable_mask __initdata;

/**
 * cgroup_init - cgroup initialization
 *
 * Register cgroup filesystem and /proc file, and initialize
 * any subsystems that didn't request early init.
 */
int __init cgroup_init(void)
{
        struct cgroup_subsys *ss;
        unsigned long key;
        int ssid, err;

        BUG_ON(percpu_init_rwsem(&cgroup_threadgroup_rwsem));
        BUG_ON(cgroup_init_cftypes(NULL, cgroup_dfl_base_files));
        BUG_ON(cgroup_init_cftypes(NULL, cgroup_legacy_base_files));

        mutex_lock(&cgroup_mutex);

        /* Add init_css_set to the hash table */
        key = css_set_hash(init_css_set.subsys);
        hash_add(css_set_table, &init_css_set.hlist, key);

        BUG_ON(cgroup_setup_root(&cgrp_dfl_root, 0));

        mutex_unlock(&cgroup_mutex);

        for_each_subsys(ss, ssid) {
                if (ss->early_init) {
                        struct cgroup_subsys_state *css =
                                init_css_set.subsys[ss->id];

                        css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2,
                                                   GFP_KERNEL);
                        BUG_ON(css->id < 0);
                } else {
                        cgroup_init_subsys(ss, false);
                }

                list_add_tail(&init_css_set.e_cset_node[ssid],
                              &cgrp_dfl_root.cgrp.e_csets[ssid]);

                /*
                 * Setting dfl_root subsys_mask needs to consider the
                 * disabled flag and cftype registration needs kmalloc,
                 * both of which aren't available during early_init.
                 */
                if (cgroup_disable_mask & (1 << ssid)) {
                        static_branch_disable(cgroup_subsys_enabled_key[ssid]);
                        printk(KERN_INFO "Disabling %s control group subsystem\n",
                               ss->name);
                        continue;
                }

                cgrp_dfl_root.subsys_mask |= 1 << ss->id;

                if (cgroup_legacy_files_on_dfl && !ss->dfl_cftypes)
                        ss->dfl_cftypes = ss->legacy_cftypes;

                if (!ss->dfl_cftypes)
                        cgrp_dfl_root_inhibit_ss_mask |= 1 << ss->id;

                if (ss->dfl_cftypes == ss->legacy_cftypes) {
                        WARN_ON(cgroup_add_cftypes(ss, ss->dfl_cftypes));
                } else {
                        WARN_ON(cgroup_add_dfl_cftypes(ss, ss->dfl_cftypes));
                        WARN_ON(cgroup_add_legacy_cftypes(ss, ss->legacy_cftypes));
                }

                if (ss->bind)
                        ss->bind(init_css_set.subsys[ssid]);
        }

        err = sysfs_create_mount_point(fs_kobj, "cgroup");
        if (err)
                return err;

        err = register_filesystem(&cgroup_fs_type);
        if (err < 0) {
                sysfs_remove_mount_point(fs_kobj, "cgroup");
                return err;
        }

        proc_create("cgroups", 0, NULL, &proc_cgroupstats_operations);
        return 0;
}

static int __init cgroup_wq_init(void)
{
        /*
         * There isn't much point in executing destruction path in
         * parallel.  Good chunk is serialized with cgroup_mutex anyway.
         * Use 1 for @max_active.
         *
         * We would prefer to do this in cgroup_init() above, but that
         * is called before init_workqueues(): so leave this until after.
         */
        cgroup_destroy_wq = alloc_workqueue("cgroup_destroy", 0, 1);
        BUG_ON(!cgroup_destroy_wq);

        /*
         * Used to destroy pidlists and separate to serve as flush domain.
         * Cap @max_active to 1 too.
         */
        cgroup_pidlist_destroy_wq = alloc_workqueue("cgroup_pidlist_destroy",
                                                    0, 1);
        BUG_ON(!cgroup_pidlist_destroy_wq);

        return 0;
}
core_initcall(cgroup_wq_init);

