Merge tag 'iomap-5.4-merge-6' of git://git.kernel.org/pub/scm/fs/xfs/xfs-linux

[linux-2.6-block.git] / kernel / locking / rwsem.c

// SPDX-License-Identifier: GPL-2.0
/* kernel/rwsem.c: R/W semaphores, public implementation
 *
 * Written by David Howells (dhowells@redhat.com).
 * Derived from asm-i386/semaphore.h
 *
 * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
 * and Michel Lespinasse <walken@google.com>
 *
 * Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
 * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
 *
 * Rwsem count bit fields re-definition and rwsem rearchitecture by
 * Waiman Long <longman@redhat.com> and
 * Peter Zijlstra <peterz@infradead.org>.
 */

#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/sched/rt.h>
#include <linux/sched/task.h>
#include <linux/sched/debug.h>
#include <linux/sched/wake_q.h>
#include <linux/sched/signal.h>
#include <linux/sched/clock.h>
#include <linux/export.h>
#include <linux/rwsem.h>
#include <linux/atomic.h>

#include "rwsem.h"
#include "lock_events.h"

/*
 * The least significant 3 bits of the owner value has the following
 * meanings when set.
 *  - Bit 0: RWSEM_READER_OWNED - The rwsem is owned by readers
 *  - Bit 1: RWSEM_RD_NONSPINNABLE - Readers cannot spin on this lock.
 *  - Bit 2: RWSEM_WR_NONSPINNABLE - Writers cannot spin on this lock.
 *
 * When the rwsem is either owned by an anonymous writer, or it is
 * reader-owned, but a spinning writer has timed out, both nonspinnable
 * bits will be set to disable optimistic spinning by readers and writers.
 * In the later case, the last unlocking reader should then check the
 * writer nonspinnable bit and clear it only to give writers preference
 * to acquire the lock via optimistic spinning, but not readers. Similar
 * action is also done in the reader slowpath.

 * When a writer acquires a rwsem, it puts its task_struct pointer
 * into the owner field. It is cleared after an unlock.
 *
 * When a reader acquires a rwsem, it will also puts its task_struct
 * pointer into the owner field with the RWSEM_READER_OWNED bit set.
 * On unlock, the owner field will largely be left untouched. So
 * for a free or reader-owned rwsem, the owner value may contain
 * information about the last reader that acquires the rwsem.
 *
 * That information may be helpful in debugging cases where the system
 * seems to hang on a reader owned rwsem especially if only one reader
 * is involved. Ideally we would like to track all the readers that own
 * a rwsem, but the overhead is simply too big.
 *
 * Reader optimistic spinning is helpful when the reader critical section
 * is short and there aren't that many readers around. It makes readers
 * relatively more preferred than writers. When a writer times out spinning
 * on a reader-owned lock and set the nospinnable bits, there are two main
 * reasons for that.
 *
 *  1) The reader critical section is long, perhaps the task sleeps after
 *     acquiring the read lock.
 *  2) There are just too many readers contending the lock causing it to
 *     take a while to service all of them.
 *
 * In the former case, long reader critical section will impede the progress
 * of writers which is usually more important for system performance. In
 * the later case, reader optimistic spinning tends to make the reader
 * groups that contain readers that acquire the lock together smaller
 * leading to more of them. That may hurt performance in some cases. In
 * other words, the setting of nonspinnable bits indicates that reader
 * optimistic spinning may not be helpful for those workloads that cause
 * it.
 *
 * Therefore, any writers that had observed the setting of the writer
 * nonspinnable bit for a given rwsem after they fail to acquire the lock
 * via optimistic spinning will set the reader nonspinnable bit once they
 * acquire the write lock. Similarly, readers that observe the setting
 * of reader nonspinnable bit at slowpath entry will set the reader
 * nonspinnable bits when they acquire the read lock via the wakeup path.
 *
 * Once the reader nonspinnable bit is on, it will only be reset when
 * a writer is able to acquire the rwsem in the fast path or somehow a
 * reader or writer in the slowpath doesn't observe the nonspinable bit.
 *
 * This is to discourage reader optmistic spinning on that particular
 * rwsem and make writers more preferred. This adaptive disabling of reader
 * optimistic spinning will alleviate the negative side effect of this
 * feature.
 */
#define RWSEM_READER_OWNED      (1UL << 0)
#define RWSEM_RD_NONSPINNABLE   (1UL << 1)
#define RWSEM_WR_NONSPINNABLE   (1UL << 2)
#define RWSEM_NONSPINNABLE      (RWSEM_RD_NONSPINNABLE | RWSEM_WR_NONSPINNABLE)
#define RWSEM_OWNER_FLAGS_MASK  (RWSEM_READER_OWNED | RWSEM_NONSPINNABLE)

#ifdef CONFIG_DEBUG_RWSEMS
# define DEBUG_RWSEMS_WARN_ON(c, sem)   do {                    \
        if (!debug_locks_silent &&                              \
            WARN_ONCE(c, "DEBUG_RWSEMS_WARN_ON(%s): count = 0x%lx, magic = 0x%lx, owner = 0x%lx, curr 0x%lx, list %sempty\n",\
                #c, atomic_long_read(&(sem)->count),            \
                (unsigned long) sem->magic,                     \
                atomic_long_read(&(sem)->owner), (long)current, \
                list_empty(&(sem)->wait_list) ? "" : "not "))   \
                        debug_locks_off();                      \
        } while (0)
#else
# define DEBUG_RWSEMS_WARN_ON(c, sem)
#endif

/*
 * On 64-bit architectures, the bit definitions of the count are:
 *
 * Bit  0    - writer locked bit
 * Bit  1    - waiters present bit
 * Bit  2    - lock handoff bit
 * Bits 3-7  - reserved
 * Bits 8-62 - 55-bit reader count
 * Bit  63   - read fail bit
 *
 * On 32-bit architectures, the bit definitions of the count are:
 *
 * Bit  0    - writer locked bit
 * Bit  1    - waiters present bit
 * Bit  2    - lock handoff bit
 * Bits 3-7  - reserved
 * Bits 8-30 - 23-bit reader count
 * Bit  31   - read fail bit
 *
 * It is not likely that the most significant bit (read fail bit) will ever
 * be set. This guard bit is still checked anyway in the down_read() fastpath
 * just in case we need to use up more of the reader bits for other purpose
 * in the future.
 *
 * atomic_long_fetch_add() is used to obtain reader lock, whereas
 * atomic_long_cmpxchg() will be used to obtain writer lock.
 *
 * There are three places where the lock handoff bit may be set or cleared.
 * 1) rwsem_mark_wake() for readers.
 * 2) rwsem_try_write_lock() for writers.
 * 3) Error path of rwsem_down_write_slowpath().
 *
 * For all the above cases, wait_lock will be held. A writer must also
 * be the first one in the wait_list to be eligible for setting the handoff
 * bit. So concurrent setting/clearing of handoff bit is not possible.
 */
#define RWSEM_WRITER_LOCKED     (1UL << 0)
#define RWSEM_FLAG_WAITERS      (1UL << 1)
#define RWSEM_FLAG_HANDOFF      (1UL << 2)
#define RWSEM_FLAG_READFAIL     (1UL << (BITS_PER_LONG - 1))

