Merge tag 'drm-intel-next-2023-03-23' of git://anongit.freedesktop.org/drm/drm-intel...

[linux-block.git] / kernel / printk / printk.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 *  linux/kernel/printk.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *
 * Modified to make sys_syslog() more flexible: added commands to
 * return the last 4k of kernel messages, regardless of whether
 * they've been read or not.  Added option to suppress kernel printk's
 * to the console.  Added hook for sending the console messages
 * elsewhere, in preparation for a serial line console (someday).
 * Ted Ts'o, 2/11/93.
 * Modified for sysctl support, 1/8/97, Chris Horn.
 * Fixed SMP synchronization, 08/08/99, Manfred Spraul
 *     manfred@colorfullife.com
 * Rewrote bits to get rid of console_lock
 *      01Mar01 Andrew Morton
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/kernel.h>
#include <linux/mm.h>
#include <linux/tty.h>
#include <linux/tty_driver.h>
#include <linux/console.h>
#include <linux/init.h>
#include <linux/jiffies.h>
#include <linux/nmi.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/delay.h>
#include <linux/smp.h>
#include <linux/security.h>
#include <linux/memblock.h>
#include <linux/syscalls.h>
#include <linux/crash_core.h>
#include <linux/ratelimit.h>
#include <linux/kmsg_dump.h>
#include <linux/syslog.h>
#include <linux/cpu.h>
#include <linux/rculist.h>
#include <linux/poll.h>
#include <linux/irq_work.h>
#include <linux/ctype.h>
#include <linux/uio.h>
#include <linux/sched/clock.h>
#include <linux/sched/debug.h>
#include <linux/sched/task_stack.h>

#include <linux/uaccess.h>
#include <asm/sections.h>

#include <trace/events/initcall.h>
#define CREATE_TRACE_POINTS
#include <trace/events/printk.h>

#include "printk_ringbuffer.h"
#include "console_cmdline.h"
#include "braille.h"
#include "internal.h"

int console_printk[4] = {
        CONSOLE_LOGLEVEL_DEFAULT,       /* console_loglevel */
        MESSAGE_LOGLEVEL_DEFAULT,       /* default_message_loglevel */
        CONSOLE_LOGLEVEL_MIN,           /* minimum_console_loglevel */
        CONSOLE_LOGLEVEL_DEFAULT,       /* default_console_loglevel */
};
EXPORT_SYMBOL_GPL(console_printk);

atomic_t ignore_console_lock_warning __read_mostly = ATOMIC_INIT(0);
EXPORT_SYMBOL(ignore_console_lock_warning);

/*
 * Low level drivers may need that to know if they can schedule in
 * their unblank() callback or not. So let's export it.
 */
int oops_in_progress;
EXPORT_SYMBOL(oops_in_progress);

/*
 * console_mutex protects console_list updates and console->flags updates.
 * The flags are synchronized only for consoles that are registered, i.e.
 * accessible via the console list.
 */
static DEFINE_MUTEX(console_mutex);

/*
 * console_sem protects updates to console->seq and console_suspended,
 * and also provides serialization for console printing.
 */
static DEFINE_SEMAPHORE(console_sem);
HLIST_HEAD(console_list);
EXPORT_SYMBOL_GPL(console_list);
DEFINE_STATIC_SRCU(console_srcu);

/*
 * System may need to suppress printk message under certain
 * circumstances, like after kernel panic happens.
 */
int __read_mostly suppress_printk;

/*
 * During panic, heavy printk by other CPUs can delay the
 * panic and risk deadlock on console resources.
 */
static int __read_mostly suppress_panic_printk;

#ifdef CONFIG_LOCKDEP
static struct lockdep_map console_lock_dep_map = {
        .name = "console_lock"
};

void lockdep_assert_console_list_lock_held(void)
{
        lockdep_assert_held(&console_mutex);
}
EXPORT_SYMBOL(lockdep_assert_console_list_lock_held);
#endif