/*
 * proc_cgroup_show()
 *  - Print task's cgroup paths into seq_file, one line for each hierarchy
 *  - Used for /proc/<pid>/cgroup.
 */
int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns,
                     struct pid *pid, struct task_struct *tsk)
{
        char *buf, *path;
        int retval;
        struct cgroup_root *root;

        retval = -ENOMEM;
        buf = kmalloc(PATH_MAX, GFP_KERNEL);
        if (!buf)
                goto out;

        mutex_lock(&cgroup_mutex);
        down_read(&css_set_rwsem);

        for_each_root(root) {
                struct cgroup_subsys *ss;
                struct cgroup *cgrp;
                int ssid, count = 0;

                if (root == &cgrp_dfl_root && !cgrp_dfl_root_visible)
                        continue;

                seq_printf(m, "%d:", root->hierarchy_id);
                if (root != &cgrp_dfl_root)
                        for_each_subsys(ss, ssid)
                                if (root->subsys_mask & (1 << ssid))
                                        seq_printf(m, "%s%s", count++ ? "," : "",
                                                   ss->legacy_name);
                if (strlen(root->name))
                        seq_printf(m, "%sname=%s", count ? "," : "",
                                   root->name);
                seq_putc(m, ':');
                cgrp = task_cgroup_from_root(tsk, root);
                path = cgroup_path(cgrp, buf, PATH_MAX);
                if (!path) {
                        retval = -ENAMETOOLONG;
                        goto out_unlock;
                }
                seq_puts(m, path);
                seq_putc(m, '\n');
        }

        retval = 0;
out_unlock:
        up_read(&css_set_rwsem);
        mutex_unlock(&cgroup_mutex);
        kfree(buf);
out:
        return retval;
}

/* Display information about each subsystem and each hierarchy */
static int proc_cgroupstats_show(struct seq_file *m, void *v)
{
        struct cgroup_subsys *ss;
        int i;

        seq_puts(m, "#subsys_name\thierarchy\tnum_cgroups\tenabled\n");
        /*
         * ideally we don't want subsystems moving around while we do this.
         * cgroup_mutex is also necessary to guarantee an atomic snapshot of
         * subsys/hierarchy state.
         */
        mutex_lock(&cgroup_mutex);

        for_each_subsys(ss, i)
                seq_printf(m, "%s\t%d\t%d\t%d\n",
                           ss->legacy_name, ss->root->hierarchy_id,
                           atomic_read(&ss->root->nr_cgrps),
                           cgroup_ssid_enabled(i));

        mutex_unlock(&cgroup_mutex);
        return 0;
}

static int cgroupstats_open(struct inode *inode, struct file *file)
{
        return single_open(file, proc_cgroupstats_show, NULL);
}

static const struct file_operations proc_cgroupstats_operations = {
        .open = cgroupstats_open,
        .read = seq_read,
        .llseek = seq_lseek,
        .release = single_release,
};

static void **subsys_canfork_priv_p(void *ss_priv[CGROUP_CANFORK_COUNT], int i)
{
        if (CGROUP_CANFORK_START <= i && i < CGROUP_CANFORK_END)
                return &ss_priv[i - CGROUP_CANFORK_START];
        return NULL;
}

static void *subsys_canfork_priv(void *ss_priv[CGROUP_CANFORK_COUNT], int i)
{
        void **private = subsys_canfork_priv_p(ss_priv, i);
        return private ? *private : NULL;
}

/**
 * cgroup_fork - initialize cgroup related fields during copy_process()
 * @child: pointer to task_struct of forking parent process.
 *
 * A task is associated with the init_css_set until cgroup_post_fork()
 * attaches it to the parent's css_set.  Empty cg_list indicates that
 * @child isn't holding reference to its css_set.
 */
void cgroup_fork(struct task_struct *child)
{
        RCU_INIT_POINTER(child->cgroups, &init_css_set);
        INIT_LIST_HEAD(&child->cg_list);
}

/**
 * cgroup_can_fork - called on a new task before the process is exposed
 * @child: the task in question.
 *
 * This calls the subsystem can_fork() callbacks. If the can_fork() callback
 * returns an error, the fork aborts with that error code. This allows for
 * a cgroup subsystem to conditionally allow or deny new forks.
 */
int cgroup_can_fork(struct task_struct *child,
                    void *ss_priv[CGROUP_CANFORK_COUNT])
{
        struct cgroup_subsys *ss;
        int i, j, ret;

        for_each_subsys_which(ss, i, &have_canfork_callback) {
                ret = ss->can_fork(child, subsys_canfork_priv_p(ss_priv, i));
                if (ret)
                        goto out_revert;
        }

        return 0;

out_revert:
        for_each_subsys(ss, j) {
                if (j >= i)
                        break;
                if (ss->cancel_fork)
                        ss->cancel_fork(child, subsys_canfork_priv(ss_priv, j));
        }

        return ret;
}

/**
 * cgroup_cancel_fork - called if a fork failed after cgroup_can_fork()
 * @child: the task in question
 *
 * This calls the cancel_fork() callbacks if a fork failed *after*
 * cgroup_can_fork() succeded.
 */
void cgroup_cancel_fork(struct task_struct *child,
                        void *ss_priv[CGROUP_CANFORK_COUNT])
{
        struct cgroup_subsys *ss;
        int i;

        for_each_subsys(ss, i)
                if (ss->cancel_fork)
                        ss->cancel_fork(child, subsys_canfork_priv(ss_priv, i));
}