#define RWSEM_READER_SHIFT      8
#define RWSEM_READER_BIAS       (1UL << RWSEM_READER_SHIFT)
#define RWSEM_READER_MASK       (~(RWSEM_READER_BIAS - 1))
#define RWSEM_WRITER_MASK       RWSEM_WRITER_LOCKED
#define RWSEM_LOCK_MASK         (RWSEM_WRITER_MASK|RWSEM_READER_MASK)
#define RWSEM_READ_FAILED_MASK  (RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\
                                 RWSEM_FLAG_HANDOFF|RWSEM_FLAG_READFAIL)

/*
 * All writes to owner are protected by WRITE_ONCE() to make sure that
 * store tearing can't happen as optimistic spinners may read and use
 * the owner value concurrently without lock. Read from owner, however,
 * may not need READ_ONCE() as long as the pointer value is only used
 * for comparison and isn't being dereferenced.
 */
static inline void rwsem_set_owner(struct rw_semaphore *sem)
{
        atomic_long_set(&sem->owner, (long)current);
}

static inline void rwsem_clear_owner(struct rw_semaphore *sem)
{
        atomic_long_set(&sem->owner, 0);
}

/*
 * Test the flags in the owner field.
 */
static inline bool rwsem_test_oflags(struct rw_semaphore *sem, long flags)
{
        return atomic_long_read(&sem->owner) & flags;
}

/*
 * The task_struct pointer of the last owning reader will be left in
 * the owner field.
 *
 * Note that the owner value just indicates the task has owned the rwsem
 * previously, it may not be the real owner or one of the real owners
 * anymore when that field is examined, so take it with a grain of salt.
 *
 * The reader non-spinnable bit is preserved.
 */
static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
                                            struct task_struct *owner)
{
        unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED |
                (atomic_long_read(&sem->owner) & RWSEM_RD_NONSPINNABLE);

        atomic_long_set(&sem->owner, val);
}

static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
{
        __rwsem_set_reader_owned(sem, current);
}

/*
 * Return true if the rwsem is owned by a reader.
 */
static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem)
{
#ifdef CONFIG_DEBUG_RWSEMS
        /*
         * Check the count to see if it is write-locked.
         */
        long count = atomic_long_read(&sem->count);

        if (count & RWSEM_WRITER_MASK)
                return false;
#endif
        return rwsem_test_oflags(sem, RWSEM_READER_OWNED);
}

#ifdef CONFIG_DEBUG_RWSEMS
/*
 * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
 * is a task pointer in owner of a reader-owned rwsem, it will be the
 * real owner or one of the real owners. The only exception is when the
 * unlock is done by up_read_non_owner().
 */
static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
{
        unsigned long val = atomic_long_read(&sem->owner);

        while ((val & ~RWSEM_OWNER_FLAGS_MASK) == (unsigned long)current) {
                if (atomic_long_try_cmpxchg(&sem->owner, &val,
                                            val & RWSEM_OWNER_FLAGS_MASK))
                        return;
        }
}
#else
static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
{
}
#endif

/*
 * Set the RWSEM_NONSPINNABLE bits if the RWSEM_READER_OWNED flag
 * remains set. Otherwise, the operation will be aborted.
 */
static inline void rwsem_set_nonspinnable(struct rw_semaphore *sem)
{
        unsigned long owner = atomic_long_read(&sem->owner);

        do {
                if (!(owner & RWSEM_READER_OWNED))
                        break;
                if (owner & RWSEM_NONSPINNABLE)
                        break;
        } while (!atomic_long_try_cmpxchg(&sem->owner, &owner,
                                          owner | RWSEM_NONSPINNABLE));
}

static inline bool rwsem_read_trylock(struct rw_semaphore *sem)
{
        long cnt = atomic_long_add_return_acquire(RWSEM_READER_BIAS, &sem->count);
        if (WARN_ON_ONCE(cnt < 0))
                rwsem_set_nonspinnable(sem);
        return !(cnt & RWSEM_READ_FAILED_MASK);
}

/*
 * Return just the real task structure pointer of the owner
 */
static inline struct task_struct *rwsem_owner(struct rw_semaphore *sem)
{
        return (struct task_struct *)
                (atomic_long_read(&sem->owner) & ~RWSEM_OWNER_FLAGS_MASK);
}

/*
 * Return the real task structure pointer of the owner and the embedded
 * flags in the owner. pflags must be non-NULL.
 */
static inline struct task_struct *
rwsem_owner_flags(struct rw_semaphore *sem, unsigned long *pflags)
{
        unsigned long owner = atomic_long_read(&sem->owner);

        *pflags = owner & RWSEM_OWNER_FLAGS_MASK;
        return (struct task_struct *)(owner & ~RWSEM_OWNER_FLAGS_MASK);
}

/*
 * Guide to the rw_semaphore's count field.
 *
 * When the RWSEM_WRITER_LOCKED bit in count is set, the lock is owned
 * by a writer.
 *
 * The lock is owned by readers when
 * (1) the RWSEM_WRITER_LOCKED isn't set in count,
 * (2) some of the reader bits are set in count, and
 * (3) the owner field has RWSEM_READ_OWNED bit set.
 *
 * Having some reader bits set is not enough to guarantee a readers owned
 * lock as the readers may be in the process of backing out from the count
 * and a writer has just released the lock. So another writer may steal
 * the lock immediately after that.
 */