#ifdef CONFIG_DEBUG_LOCK_ALLOC
bool console_srcu_read_lock_is_held(void)
{
        return srcu_read_lock_held(&console_srcu);
}
EXPORT_SYMBOL(console_srcu_read_lock_is_held);
#endif

enum devkmsg_log_bits {
        __DEVKMSG_LOG_BIT_ON = 0,
        __DEVKMSG_LOG_BIT_OFF,
        __DEVKMSG_LOG_BIT_LOCK,
};

enum devkmsg_log_masks {
        DEVKMSG_LOG_MASK_ON             = BIT(__DEVKMSG_LOG_BIT_ON),
        DEVKMSG_LOG_MASK_OFF            = BIT(__DEVKMSG_LOG_BIT_OFF),
        DEVKMSG_LOG_MASK_LOCK           = BIT(__DEVKMSG_LOG_BIT_LOCK),
};

/* Keep both the 'on' and 'off' bits clear, i.e. ratelimit by default: */
#define DEVKMSG_LOG_MASK_DEFAULT        0

static unsigned int __read_mostly devkmsg_log = DEVKMSG_LOG_MASK_DEFAULT;

static int __control_devkmsg(char *str)
{
        size_t len;

        if (!str)
                return -EINVAL;

        len = str_has_prefix(str, "on");
        if (len) {
                devkmsg_log = DEVKMSG_LOG_MASK_ON;
                return len;
        }

        len = str_has_prefix(str, "off");
        if (len) {
                devkmsg_log = DEVKMSG_LOG_MASK_OFF;
                return len;
        }

        len = str_has_prefix(str, "ratelimit");
        if (len) {
                devkmsg_log = DEVKMSG_LOG_MASK_DEFAULT;
                return len;
        }

        return -EINVAL;
}

static int __init control_devkmsg(char *str)
{
        if (__control_devkmsg(str) < 0) {
                pr_warn("printk.devkmsg: bad option string '%s'\n", str);
                return 1;
        }

        /*
         * Set sysctl string accordingly:
         */
        if (devkmsg_log == DEVKMSG_LOG_MASK_ON)
                strcpy(devkmsg_log_str, "on");
        else if (devkmsg_log == DEVKMSG_LOG_MASK_OFF)
                strcpy(devkmsg_log_str, "off");
        /* else "ratelimit" which is set by default. */

        /*
         * Sysctl cannot change it anymore. The kernel command line setting of
         * this parameter is to force the setting to be permanent throughout the
         * runtime of the system. This is a precation measure against userspace
         * trying to be a smarta** and attempting to change it up on us.
         */
        devkmsg_log |= DEVKMSG_LOG_MASK_LOCK;

        return 1;
}
__setup("printk.devkmsg=", control_devkmsg);

char devkmsg_log_str[DEVKMSG_STR_MAX_SIZE] = "ratelimit";
#if defined(CONFIG_PRINTK) && defined(CONFIG_SYSCTL)
int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
                              void *buffer, size_t *lenp, loff_t *ppos)
{
        char old_str[DEVKMSG_STR_MAX_SIZE];
        unsigned int old;
        int err;

        if (write) {
                if (devkmsg_log & DEVKMSG_LOG_MASK_LOCK)
                        return -EINVAL;

                old = devkmsg_log;
                strncpy(old_str, devkmsg_log_str, DEVKMSG_STR_MAX_SIZE);
        }

        err = proc_dostring(table, write, buffer, lenp, ppos);
        if (err)
                return err;

        if (write) {
                err = __control_devkmsg(devkmsg_log_str);

                /*
                 * Do not accept an unknown string OR a known string with
                 * trailing crap...
                 */
                if (err < 0 || (err + 1 != *lenp)) {

                        /* ... and restore old setting. */
                        devkmsg_log = old;
                        strncpy(devkmsg_log_str, old_str, DEVKMSG_STR_MAX_SIZE);

                        return -EINVAL;
                }
        }

        return 0;
}
#endif /* CONFIG_PRINTK && CONFIG_SYSCTL */

/**
 * console_list_lock - Lock the console list
 *
 * For console list or console->flags updates
 */
void console_list_lock(void)
{
        /*
         * In unregister_console() and console_force_preferred_locked(),
         * synchronize_srcu() is called with the console_list_lock held.
         * Therefore it is not allowed that the console_list_lock is taken
         * with the srcu_lock held.
         *
         * Detecting if this context is really in the read-side critical
         * section is only possible if the appropriate debug options are
         * enabled.
         */
        WARN_ON_ONCE(debug_lockdep_rcu_enabled() &&
                     srcu_read_lock_held(&console_srcu));

        mutex_lock(&console_mutex);
}
EXPORT_SYMBOL(console_list_lock);