/**
 * cgroup_post_fork - called on a new task after adding it to the task list
 * @child: the task in question
 *
 * Adds the task to the list running through its css_set if necessary and
 * call the subsystem fork() callbacks.  Has to be after the task is
 * visible on the task list in case we race with the first call to
 * cgroup_task_iter_start() - to guarantee that the new task ends up on its
 * list.
 */
void cgroup_post_fork(struct task_struct *child,
                      void *old_ss_priv[CGROUP_CANFORK_COUNT])
{
        struct cgroup_subsys *ss;
        int i;

        /*
         * This may race against cgroup_enable_task_cg_lists().  As that
         * function sets use_task_css_set_links before grabbing
         * tasklist_lock and we just went through tasklist_lock to add
         * @child, it's guaranteed that either we see the set
         * use_task_css_set_links or cgroup_enable_task_cg_lists() sees
         * @child during its iteration.
         *
         * If we won the race, @child is associated with %current's
         * css_set.  Grabbing css_set_rwsem guarantees both that the
         * association is stable, and, on completion of the parent's
         * migration, @child is visible in the source of migration or
         * already in the destination cgroup.  This guarantee is necessary
         * when implementing operations which need to migrate all tasks of
         * a cgroup to another.
         *
         * Note that if we lose to cgroup_enable_task_cg_lists(), @child
         * will remain in init_css_set.  This is safe because all tasks are
         * in the init_css_set before cg_links is enabled and there's no
         * operation which transfers all tasks out of init_css_set.
         */
        if (use_task_css_set_links) {
                struct css_set *cset;

                down_write(&css_set_rwsem);
                cset = task_css_set(current);
                if (list_empty(&child->cg_list)) {
                        get_css_set(cset);
                        css_set_move_task(child, NULL, cset, false);
                }
                up_write(&css_set_rwsem);
        }

        /*
         * Call ss->fork().  This must happen after @child is linked on
         * css_set; otherwise, @child might change state between ->fork()
         * and addition to css_set.
         */
        for_each_subsys_which(ss, i, &have_fork_callback)
                ss->fork(child, subsys_canfork_priv(old_ss_priv, i));
}

/**
 * cgroup_exit - detach cgroup from exiting task
 * @tsk: pointer to task_struct of exiting process
 *
 * Description: Detach cgroup from @tsk and release it.
 *
 * Note that cgroups marked notify_on_release force every task in
 * them to take the global cgroup_mutex mutex when exiting.
 * This could impact scaling on very large systems.  Be reluctant to
 * use notify_on_release cgroups where very high task exit scaling
 * is required on large systems.
 *
 * We set the exiting tasks cgroup to the root cgroup (top_cgroup).  We
 * call cgroup_exit() while the task is still competent to handle
 * notify_on_release(), then leave the task attached to the root cgroup in
 * each hierarchy for the remainder of its exit.  No need to bother with
 * init_css_set refcnting.  init_css_set never goes away and we can't race
 * with migration path - PF_EXITING is visible to migration path.
 */
void cgroup_exit(struct task_struct *tsk)
{
        struct cgroup_subsys *ss;
        struct css_set *cset;
        bool put_cset = false;
        int i;

        /*
         * Unlink from @tsk from its css_set.  As migration path can't race
         * with us, we can check css_set and cg_list without synchronization.
         */
        cset = task_css_set(tsk);

        if (!list_empty(&tsk->cg_list)) {
                down_write(&css_set_rwsem);
                css_set_move_task(tsk, cset, NULL, false);
                up_write(&css_set_rwsem);
                put_cset = true;
        }

        /* Reassign the task to the init_css_set. */
        RCU_INIT_POINTER(tsk->cgroups, &init_css_set);

        /* see cgroup_post_fork() for details */
        for_each_subsys_which(ss, i, &have_exit_callback) {
                struct cgroup_subsys_state *old_css = cset->subsys[i];
                struct cgroup_subsys_state *css = task_css(tsk, i);

                ss->exit(css, old_css, tsk);
        }

        if (put_cset)
                put_css_set(cset);
}

static void check_for_release(struct cgroup *cgrp)
{
        if (notify_on_release(cgrp) && !cgroup_is_populated(cgrp) &&
            !css_has_online_children(&cgrp->self) && !cgroup_is_dead(cgrp))
                schedule_work(&cgrp->release_agent_work);
}