/*
 * Initialize an rwsem:
 */
void __init_rwsem(struct rw_semaphore *sem, const char *name,
                  struct lock_class_key *key)
{
#ifdef CONFIG_DEBUG_LOCK_ALLOC
        /*
         * Make sure we are not reinitializing a held semaphore:
         */
        debug_check_no_locks_freed((void *)sem, sizeof(*sem));
        lockdep_init_map(&sem->dep_map, name, key, 0);
#endif
#ifdef CONFIG_DEBUG_RWSEMS
        sem->magic = sem;
#endif
        atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
        raw_spin_lock_init(&sem->wait_lock);
        INIT_LIST_HEAD(&sem->wait_list);
        atomic_long_set(&sem->owner, 0L);
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
        osq_lock_init(&sem->osq);
#endif
}
EXPORT_SYMBOL(__init_rwsem);

enum rwsem_waiter_type {
        RWSEM_WAITING_FOR_WRITE,
        RWSEM_WAITING_FOR_READ
};

struct rwsem_waiter {
        struct list_head list;
        struct task_struct *task;
        enum rwsem_waiter_type type;
        unsigned long timeout;
        unsigned long last_rowner;
};
#define rwsem_first_waiter(sem) \
        list_first_entry(&sem->wait_list, struct rwsem_waiter, list)

enum rwsem_wake_type {
        RWSEM_WAKE_ANY,         /* Wake whatever's at head of wait list */
        RWSEM_WAKE_READERS,     /* Wake readers only */
        RWSEM_WAKE_READ_OWNED   /* Waker thread holds the read lock */
};

enum writer_wait_state {
        WRITER_NOT_FIRST,       /* Writer is not first in wait list */
        WRITER_FIRST,           /* Writer is first in wait list     */
        WRITER_HANDOFF          /* Writer is first & handoff needed */
};

/*
 * The typical HZ value is either 250 or 1000. So set the minimum waiting
 * time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait
 * queue before initiating the handoff protocol.
 */
#define RWSEM_WAIT_TIMEOUT      DIV_ROUND_UP(HZ, 250)

/*
 * Magic number to batch-wakeup waiting readers, even when writers are
 * also present in the queue. This both limits the amount of work the
 * waking thread must do and also prevents any potential counter overflow,
 * however unlikely.
 */
#define MAX_READERS_WAKEUP      0x100

/*
 * handle the lock release when processes blocked on it that can now run
 * - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
 *   have been set.
 * - there must be someone on the queue
 * - the wait_lock must be held by the caller
 * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
 *   to actually wakeup the blocked task(s) and drop the reference count,
 *   preferably when the wait_lock is released
 * - woken process blocks are discarded from the list after having task zeroed
 * - writers are only marked woken if downgrading is false
 */
static void rwsem_mark_wake(struct rw_semaphore *sem,
                            enum rwsem_wake_type wake_type,
                            struct wake_q_head *wake_q)
{
        struct rwsem_waiter *waiter, *tmp;
        long oldcount, woken = 0, adjustment = 0;
        struct list_head wlist;

        lockdep_assert_held(&sem->wait_lock);

        /*
         * Take a peek at the queue head waiter such that we can determine
         * the wakeup(s) to perform.
         */
        waiter = rwsem_first_waiter(sem);

        if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
                if (wake_type == RWSEM_WAKE_ANY) {
                        /*
                         * Mark writer at the front of the queue for wakeup.
                         * Until the task is actually later awoken later by
                         * the caller, other writers are able to steal it.
                         * Readers, on the other hand, will block as they
                         * will notice the queued writer.
                         */
                        wake_q_add(wake_q, waiter->task);
                        lockevent_inc(rwsem_wake_writer);
                }

                return;
        }

        /*
         * No reader wakeup if there are too many of them already.
         */
        if (unlikely(atomic_long_read(&sem->count) < 0))
                return;

        /*
         * Writers might steal the lock before we grant it to the next reader.
         * We prefer to do the first reader grant before counting readers
         * so we can bail out early if a writer stole the lock.
         */
        if (wake_type != RWSEM_WAKE_READ_OWNED) {
                struct task_struct *owner;

                adjustment = RWSEM_READER_BIAS;
                oldcount = atomic_long_fetch_add(adjustment, &sem->count);
                if (unlikely(oldcount & RWSEM_WRITER_MASK)) {
                        /*
                         * When we've been waiting "too" long (for writers
                         * to give up the lock), request a HANDOFF to
                         * force the issue.
                         */
                        if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
                            time_after(jiffies, waiter->timeout)) {
                                adjustment -= RWSEM_FLAG_HANDOFF;
                                lockevent_inc(rwsem_rlock_handoff);
                        }

                        atomic_long_add(-adjustment, &sem->count);
                        return;
                }
                /*
                 * Set it to reader-owned to give spinners an early
                 * indication that readers now have the lock.
                 * The reader nonspinnable bit seen at slowpath entry of
                 * the reader is copied over.
                 */
                owner = waiter->task;
                if (waiter->last_rowner & RWSEM_RD_NONSPINNABLE) {
                        owner = (void *)((unsigned long)owner | RWSEM_RD_NONSPINNABLE);
                        lockevent_inc(rwsem_opt_norspin);
                }
                __rwsem_set_reader_owned(sem, owner);
        }

        /*
         * Grant up to MAX_READERS_WAKEUP read locks to all the readers in the
         * queue. We know that the woken will be at least 1 as we accounted
         * for above. Note we increment the 'active part' of the count by the
         * number of readers before waking any processes up.
         *
         * This is an adaptation of the phase-fair R/W locks where at the
         * reader phase (first waiter is a reader), all readers are eligible
         * to acquire the lock at the same time irrespective of their order
         * in the queue. The writers acquire the lock according to their
         * order in the queue.
         *
         * We have to do wakeup in 2 passes to prevent the possibility that
         * the reader count may be decremented before it is incremented. It
         * is because the to-be-woken waiter may not have slept yet. So it
         * may see waiter->task got cleared, finish its critical section and
         * do an unlock before the reader count increment.
         *
         * 1) Collect the read-waiters in a separate list, count them and
         *    fully increment the reader count in rwsem.
         * 2) For each waiters in the new list, clear waiter->task and
         *    put them into wake_q to be woken up later.
         */
        INIT_LIST_HEAD(&wlist);
        list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
                if (waiter->type == RWSEM_WAITING_FOR_WRITE)
                        continue;

                woken++;
                list_move_tail(&waiter->list, &wlist);

                /*
                 * Limit # of readers that can be woken up per wakeup call.
                 */
                if (woken >= MAX_READERS_WAKEUP)
                        break;
        }

        adjustment = woken * RWSEM_READER_BIAS - adjustment;
        lockevent_cond_inc(rwsem_wake_reader, woken);
        if (list_empty(&sem->wait_list)) {
                /* hit end of list above */
                adjustment -= RWSEM_FLAG_WAITERS;
        }