/**
 * console_list_unlock - Unlock the console list
 *
 * Counterpart to console_list_lock()
 */
void console_list_unlock(void)
{
        mutex_unlock(&console_mutex);
}
EXPORT_SYMBOL(console_list_unlock);

/**
 * console_srcu_read_lock - Register a new reader for the
 *      SRCU-protected console list
 *
 * Use for_each_console_srcu() to iterate the console list
 *
 * Context: Any context.
 * Return: A cookie to pass to console_srcu_read_unlock().
 */
int console_srcu_read_lock(void)
{
        return srcu_read_lock_nmisafe(&console_srcu);
}
EXPORT_SYMBOL(console_srcu_read_lock);

/**
 * console_srcu_read_unlock - Unregister an old reader from
 *      the SRCU-protected console list
 * @cookie: cookie returned from console_srcu_read_lock()
 *
 * Counterpart to console_srcu_read_lock()
 */
void console_srcu_read_unlock(int cookie)
{
        srcu_read_unlock_nmisafe(&console_srcu, cookie);
}
EXPORT_SYMBOL(console_srcu_read_unlock);

/*
 * Helper macros to handle lockdep when locking/unlocking console_sem. We use
 * macros instead of functions so that _RET_IP_ contains useful information.
 */
#define down_console_sem() do { \
        down(&console_sem);\
        mutex_acquire(&console_lock_dep_map, 0, 0, _RET_IP_);\
} while (0)

static int __down_trylock_console_sem(unsigned long ip)
{
        int lock_failed;
        unsigned long flags;

        /*
         * Here and in __up_console_sem() we need to be in safe mode,
         * because spindump/WARN/etc from under console ->lock will
         * deadlock in printk()->down_trylock_console_sem() otherwise.
         */
        printk_safe_enter_irqsave(flags);
        lock_failed = down_trylock(&console_sem);
        printk_safe_exit_irqrestore(flags);

        if (lock_failed)
                return 1;
        mutex_acquire(&console_lock_dep_map, 0, 1, ip);
        return 0;
}
#define down_trylock_console_sem() __down_trylock_console_sem(_RET_IP_)

static void __up_console_sem(unsigned long ip)
{
        unsigned long flags;

        mutex_release(&console_lock_dep_map, ip);

        printk_safe_enter_irqsave(flags);
        up(&console_sem);
        printk_safe_exit_irqrestore(flags);
}
#define up_console_sem() __up_console_sem(_RET_IP_)

static bool panic_in_progress(void)
{
        return unlikely(atomic_read(&panic_cpu) != PANIC_CPU_INVALID);
}

/*
 * This is used for debugging the mess that is the VT code by
 * keeping track if we have the console semaphore held. It's
 * definitely not the perfect debug tool (we don't know if _WE_
 * hold it and are racing, but it helps tracking those weird code
 * paths in the console code where we end up in places I want
 * locked without the console semaphore held).
 */
static int console_locked, console_suspended;

/*
 *      Array of consoles built from command line options (console=)
 */

#define MAX_CMDLINECONSOLES 8

static struct console_cmdline console_cmdline[MAX_CMDLINECONSOLES];

static int preferred_console = -1;
int console_set_on_cmdline;
EXPORT_SYMBOL(console_set_on_cmdline);

/* Flag: console code may call schedule() */
static int console_may_schedule;

enum con_msg_format_flags {
        MSG_FORMAT_DEFAULT      = 0,
        MSG_FORMAT_SYSLOG       = (1 << 0),
};

static int console_msg_format = MSG_FORMAT_DEFAULT;