/*
 * Notify userspace when a cgroup is released, by running the
 * configured release agent with the name of the cgroup (path
 * relative to the root of cgroup file system) as the argument.
 *
 * Most likely, this user command will try to rmdir this cgroup.
 *
 * This races with the possibility that some other task will be
 * attached to this cgroup before it is removed, or that some other
 * user task will 'mkdir' a child cgroup of this cgroup.  That's ok.
 * The presumed 'rmdir' will fail quietly if this cgroup is no longer
 * unused, and this cgroup will be reprieved from its death sentence,
 * to continue to serve a useful existence.  Next time it's released,
 * we will get notified again, if it still has 'notify_on_release' set.
 *
 * The final arg to call_usermodehelper() is UMH_WAIT_EXEC, which
 * means only wait until the task is successfully execve()'d.  The
 * separate release agent task is forked by call_usermodehelper(),
 * then control in this thread returns here, without waiting for the
 * release agent task.  We don't bother to wait because the caller of
 * this routine has no use for the exit status of the release agent
 * task, so no sense holding our caller up for that.
 */
static void cgroup_release_agent(struct work_struct *work)
{
        struct cgroup *cgrp =
                container_of(work, struct cgroup, release_agent_work);
        char *pathbuf = NULL, *agentbuf = NULL, *path;
        char *argv[3], *envp[3];

        mutex_lock(&cgroup_mutex);

        pathbuf = kmalloc(PATH_MAX, GFP_KERNEL);
        agentbuf = kstrdup(cgrp->root->release_agent_path, GFP_KERNEL);
        if (!pathbuf || !agentbuf)
                goto out;

        path = cgroup_path(cgrp, pathbuf, PATH_MAX);
        if (!path)
                goto out;

        argv[0] = agentbuf;
        argv[1] = path;
        argv[2] = NULL;

        /* minimal command environment */
        envp[0] = "HOME=/";
        envp[1] = "PATH=/sbin:/bin:/usr/sbin:/usr/bin";
        envp[2] = NULL;

        mutex_unlock(&cgroup_mutex);
        call_usermodehelper(argv[0], argv, envp, UMH_WAIT_EXEC);
        goto out_free;
out:
        mutex_unlock(&cgroup_mutex);
out_free:
        kfree(agentbuf);
        kfree(pathbuf);
}

static int __init cgroup_disable(char *str)
{
        struct cgroup_subsys *ss;
        char *token;
        int i;

        while ((token = strsep(&str, ",")) != NULL) {
                if (!*token)
                        continue;

                for_each_subsys(ss, i) {
                        if (strcmp(token, ss->name) &&
                            strcmp(token, ss->legacy_name))
                                continue;
                        cgroup_disable_mask |= 1 << i;
                }
        }
        return 1;
}
__setup("cgroup_disable=", cgroup_disable);

static int __init cgroup_set_legacy_files_on_dfl(char *str)
{
        printk("cgroup: using legacy files on the default hierarchy\n");
        cgroup_legacy_files_on_dfl = true;
        return 0;
}
__setup("cgroup__DEVEL__legacy_files_on_dfl", cgroup_set_legacy_files_on_dfl);

/**
 * css_tryget_online_from_dir - get corresponding css from a cgroup dentry
 * @dentry: directory dentry of interest
 * @ss: subsystem of interest
 *
 * If @dentry is a directory for a cgroup which has @ss enabled on it, try
 * to get the corresponding css and return it.  If such css doesn't exist
 * or can't be pinned, an ERR_PTR value is returned.
 */
struct cgroup_subsys_state *css_tryget_online_from_dir(struct dentry *dentry,
                                                       struct cgroup_subsys *ss)
{
        struct kernfs_node *kn = kernfs_node_from_dentry(dentry);
        struct cgroup_subsys_state *css = NULL;
        struct cgroup *cgrp;

        /* is @dentry a cgroup dir? */
        if (dentry->d_sb->s_type != &cgroup_fs_type || !kn ||
            kernfs_type(kn) != KERNFS_DIR)
                return ERR_PTR(-EBADF);

        rcu_read_lock();

        /*
         * This path doesn't originate from kernfs and @kn could already
         * have been or be removed at any point.  @kn->priv is RCU
         * protected for this access.  See css_release_work_fn() for details.
         */
        cgrp = rcu_dereference(kn->priv);
        if (cgrp)
                css = cgroup_css(cgrp, ss);

        if (!css || !css_tryget_online(css))
                css = ERR_PTR(-ENOENT);

        rcu_read_unlock();
        return css;
}

/**
 * css_from_id - lookup css by id
 * @id: the cgroup id
 * @ss: cgroup subsys to be looked into
 *
 * Returns the css if there's valid one with @id, otherwise returns NULL.
 * Should be called under rcu_read_lock().
 */
struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss)
{
        WARN_ON_ONCE(!rcu_read_lock_held());
        return id > 0 ? idr_find(&ss->css_idr, id) : NULL;
}