        /*
         * When we've woken a reader, we no longer need to force writers
         * to give up the lock and we can clear HANDOFF.
         */
        if (woken && (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF))
                adjustment -= RWSEM_FLAG_HANDOFF;

        if (adjustment)
                atomic_long_add(adjustment, &sem->count);

        /* 2nd pass */
        list_for_each_entry_safe(waiter, tmp, &wlist, list) {
                struct task_struct *tsk;

                tsk = waiter->task;
                get_task_struct(tsk);

                /*
                 * Ensure calling get_task_struct() before setting the reader
                 * waiter to nil such that rwsem_down_read_slowpath() cannot
                 * race with do_exit() by always holding a reference count
                 * to the task to wakeup.
                 */
                smp_store_release(&waiter->task, NULL);
                /*
                 * Ensure issuing the wakeup (either by us or someone else)
                 * after setting the reader waiter to nil.
                 */
                wake_q_add_safe(wake_q, tsk);
        }
}

/*
 * This function must be called with the sem->wait_lock held to prevent
 * race conditions between checking the rwsem wait list and setting the
 * sem->count accordingly.
 *
 * If wstate is WRITER_HANDOFF, it will make sure that either the handoff
 * bit is set or the lock is acquired with handoff bit cleared.
 */
static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
                                        enum writer_wait_state wstate)
{
        long count, new;

        lockdep_assert_held(&sem->wait_lock);

        count = atomic_long_read(&sem->count);
        do {
                bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);

                if (has_handoff && wstate == WRITER_NOT_FIRST)
                        return false;

                new = count;

                if (count & RWSEM_LOCK_MASK) {
                        if (has_handoff || (wstate != WRITER_HANDOFF))
                                return false;

                        new |= RWSEM_FLAG_HANDOFF;
                } else {
                        new |= RWSEM_WRITER_LOCKED;
                        new &= ~RWSEM_FLAG_HANDOFF;

                        if (list_is_singular(&sem->wait_list))
                                new &= ~RWSEM_FLAG_WAITERS;
                }
        } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));

        /*
         * We have either acquired the lock with handoff bit cleared or
         * set the handoff bit.
         */
        if (new & RWSEM_FLAG_HANDOFF)
                return false;

        rwsem_set_owner(sem);
        return true;
}

#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
/*
 * Try to acquire read lock before the reader is put on wait queue.
 * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff
 * is ongoing.
 */
static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
{
        long count = atomic_long_read(&sem->count);

        if (count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))
                return false;

        count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
        if (!(count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
                rwsem_set_reader_owned(sem);
                lockevent_inc(rwsem_opt_rlock);
                return true;
        }

        /* Back out the change */
        atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
        return false;
}

/*
 * Try to acquire write lock before the writer has been put on wait queue.
 */
static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
{
        long count = atomic_long_read(&sem->count);

        while (!(count & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))) {
                if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
                                        count | RWSEM_WRITER_LOCKED)) {
                        rwsem_set_owner(sem);
                        lockevent_inc(rwsem_opt_wlock);
                        return true;
                }
        }
        return false;
}

static inline bool owner_on_cpu(struct task_struct *owner)
{
        /*
         * As lock holder preemption issue, we both skip spinning if
         * task is not on cpu or its cpu is preempted
         */
        return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
}

static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
                                           unsigned long nonspinnable)
{
        struct task_struct *owner;
        unsigned long flags;
        bool ret = true;

        BUILD_BUG_ON(!(RWSEM_OWNER_UNKNOWN & RWSEM_NONSPINNABLE));

        if (need_resched()) {
                lockevent_inc(rwsem_opt_fail);
                return false;
        }

        preempt_disable();
        rcu_read_lock();
        owner = rwsem_owner_flags(sem, &flags);
        /*
         * Don't check the read-owner as the entry may be stale.
         */
        if ((flags & nonspinnable) ||
            (owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner)))
                ret = false;
        rcu_read_unlock();
        preempt_enable();

        lockevent_cond_inc(rwsem_opt_fail, !ret);
        return ret;
}

/*
 * The rwsem_spin_on_owner() function returns the folowing 4 values
 * depending on the lock owner state.
 *   OWNER_NULL  : owner is currently NULL
 *   OWNER_WRITER: when owner changes and is a writer
 *   OWNER_READER: when owner changes and the new owner may be a reader.
 *   OWNER_NONSPINNABLE:
 *                 when optimistic spinning has to stop because either the
 *                 owner stops running, is unknown, or its timeslice has
 *                 been used up.
 */
enum owner_state {
        OWNER_NULL              = 1 << 0,
        OWNER_WRITER            = 1 << 1,
        OWNER_READER            = 1 << 2,
        OWNER_NONSPINNABLE      = 1 << 3,
};
#define OWNER_SPINNABLE         (OWNER_NULL | OWNER_WRITER | OWNER_READER)

static inline enum owner_state
rwsem_owner_state(struct task_struct *owner, unsigned long flags, unsigned long nonspinnable)
{
        if (flags & nonspinnable)
                return OWNER_NONSPINNABLE;

        if (flags & RWSEM_READER_OWNED)
                return OWNER_READER;

        return owner ? OWNER_WRITER : OWNER_NULL;
}

static noinline enum owner_state
rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
{
        struct task_struct *new, *owner;
        unsigned long flags, new_flags;
        enum owner_state state;

        owner = rwsem_owner_flags(sem, &flags);
        state = rwsem_owner_state(owner, flags, nonspinnable);
        if (state != OWNER_WRITER)
                return state;

        rcu_read_lock();
        for (;;) {
                /*
                 * When a waiting writer set the handoff flag, it may spin
                 * on the owner as well. Once that writer acquires the lock,
                 * we can spin on it. So we don't need to quit even when the
                 * handoff bit is set.
                 */
                new = rwsem_owner_flags(sem, &new_flags);
                if ((new != owner) || (new_flags != flags)) {
                        state = rwsem_owner_state(new, new_flags, nonspinnable);
                        break;
                }