/*
 * The printk log buffer consists of a sequenced collection of records, each
 * containing variable length message text. Every record also contains its
 * own meta-data (@info).
 *
 * Every record meta-data carries the timestamp in microseconds, as well as
 * the standard userspace syslog level and syslog facility. The usual kernel
 * messages use LOG_KERN; userspace-injected messages always carry a matching
 * syslog facility, by default LOG_USER. The origin of every message can be
 * reliably determined that way.
 *
 * The human readable log message of a record is available in @text, the
 * length of the message text in @text_len. The stored message is not
 * terminated.
 *
 * Optionally, a record can carry a dictionary of properties (key/value
 * pairs), to provide userspace with a machine-readable message context.
 *
 * Examples for well-defined, commonly used property names are:
 *   DEVICE=b12:8               device identifier
 *                                b12:8         block dev_t
 *                                c127:3        char dev_t
 *                                n8            netdev ifindex
 *                                +sound:card0  subsystem:devname
 *   SUBSYSTEM=pci              driver-core subsystem name
 *
 * Valid characters in property names are [a-zA-Z0-9.-_]. Property names
 * and values are terminated by a '\0' character.
 *
 * Example of record values:
 *   record.text_buf                = "it's a line" (unterminated)
 *   record.info.seq                = 56
 *   record.info.ts_nsec            = 36863
 *   record.info.text_len           = 11
 *   record.info.facility           = 0 (LOG_KERN)
 *   record.info.flags              = 0
 *   record.info.level              = 3 (LOG_ERR)
 *   record.info.caller_id          = 299 (task 299)
 *   record.info.dev_info.subsystem = "pci" (terminated)
 *   record.info.dev_info.device    = "+pci:0000:00:01.0" (terminated)
 *
 * The 'struct printk_info' buffer must never be directly exported to
 * userspace, it is a kernel-private implementation detail that might
 * need to be changed in the future, when the requirements change.
 *
 * /dev/kmsg exports the structured data in the following line format:
 *   "<level>,<sequnum>,<timestamp>,<contflag>[,additional_values, ... ];<message text>\n"
 *
 * Users of the export format should ignore possible additional values
 * separated by ',', and find the message after the ';' character.
 *
 * The optional key/value pairs are attached as continuation lines starting
 * with a space character and terminated by a newline. All possible
 * non-prinatable characters are escaped in the "\xff" notation.
 */

/* syslog_lock protects syslog_* variables and write access to clear_seq. */
static DEFINE_MUTEX(syslog_lock);

#ifdef CONFIG_PRINTK
DECLARE_WAIT_QUEUE_HEAD(log_wait);
/* All 3 protected by @syslog_lock. */
/* the next printk record to read by syslog(READ) or /proc/kmsg */
static u64 syslog_seq;
static size_t syslog_partial;
static bool syslog_time;

struct latched_seq {
        seqcount_latch_t        latch;
        u64                     val[2];
};

/*
 * The next printk record to read after the last 'clear' command. There are
 * two copies (updated with seqcount_latch) so that reads can locklessly
 * access a valid value. Writers are synchronized by @syslog_lock.
 */
static struct latched_seq clear_seq = {
        .latch          = SEQCNT_LATCH_ZERO(clear_seq.latch),
        .val[0]         = 0,
        .val[1]         = 0,
};

#define LOG_LEVEL(v)            ((v) & 0x07)
#define LOG_FACILITY(v)         ((v) >> 3 & 0xff)

/* record buffer */
#define LOG_ALIGN __alignof__(unsigned long)
#define __LOG_BUF_LEN (1 << CONFIG_LOG_BUF_SHIFT)
#define LOG_BUF_LEN_MAX (u32)(1 << 31)
static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
static char *log_buf = __log_buf;
static u32 log_buf_len = __LOG_BUF_LEN;

/*
 * Define the average message size. This only affects the number of
 * descriptors that will be available. Underestimating is better than
 * overestimating (too many available descriptors is better than not enough).
 */
#define PRB_AVGBITS 5   /* 32 character average length */

#if CONFIG_LOG_BUF_SHIFT <= PRB_AVGBITS
#error CONFIG_LOG_BUF_SHIFT value too small.
#endif
_DEFINE_PRINTKRB(printk_rb_static, CONFIG_LOG_BUF_SHIFT - PRB_AVGBITS,
                 PRB_AVGBITS, &__log_buf[0]);

static struct printk_ringbuffer printk_rb_dynamic;

static struct printk_ringbuffer *prb = &printk_rb_static;

/*
 * We cannot access per-CPU data (e.g. per-CPU flush irq_work) before
 * per_cpu_areas are initialised. This variable is set to true when
 * it's safe to access per-CPU data.
 */
static bool __printk_percpu_data_ready __ro_after_init;

bool printk_percpu_data_ready(void)
{
        return __printk_percpu_data_ready;
}