#ifdef CONFIG_CGROUP_DEBUG
static struct cgroup_subsys_state *
debug_css_alloc(struct cgroup_subsys_state *parent_css)
{
        struct cgroup_subsys_state *css = kzalloc(sizeof(*css), GFP_KERNEL);

        if (!css)
                return ERR_PTR(-ENOMEM);

        return css;
}

static void debug_css_free(struct cgroup_subsys_state *css)
{
        kfree(css);
}

static u64 debug_taskcount_read(struct cgroup_subsys_state *css,
                                struct cftype *cft)
{
        return cgroup_task_count(css->cgroup);
}

static u64 current_css_set_read(struct cgroup_subsys_state *css,
                                struct cftype *cft)
{
        return (u64)(unsigned long)current->cgroups;
}

static u64 current_css_set_refcount_read(struct cgroup_subsys_state *css,
                                         struct cftype *cft)
{
        u64 count;

        rcu_read_lock();
        count = atomic_read(&task_css_set(current)->refcount);
        rcu_read_unlock();
        return count;
}

static int current_css_set_cg_links_read(struct seq_file *seq, void *v)
{
        struct cgrp_cset_link *link;
        struct css_set *cset;
        char *name_buf;

        name_buf = kmalloc(NAME_MAX + 1, GFP_KERNEL);
        if (!name_buf)
                return -ENOMEM;

        down_read(&css_set_rwsem);
        rcu_read_lock();
        cset = rcu_dereference(current->cgroups);
        list_for_each_entry(link, &cset->cgrp_links, cgrp_link) {
                struct cgroup *c = link->cgrp;

                cgroup_name(c, name_buf, NAME_MAX + 1);
                seq_printf(seq, "Root %d group %s\n",
                           c->root->hierarchy_id, name_buf);
        }
        rcu_read_unlock();
        up_read(&css_set_rwsem);
        kfree(name_buf);
        return 0;
}

#define MAX_TASKS_SHOWN_PER_CSS 25
static int cgroup_css_links_read(struct seq_file *seq, void *v)
{
        struct cgroup_subsys_state *css = seq_css(seq);
        struct cgrp_cset_link *link;

        down_read(&css_set_rwsem);
        list_for_each_entry(link, &css->cgroup->cset_links, cset_link) {
                struct css_set *cset = link->cset;
                struct task_struct *task;
                int count = 0;

                seq_printf(seq, "css_set %p\n", cset);

                list_for_each_entry(task, &cset->tasks, cg_list) {
                        if (count++ > MAX_TASKS_SHOWN_PER_CSS)
                                goto overflow;
                        seq_printf(seq, "  task %d\n", task_pid_vnr(task));
                }

                list_for_each_entry(task, &cset->mg_tasks, cg_list) {
                        if (count++ > MAX_TASKS_SHOWN_PER_CSS)
                                goto overflow;
                        seq_printf(seq, "  task %d\n", task_pid_vnr(task));
                }
                continue;
        overflow:
                seq_puts(seq, "  ...\n");
        }
        up_read(&css_set_rwsem);
        return 0;
}

static u64 releasable_read(struct cgroup_subsys_state *css, struct cftype *cft)
{
        return (!cgroup_is_populated(css->cgroup) &&
                !css_has_online_children(&css->cgroup->self));
}

static struct cftype debug_files[] =  {
        {
                .name = "taskcount",
                .read_u64 = debug_taskcount_read,
        },

        {
                .name = "current_css_set",
                .read_u64 = current_css_set_read,
        },

        {
                .name = "current_css_set_refcount",
                .read_u64 = current_css_set_refcount_read,
        },

        {
                .name = "current_css_set_cg_links",
                .seq_show = current_css_set_cg_links_read,
        },

        {
                .name = "cgroup_css_links",
                .seq_show = cgroup_css_links_read,
        },

        {
                .name = "releasable",
                .read_u64 = releasable_read,
        },

        { }     /* terminate */
};

struct cgroup_subsys debug_cgrp_subsys = {
        .css_alloc = debug_css_alloc,
        .css_free = debug_css_free,
        .legacy_cftypes = debug_files,
};
#endif /* CONFIG_CGROUP_DEBUG */