                /*
                 * Ensure we emit the owner->on_cpu, dereference _after_
                 * checking sem->owner still matches owner, if that fails,
                 * owner might point to free()d memory, if it still matches,
                 * the rcu_read_lock() ensures the memory stays valid.
                 */
                barrier();

                if (need_resched() || !owner_on_cpu(owner)) {
                        state = OWNER_NONSPINNABLE;
                        break;
                }

                cpu_relax();
        }
        rcu_read_unlock();

        return state;
}

/*
 * Calculate reader-owned rwsem spinning threshold for writer
 *
 * The more readers own the rwsem, the longer it will take for them to
 * wind down and free the rwsem. So the empirical formula used to
 * determine the actual spinning time limit here is:
 *
 *   Spinning threshold = (10 + nr_readers/2)us
 *
 * The limit is capped to a maximum of 25us (30 readers). This is just
 * a heuristic and is subjected to change in the future.
 */
static inline u64 rwsem_rspin_threshold(struct rw_semaphore *sem)
{
        long count = atomic_long_read(&sem->count);
        int readers = count >> RWSEM_READER_SHIFT;
        u64 delta;

        if (readers > 30)
                readers = 30;
        delta = (20 + readers) * NSEC_PER_USEC / 2;

        return sched_clock() + delta;
}

static bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
{
        bool taken = false;
        int prev_owner_state = OWNER_NULL;
        int loop = 0;
        u64 rspin_threshold = 0;
        unsigned long nonspinnable = wlock ? RWSEM_WR_NONSPINNABLE
                                           : RWSEM_RD_NONSPINNABLE;

        preempt_disable();

        /* sem->wait_lock should not be held when doing optimistic spinning */
        if (!osq_lock(&sem->osq))
                goto done;

        /*
         * Optimistically spin on the owner field and attempt to acquire the
         * lock whenever the owner changes. Spinning will be stopped when:
         *  1) the owning writer isn't running; or
         *  2) readers own the lock and spinning time has exceeded limit.
         */
        for (;;) {
                enum owner_state owner_state;

                owner_state = rwsem_spin_on_owner(sem, nonspinnable);
                if (!(owner_state & OWNER_SPINNABLE))
                        break;

                /*
                 * Try to acquire the lock
                 */
                taken = wlock ? rwsem_try_write_lock_unqueued(sem)
                              : rwsem_try_read_lock_unqueued(sem);

                if (taken)
                        break;

                /*
                 * Time-based reader-owned rwsem optimistic spinning
                 */
                if (wlock && (owner_state == OWNER_READER)) {
                        /*
                         * Re-initialize rspin_threshold every time when
                         * the owner state changes from non-reader to reader.
                         * This allows a writer to steal the lock in between
                         * 2 reader phases and have the threshold reset at
                         * the beginning of the 2nd reader phase.
                         */
                        if (prev_owner_state != OWNER_READER) {
                                if (rwsem_test_oflags(sem, nonspinnable))
                                        break;
                                rspin_threshold = rwsem_rspin_threshold(sem);
                                loop = 0;
                        }

                        /*
                         * Check time threshold once every 16 iterations to
                         * avoid calling sched_clock() too frequently so
                         * as to reduce the average latency between the times
                         * when the lock becomes free and when the spinner
                         * is ready to do a trylock.
                         */
                        else if (!(++loop & 0xf) && (sched_clock() > rspin_threshold)) {
                                rwsem_set_nonspinnable(sem);
                                lockevent_inc(rwsem_opt_nospin);
                                break;
                        }
                }

                /*
                 * An RT task cannot do optimistic spinning if it cannot
                 * be sure the lock holder is running or live-lock may
                 * happen if the current task and the lock holder happen
                 * to run in the same CPU. However, aborting optimistic
                 * spinning while a NULL owner is detected may miss some
                 * opportunity where spinning can continue without causing
                 * problem.
                 *
                 * There are 2 possible cases where an RT task may be able
                 * to continue spinning.
                 *
                 * 1) The lock owner is in the process of releasing the
                 *    lock, sem->owner is cleared but the lock has not
                 *    been released yet.
                 * 2) The lock was free and owner cleared, but another
                 *    task just comes in and acquire the lock before
                 *    we try to get it. The new owner may be a spinnable
                 *    writer.
                 *
                 * To take advantage of two scenarios listed agove, the RT
                 * task is made to retry one more time to see if it can
                 * acquire the lock or continue spinning on the new owning
                 * writer. Of course, if the time lag is long enough or the
                 * new owner is not a writer or spinnable, the RT task will
                 * quit spinning.
                 *
                 * If the owner is a writer, the need_resched() check is
                 * done inside rwsem_spin_on_owner(). If the owner is not
                 * a writer, need_resched() check needs to be done here.
                 */
                if (owner_state != OWNER_WRITER) {
                        if (need_resched())
                                break;
                        if (rt_task(current) &&
                           (prev_owner_state != OWNER_WRITER))
                                break;
                }
                prev_owner_state = owner_state;

                /*
                 * The cpu_relax() call is a compiler barrier which forces
                 * everything in this loop to be re-loaded. We don't need
                 * memory barriers as we'll eventually observe the right
                 * values at the cost of a few extra spins.
                 */
                cpu_relax();
        }
        osq_unlock(&sem->osq);
done:
        preempt_enable();
        lockevent_cond_inc(rwsem_opt_fail, !taken);
        return taken;
}

/*
 * Clear the owner's RWSEM_WR_NONSPINNABLE bit if it is set. This should
 * only be called when the reader count reaches 0.
 *
 * This give writers better chance to acquire the rwsem first before
 * readers when the rwsem was being held by readers for a relatively long
 * period of time. Race can happen that an optimistic spinner may have
 * just stolen the rwsem and set the owner, but just clearing the
 * RWSEM_WR_NONSPINNABLE bit will do no harm anyway.
 */
static inline void clear_wr_nonspinnable(struct rw_semaphore *sem)
{
        if (rwsem_test_oflags(sem, RWSEM_WR_NONSPINNABLE))
                atomic_long_andnot(RWSEM_WR_NONSPINNABLE, &sem->owner);
}