/* Must be called under syslog_lock. */
static void latched_seq_write(struct latched_seq *ls, u64 val)
{
        raw_write_seqcount_latch(&ls->latch);
        ls->val[0] = val;
        raw_write_seqcount_latch(&ls->latch);
        ls->val[1] = val;
}

/* Can be called from any context. */
static u64 latched_seq_read_nolock(struct latched_seq *ls)
{
        unsigned int seq;
        unsigned int idx;
        u64 val;

        do {
                seq = raw_read_seqcount_latch(&ls->latch);
                idx = seq & 0x1;
                val = ls->val[idx];
        } while (read_seqcount_latch_retry(&ls->latch, seq));

        return val;
}

/* Return log buffer address */
char *log_buf_addr_get(void)
{
        return log_buf;
}

/* Return log buffer size */
u32 log_buf_len_get(void)
{
        return log_buf_len;
}

/*
 * Define how much of the log buffer we could take at maximum. The value
 * must be greater than two. Note that only half of the buffer is available
 * when the index points to the middle.
 */
#define MAX_LOG_TAKE_PART 4
static const char trunc_msg[] = "<truncated>";

static void truncate_msg(u16 *text_len, u16 *trunc_msg_len)
{
        /*
         * The message should not take the whole buffer. Otherwise, it might
         * get removed too soon.
         */
        u32 max_text_len = log_buf_len / MAX_LOG_TAKE_PART;

        if (*text_len > max_text_len)
                *text_len = max_text_len;

        /* enable the warning message (if there is room) */
        *trunc_msg_len = strlen(trunc_msg);
        if (*text_len >= *trunc_msg_len)
                *text_len -= *trunc_msg_len;
        else
                *trunc_msg_len = 0;
}

int dmesg_restrict = IS_ENABLED(CONFIG_SECURITY_DMESG_RESTRICT);

static int syslog_action_restricted(int type)
{
        if (dmesg_restrict)
                return 1;
        /*
         * Unless restricted, we allow "read all" and "get buffer size"
         * for everybody.
         */
        return type != SYSLOG_ACTION_READ_ALL &&
               type != SYSLOG_ACTION_SIZE_BUFFER;
}

static int check_syslog_permissions(int type, int source)
{
        /*
         * If this is from /proc/kmsg and we've already opened it, then we've
         * already done the capabilities checks at open time.
         */
        if (source == SYSLOG_FROM_PROC && type != SYSLOG_ACTION_OPEN)
                goto ok;

        if (syslog_action_restricted(type)) {
                if (capable(CAP_SYSLOG))
                        goto ok;
                /*
                 * For historical reasons, accept CAP_SYS_ADMIN too, with
                 * a warning.
                 */
                if (capable(CAP_SYS_ADMIN)) {
                        pr_warn_once("%s (%d): Attempt to access syslog with "
                                     "CAP_SYS_ADMIN but no CAP_SYSLOG "
                                     "(deprecated).\n",
                                 current->comm, task_pid_nr(current));
                        goto ok;
                }
                return -EPERM;
        }
ok:
        return security_syslog(type);
}

static void append_char(char **pp, char *e, char c)
{
        if (*pp < e)
                *(*pp)++ = c;
}

static ssize_t info_print_ext_header(char *buf, size_t size,
                                     struct printk_info *info)
{
        u64 ts_usec = info->ts_nsec;
        char caller[20];
#ifdef CONFIG_PRINTK_CALLER
        u32 id = info->caller_id;

        snprintf(caller, sizeof(caller), ",caller=%c%u",
                 id & 0x80000000 ? 'C' : 'T', id & ~0x80000000);
#else
        caller[0] = '\0';
#endif

        do_div(ts_usec, 1000);

        return scnprintf(buf, size, "%u,%llu,%llu,%c%s;",
                         (info->facility << 3) | info->level, info->seq,
                         ts_usec, info->flags & LOG_CONT ? 'c' : '-', caller);
}

static ssize_t msg_add_ext_text(char *buf, size_t size,
                                const char *text, size_t text_len,
                                unsigned char endc)
{
        char *p = buf, *e = buf + size;
        size_t i;