/*
 * This function is called when the reader fails to acquire the lock via
 * optimistic spinning. In this case we will still attempt to do a trylock
 * when comparing the rwsem state right now with the state when entering
 * the slowpath indicates that the reader is still in a valid reader phase.
 * This happens when the following conditions are true:
 *
 * 1) The lock is currently reader owned, and
 * 2) The lock is previously not reader-owned or the last read owner changes.
 *
 * In the former case, we have transitioned from a writer phase to a
 * reader-phase while spinning. In the latter case, it means the reader
 * phase hasn't ended when we entered the optimistic spinning loop. In
 * both cases, the reader is eligible to acquire the lock. This is the
 * secondary path where a read lock is acquired optimistically.
 *
 * The reader non-spinnable bit wasn't set at time of entry or it will
 * not be here at all.
 */
static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
                                              unsigned long last_rowner)
{
        unsigned long owner = atomic_long_read(&sem->owner);

        if (!(owner & RWSEM_READER_OWNED))
                return false;

        if (((owner ^ last_rowner) & ~RWSEM_OWNER_FLAGS_MASK) &&
            rwsem_try_read_lock_unqueued(sem)) {
                lockevent_inc(rwsem_opt_rlock2);
                lockevent_add(rwsem_opt_fail, -1);
                return true;
        }
        return false;
}
#else
static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
                                           unsigned long nonspinnable)
{
        return false;
}

static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
{
        return false;
}

static inline void clear_wr_nonspinnable(struct rw_semaphore *sem) { }

static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
                                              unsigned long last_rowner)
{
        return false;
}

static inline int
rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
{
        return 0;
}
#define OWNER_NULL      1
#endif

/*
 * Wait for the read lock to be granted
 */
static struct rw_semaphore __sched *
rwsem_down_read_slowpath(struct rw_semaphore *sem, int state)
{
        long count, adjustment = -RWSEM_READER_BIAS;
        struct rwsem_waiter waiter;
        DEFINE_WAKE_Q(wake_q);
        bool wake = false;

        /*
         * Save the current read-owner of rwsem, if available, and the
         * reader nonspinnable bit.
         */
        waiter.last_rowner = atomic_long_read(&sem->owner);
        if (!(waiter.last_rowner & RWSEM_READER_OWNED))
                waiter.last_rowner &= RWSEM_RD_NONSPINNABLE;

        if (!rwsem_can_spin_on_owner(sem, RWSEM_RD_NONSPINNABLE))
                goto queue;

        /*
         * Undo read bias from down_read() and do optimistic spinning.
         */
        atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
        adjustment = 0;
        if (rwsem_optimistic_spin(sem, false)) {
                /* rwsem_optimistic_spin() implies ACQUIRE on success */
                /*
                 * Wake up other readers in the wait list if the front
                 * waiter is a reader.
                 */
                if ((atomic_long_read(&sem->count) & RWSEM_FLAG_WAITERS)) {
                        raw_spin_lock_irq(&sem->wait_lock);
                        if (!list_empty(&sem->wait_list))
                                rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
                                                &wake_q);
                        raw_spin_unlock_irq(&sem->wait_lock);
                        wake_up_q(&wake_q);
                }
                return sem;
        } else if (rwsem_reader_phase_trylock(sem, waiter.last_rowner)) {
                /* rwsem_reader_phase_trylock() implies ACQUIRE on success */
                return sem;
        }

queue:
        waiter.task = current;
        waiter.type = RWSEM_WAITING_FOR_READ;
        waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;

        raw_spin_lock_irq(&sem->wait_lock);
        if (list_empty(&sem->wait_list)) {
                /*
                 * In case the wait queue is empty and the lock isn't owned
                 * by a writer or has the handoff bit set, this reader can
                 * exit the slowpath and return immediately as its
                 * RWSEM_READER_BIAS has already been set in the count.
                 */
                if (adjustment && !(atomic_long_read(&sem->count) &
                     (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
                        /* Provide lock ACQUIRE */
                        smp_acquire__after_ctrl_dep();
                        raw_spin_unlock_irq(&sem->wait_lock);
                        rwsem_set_reader_owned(sem);
                        lockevent_inc(rwsem_rlock_fast);
                        return sem;
                }
                adjustment += RWSEM_FLAG_WAITERS;
        }
        list_add_tail(&waiter.list, &sem->wait_list);

        /* we're now waiting on the lock, but no longer actively locking */
        if (adjustment)
                count = atomic_long_add_return(adjustment, &sem->count);
        else
                count = atomic_long_read(&sem->count);

        /*
         * If there are no active locks, wake the front queued process(es).
         *
         * If there are no writers and we are first in the queue,
         * wake our own waiter to join the existing active readers !
         */
        if (!(count & RWSEM_LOCK_MASK)) {
                clear_wr_nonspinnable(sem);
                wake = true;
        }
        if (wake || (!(count & RWSEM_WRITER_MASK) &&
                    (adjustment & RWSEM_FLAG_WAITERS)))
                rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);

        raw_spin_unlock_irq(&sem->wait_lock);
        wake_up_q(&wake_q);

        /* wait to be given the lock */
        for (;;) {
                set_current_state(state);
                if (!smp_load_acquire(&waiter.task)) {
                        /* Matches rwsem_mark_wake()'s smp_store_release(). */
                        break;
                }
                if (signal_pending_state(state, current)) {
                        raw_spin_lock_irq(&sem->wait_lock);
                        if (waiter.task)
                                goto out_nolock;
                        raw_spin_unlock_irq(&sem->wait_lock);
                        /* Ordered by sem->wait_lock against rwsem_mark_wake(). */
                        break;
                }
                schedule();
                lockevent_inc(rwsem_sleep_reader);
        }

        __set_current_state(TASK_RUNNING);
        lockevent_inc(rwsem_rlock);
        return sem;

out_nolock:
        list_del(&waiter.list);
        if (list_empty(&sem->wait_list)) {
                atomic_long_andnot(RWSEM_FLAG_WAITERS|RWSEM_FLAG_HANDOFF,
                                   &sem->count);
        }
        raw_spin_unlock_irq(&sem->wait_lock);
        __set_current_state(TASK_RUNNING);
        lockevent_inc(rwsem_rlock_fail);
        return ERR_PTR(-EINTR);
}

/*
 * This function is called by the a write lock owner. So the owner value
 * won't get changed by others.
 */
static inline void rwsem_disable_reader_optspin(struct rw_semaphore *sem,
                                                bool disable)
{
        if (unlikely(disable)) {
                atomic_long_or(RWSEM_RD_NONSPINNABLE, &sem->owner);
                lockevent_inc(rwsem_opt_norspin);
        }
}

/*
 * Wait until we successfully acquire the write lock
 */
static struct rw_semaphore *
rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
{
        long count;
        bool disable_rspin;
        enum writer_wait_state wstate;
        struct rwsem_waiter waiter;
        struct rw_semaphore *ret = sem;
        DEFINE_WAKE_Q(wake_q);

        /* do optimistic spinning and steal lock if possible */
        if (rwsem_can_spin_on_owner(sem, RWSEM_WR_NONSPINNABLE) &&
            rwsem_optimistic_spin(sem, true)) {
                /* rwsem_optimistic_spin() implies ACQUIRE on success */
                return sem;
        }

        /*
         * Disable reader optimistic spinning for this rwsem after
         * acquiring the write lock when the setting of the nonspinnable
         * bits are observed.
         */
        disable_rspin = atomic_long_read(&sem->owner) & RWSEM_NONSPINNABLE;

        /*
         * Optimistic spinning failed, proceed to the slowpath
         * and block until we can acquire the sem.
         */
        waiter.task = current;
        waiter.type = RWSEM_WAITING_FOR_WRITE;
        waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;

        raw_spin_lock_irq(&sem->wait_lock);

        /* account for this before adding a new element to the list */
        wstate = list_empty(&sem->wait_list) ? WRITER_FIRST : WRITER_NOT_FIRST;

        list_add_tail(&waiter.list, &sem->wait_list);

        /* we're now waiting on the lock */
        if (wstate == WRITER_NOT_FIRST) {
                count = atomic_long_read(&sem->count);

                /*
                 * If there were already threads queued before us and:
                 *  1) there are no no active locks, wake the front
                 *     queued process(es) as the handoff bit might be set.
                 *  2) there are no active writers and some readers, the lock
                 *     must be read owned; so we try to wake any read lock
                 *     waiters that were queued ahead of us.
                 */
                if (count & RWSEM_WRITER_MASK)
                        goto wait;

                rwsem_mark_wake(sem, (count & RWSEM_READER_MASK)
                                        ? RWSEM_WAKE_READERS
                                        : RWSEM_WAKE_ANY, &wake_q);

                if (!wake_q_empty(&wake_q)) {
                        /*
                         * We want to minimize wait_lock hold time especially
                         * when a large number of readers are to be woken up.
                         */
                        raw_spin_unlock_irq(&sem->wait_lock);
                        wake_up_q(&wake_q);
                        wake_q_init(&wake_q);   /* Used again, reinit */
                        raw_spin_lock_irq(&sem->wait_lock);
                }
        } else {
                atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);
        }

wait:
        /* wait until we successfully acquire the lock */
        set_current_state(state);
        for (;;) {
                if (rwsem_try_write_lock(sem, wstate)) {
                        /* rwsem_try_write_lock() implies ACQUIRE on success */
                        break;
                }

                raw_spin_unlock_irq(&sem->wait_lock);

                /*
                 * After setting the handoff bit and failing to acquire
                 * the lock, attempt to spin on owner to accelerate lock
                 * transfer. If the previous owner is a on-cpu writer and it
                 * has just released the lock, OWNER_NULL will be returned.
                 * In this case, we attempt to acquire the lock again
                 * without sleeping.
                 */
                if ((wstate == WRITER_HANDOFF) &&
                    (rwsem_spin_on_owner(sem, 0) == OWNER_NULL))
                        goto trylock_again;

                /* Block until there are no active lockers. */
                for (;;) {
                        if (signal_pending_state(state, current))
                                goto out_nolock;

                        schedule();
                        lockevent_inc(rwsem_sleep_writer);
                        set_current_state(state);
                        /*
                         * If HANDOFF bit is set, unconditionally do
                         * a trylock.
                         */
                        if (wstate == WRITER_HANDOFF)
                                break;

                        if ((wstate == WRITER_NOT_FIRST) &&
                            (rwsem_first_waiter(sem) == &waiter))
                                wstate = WRITER_FIRST;

                        count = atomic_long_read(&sem->count);
                        if (!(count & RWSEM_LOCK_MASK))
                                break;

                        /*
                         * The setting of the handoff bit is deferred
                         * until rwsem_try_write_lock() is called.
                         */
                        if ((wstate == WRITER_FIRST) && (rt_task(current) ||
                            time_after(jiffies, waiter.timeout))) {
                                wstate = WRITER_HANDOFF;
                                lockevent_inc(rwsem_wlock_handoff);
                                break;
                        }
                }
trylock_again:
                raw_spin_lock_irq(&sem->wait_lock);
        }
        __set_current_state(TASK_RUNNING);
        list_del(&waiter.list);
        rwsem_disable_reader_optspin(sem, disable_rspin);
        raw_spin_unlock_irq(&sem->wait_lock);
        lockevent_inc(rwsem_wlock);

        return ret;

out_nolock:
        __set_current_state(TASK_RUNNING);
        raw_spin_lock_irq(&sem->wait_lock);
        list_del(&waiter.list);

        if (unlikely(wstate == WRITER_HANDOFF))
                atomic_long_add(-RWSEM_FLAG_HANDOFF,  &sem->count);

        if (list_empty(&sem->wait_list))
                atomic_long_andnot(RWSEM_FLAG_WAITERS, &sem->count);
        else
                rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
        raw_spin_unlock_irq(&sem->wait_lock);
        wake_up_q(&wake_q);
        lockevent_inc(rwsem_wlock_fail);

        return ERR_PTR(-EINTR);
}

/*
 * handle waking up a waiter on the semaphore
 * - up_read/up_write has decremented the active part of count if we come here
 */
static struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem, long count)
{
        unsigned long flags;
        DEFINE_WAKE_Q(wake_q);

        raw_spin_lock_irqsave(&sem->wait_lock, flags);

        if (!list_empty(&sem->wait_list))
                rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);

        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
        wake_up_q(&wake_q);

        return sem;
}

/*
 * downgrade a write lock into a read lock
 * - caller incremented waiting part of count and discovered it still negative
 * - just wake up any readers at the front of the queue
 */
static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
{
        unsigned long flags;
        DEFINE_WAKE_Q(wake_q);

        raw_spin_lock_irqsave(&sem->wait_lock, flags);

        if (!list_empty(&sem->wait_list))
                rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);

        raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
        wake_up_q(&wake_q);

        return sem;
}

/*
 * lock for reading
 */
inline void __down_read(struct rw_semaphore *sem)
{
        if (!rwsem_read_trylock(sem)) {
                rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE);
                DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
        } else {
                rwsem_set_reader_owned(sem);
        }
}

static inline int __down_read_killable(struct rw_semaphore *sem)
{
        if (!rwsem_read_trylock(sem)) {
                if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE)))
                        return -EINTR;
                DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
        } else {
                rwsem_set_reader_owned(sem);
        }
        return 0;
}

static inline int __down_read_trylock(struct rw_semaphore *sem)
{
        long tmp;

        DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);