        /* escape non-printable characters */
        for (i = 0; i < text_len; i++) {
                unsigned char c = text[i];

                if (c < ' ' || c >= 127 || c == '\\')
                        p += scnprintf(p, e - p, "\\x%02x", c);
                else
                        append_char(&p, e, c);
        }
        append_char(&p, e, endc);

        return p - buf;
}

static ssize_t msg_add_dict_text(char *buf, size_t size,
                                 const char *key, const char *val)
{
        size_t val_len = strlen(val);
        ssize_t len;

        if (!val_len)
                return 0;

        len = msg_add_ext_text(buf, size, "", 0, ' ');  /* dict prefix */
        len += msg_add_ext_text(buf + len, size - len, key, strlen(key), '=');
        len += msg_add_ext_text(buf + len, size - len, val, val_len, '\n');

        return len;
}

static ssize_t msg_print_ext_body(char *buf, size_t size,
                                  char *text, size_t text_len,
                                  struct dev_printk_info *dev_info)
{
        ssize_t len;

        len = msg_add_ext_text(buf, size, text, text_len, '\n');

        if (!dev_info)
                goto out;

        len += msg_add_dict_text(buf + len, size - len, "SUBSYSTEM",
                                 dev_info->subsystem);
        len += msg_add_dict_text(buf + len, size - len, "DEVICE",
                                 dev_info->device);
out:
        return len;
}

static bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
                                    bool is_extended, bool may_supress);

/* /dev/kmsg - userspace message inject/listen interface */
struct devkmsg_user {
        atomic64_t seq;
        struct ratelimit_state rs;
        struct mutex lock;
        struct printk_buffers pbufs;
};

static __printf(3, 4) __cold
int devkmsg_emit(int facility, int level, const char *fmt, ...)
{
        va_list args;
        int r;

        va_start(args, fmt);
        r = vprintk_emit(facility, level, NULL, fmt, args);
        va_end(args);

        return r;
}

static ssize_t devkmsg_write(struct kiocb *iocb, struct iov_iter *from)
{
        char *buf, *line;
        int level = default_message_loglevel;
        int facility = 1;       /* LOG_USER */
        struct file *file = iocb->ki_filp;
        struct devkmsg_user *user = file->private_data;
        size_t len = iov_iter_count(from);
        ssize_t ret = len;

        if (!user || len > PRINTKRB_RECORD_MAX)
                return -EINVAL;

        /* Ignore when user logging is disabled. */
        if (devkmsg_log & DEVKMSG_LOG_MASK_OFF)
                return len;

        /* Ratelimit when not explicitly enabled. */
        if (!(devkmsg_log & DEVKMSG_LOG_MASK_ON)) {
                if (!___ratelimit(&user->rs, current->comm))
                        return ret;
        }

        buf = kmalloc(len+1, GFP_KERNEL);
        if (buf == NULL)
                return -ENOMEM;

        buf[len] = '\0';
        if (!copy_from_iter_full(buf, len, from)) {
                kfree(buf);
                return -EFAULT;
        }

        /*
         * Extract and skip the syslog prefix <[0-9]*>. Coming from userspace
         * the decimal value represents 32bit, the lower 3 bit are the log
         * level, the rest are the log facility.
         *
         * If no prefix or no userspace facility is specified, we
         * enforce LOG_USER, to be able to reliably distinguish
         * kernel-generated messages from userspace-injected ones.
         */
        line = buf;
        if (line[0] == '<') {
                char *endp = NULL;
                unsigned int u;

                u = simple_strtoul(line + 1, &endp, 10);
                if (endp && endp[0] == '>') {
                        level = LOG_LEVEL(u);
                        if (LOG_FACILITY(u) != 0)
                                facility = LOG_FACILITY(u);
                        endp++;
                        line = endp;
                }
        }

        devkmsg_emit(facility, level, "%s", line);
        kfree(buf);
        return ret;
}

static ssize_t devkmsg_read(struct file *file, char __user *buf,
                            size_t count, loff_t *ppos)
{
        struct devkmsg_user *user = file->private_data;
        char *outbuf = &user->pbufs.outbuf[0];
        struct printk_message pmsg = {
                .pbufs = &user->pbufs,
        };
        ssize_t ret;

        if (!user)
                return -EBADF;

        ret = mutex_lock_interruptible(&user->lock);
        if (ret)
                return ret;

        if (!printk_get_next_message(&pmsg, atomic64_read(&user->seq), true, false)) {
                if (file->f_flags & O_NONBLOCK) {
                        ret = -EAGAIN;
                        goto out;
                }