        /*
         * Optimize for the case when the rwsem is not locked at all.
         */
        tmp = RWSEM_UNLOCKED_VALUE;
        do {
                if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
                                        tmp + RWSEM_READER_BIAS)) {
                        rwsem_set_reader_owned(sem);
                        return 1;
                }
        } while (!(tmp & RWSEM_READ_FAILED_MASK));
        return 0;
}

/*
 * lock for writing
 */
static inline void __down_write(struct rw_semaphore *sem)
{
        long tmp = RWSEM_UNLOCKED_VALUE;

        if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
                                                      RWSEM_WRITER_LOCKED)))
                rwsem_down_write_slowpath(sem, TASK_UNINTERRUPTIBLE);
        else
                rwsem_set_owner(sem);
}

static inline int __down_write_killable(struct rw_semaphore *sem)
{
        long tmp = RWSEM_UNLOCKED_VALUE;

        if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
                                                      RWSEM_WRITER_LOCKED))) {
                if (IS_ERR(rwsem_down_write_slowpath(sem, TASK_KILLABLE)))
                        return -EINTR;
        } else {
                rwsem_set_owner(sem);
        }
        return 0;
}

static inline int __down_write_trylock(struct rw_semaphore *sem)
{
        long tmp;

        DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);

        tmp  = RWSEM_UNLOCKED_VALUE;
        if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
                                            RWSEM_WRITER_LOCKED)) {
                rwsem_set_owner(sem);
                return true;
        }
        return false;
}

/*
 * unlock after reading
 */
inline void __up_read(struct rw_semaphore *sem)
{
        long tmp;

        DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
        DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);

        rwsem_clear_reader_owned(sem);
        tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
        DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);
        if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
                      RWSEM_FLAG_WAITERS)) {
                clear_wr_nonspinnable(sem);
                rwsem_wake(sem, tmp);
        }
}

/*
 * unlock after writing
 */
static inline void __up_write(struct rw_semaphore *sem)
{
        long tmp;

        DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
        /*
         * sem->owner may differ from current if the ownership is transferred
         * to an anonymous writer by setting the RWSEM_NONSPINNABLE bits.
         */
        DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) &&
                            !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE), sem);

        rwsem_clear_owner(sem);
        tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
        if (unlikely(tmp & RWSEM_FLAG_WAITERS))
                rwsem_wake(sem, tmp);
}

/*
 * downgrade write lock to read lock
 */
static inline void __downgrade_write(struct rw_semaphore *sem)
{
        long tmp;

        /*
         * When downgrading from exclusive to shared ownership,
         * anything inside the write-locked region cannot leak
         * into the read side. In contrast, anything in the
         * read-locked region is ok to be re-ordered into the
         * write side. As such, rely on RELEASE semantics.
         */
        DEBUG_RWSEMS_WARN_ON(rwsem_owner(sem) != current, sem);
        tmp = atomic_long_fetch_add_release(
                -RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
        rwsem_set_reader_owned(sem);
        if (tmp & RWSEM_FLAG_WAITERS)
                rwsem_downgrade_wake(sem);
}

/*
 * lock for reading
 */
void __sched down_read(struct rw_semaphore *sem)
{
        might_sleep();
        rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);

        LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
}
EXPORT_SYMBOL(down_read);

int __sched down_read_killable(struct rw_semaphore *sem)
{
        might_sleep();
        rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);

        if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
                rwsem_release(&sem->dep_map, 1, _RET_IP_);
                return -EINTR;
        }

        return 0;
}
EXPORT_SYMBOL(down_read_killable);

/*
 * trylock for reading -- returns 1 if successful, 0 if contention
 */
int down_read_trylock(struct rw_semaphore *sem)
{
        int ret = __down_read_trylock(sem);

        if (ret == 1)
                rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
        return ret;
}
EXPORT_SYMBOL(down_read_trylock);

/*
 * lock for writing
 */
void __sched down_write(struct rw_semaphore *sem)
{
        might_sleep();
        rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
        LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
}
EXPORT_SYMBOL(down_write);

/*
 * lock for writing
 */
int __sched down_write_killable(struct rw_semaphore *sem)
{
        might_sleep();
        rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);

        if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
                                  __down_write_killable)) {
                rwsem_release(&sem->dep_map, 1, _RET_IP_);
                return -EINTR;
        }

        return 0;
}
EXPORT_SYMBOL(down_write_killable);

/*
 * trylock for writing -- returns 1 if successful, 0 if contention
 */
int down_write_trylock(struct rw_semaphore *sem)
{
        int ret = __down_write_trylock(sem);

        if (ret == 1)
                rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);

        return ret;
}
EXPORT_SYMBOL(down_write_trylock);

/*
 * release a read lock
 */
void up_read(struct rw_semaphore *sem)
{
        rwsem_release(&sem->dep_map, 1, _RET_IP_);
        __up_read(sem);
}
EXPORT_SYMBOL(up_read);

/*
 * release a write lock
 */
void up_write(struct rw_semaphore *sem)
{
        rwsem_release(&sem->dep_map, 1, _RET_IP_);
        __up_write(sem);
}
EXPORT_SYMBOL(up_write);

/*
 * downgrade write lock to read lock
 */
void downgrade_write(struct rw_semaphore *sem)
{
        lock_downgrade(&sem->dep_map, _RET_IP_);
        __downgrade_write(sem);
}
EXPORT_SYMBOL(downgrade_write);

#ifdef CONFIG_DEBUG_LOCK_ALLOC

void down_read_nested(struct rw_semaphore *sem, int subclass)
{
        might_sleep();
        rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
        LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
}
EXPORT_SYMBOL(down_read_nested);

void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
{
        might_sleep();
        rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);
        LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
}
EXPORT_SYMBOL(_down_write_nest_lock);

void down_read_non_owner(struct rw_semaphore *sem)
{
        might_sleep();
        __down_read(sem);
        __rwsem_set_reader_owned(sem, NULL);
}
EXPORT_SYMBOL(down_read_non_owner);

void down_write_nested(struct rw_semaphore *sem, int subclass)
{
        might_sleep();
        rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
        LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
}
EXPORT_SYMBOL(down_write_nested);

int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass)
{
        might_sleep();
        rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);

        if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
                                  __down_write_killable)) {
                rwsem_release(&sem->dep_map, 1, _RET_IP_);
                return -EINTR;
        }

        return 0;
}
EXPORT_SYMBOL(down_write_killable_nested);

void up_read_non_owner(struct rw_semaphore *sem)
{
        DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
        __up_read(sem);
}
EXPORT_SYMBOL(up_read_non_owner);

#endif