                /*
                 * Guarantee this task is visible on the waitqueue before
                 * checking the wake condition.
                 *
                 * The full memory barrier within set_current_state() of
                 * prepare_to_wait_event() pairs with the full memory barrier
                 * within wq_has_sleeper().
                 *
                 * This pairs with __wake_up_klogd:A.
                 */
                ret = wait_event_interruptible(log_wait,
                                printk_get_next_message(&pmsg, atomic64_read(&user->seq), true,
                                                        false)); /* LMM(devkmsg_read:A) */
                if (ret)
                        goto out;
        }

        if (pmsg.dropped) {
                /* our last seen message is gone, return error and reset */
                atomic64_set(&user->seq, pmsg.seq);
                ret = -EPIPE;
                goto out;
        }

        atomic64_set(&user->seq, pmsg.seq + 1);

        if (pmsg.outbuf_len > count) {
                ret = -EINVAL;
                goto out;
        }

        if (copy_to_user(buf, outbuf, pmsg.outbuf_len)) {
                ret = -EFAULT;
                goto out;
        }
        ret = pmsg.outbuf_len;
out:
        mutex_unlock(&user->lock);
        return ret;
}

/*
 * Be careful when modifying this function!!!
 *
 * Only few operations are supported because the device works only with the
 * entire variable length messages (records). Non-standard values are
 * returned in the other cases and has been this way for quite some time.
 * User space applications might depend on this behavior.
 */
static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
{
        struct devkmsg_user *user = file->private_data;
        loff_t ret = 0;

        if (!user)
                return -EBADF;
        if (offset)
                return -ESPIPE;

        switch (whence) {
        case SEEK_SET:
                /* the first record */
                atomic64_set(&user->seq, prb_first_valid_seq(prb));
                break;
        case SEEK_DATA:
                /*
                 * The first record after the last SYSLOG_ACTION_CLEAR,
                 * like issued by 'dmesg -c'. Reading /dev/kmsg itself
                 * changes no global state, and does not clear anything.
                 */
                atomic64_set(&user->seq, latched_seq_read_nolock(&clear_seq));
                break;
        case SEEK_END:
                /* after the last record */
                atomic64_set(&user->seq, prb_next_seq(prb));
                break;
        default:
                ret = -EINVAL;
        }
        return ret;
}

static __poll_t devkmsg_poll(struct file *file, poll_table *wait)
{
        struct devkmsg_user *user = file->private_data;
        struct printk_info info;
        __poll_t ret = 0;

        if (!user)
                return EPOLLERR|EPOLLNVAL;

        poll_wait(file, &log_wait, wait);

        if (prb_read_valid_info(prb, atomic64_read(&user->seq), &info, NULL)) {
                /* return error when data has vanished underneath us */
                if (info.seq != atomic64_read(&user->seq))
                        ret = EPOLLIN|EPOLLRDNORM|EPOLLERR|EPOLLPRI;
                else
                        ret = EPOLLIN|EPOLLRDNORM;
        }

        return ret;
}

static int devkmsg_open(struct inode *inode, struct file *file)
{
        struct devkmsg_user *user;
        int err;

        if (devkmsg_log & DEVKMSG_LOG_MASK_OFF)
                return -EPERM;

        /* write-only does not need any file context */
        if ((file->f_flags & O_ACCMODE) != O_WRONLY) {
                err = check_syslog_permissions(SYSLOG_ACTION_READ_ALL,
                                               SYSLOG_FROM_READER);
                if (err)
                        return err;
        }

        user = kvmalloc(sizeof(struct devkmsg_user), GFP_KERNEL);
        if (!user)
                return -ENOMEM;

        ratelimit_default_init(&user->rs);
        ratelimit_set_flags(&user->rs, RATELIMIT_MSG_ON_RELEASE);

        mutex_init(&user->lock);

        atomic64_set(&user->seq, prb_first_valid_seq(prb));

        file->private_data = user;
        return 0;
}

static int devkmsg_release(struct inode *inode, struct file *file)
{
        struct devkmsg_user *user = file->private_data;

        if (!user)
                return 0;

        ratelimit_state_exit(&user->rs);

        mutex_destroy(&user->lock);
        kvfree(user);
        return 0;
}

const struct file_operations kmsg_fops = {
        .open = devkmsg_open,
        .read = devkmsg_read,
        .write_iter = devkmsg_write,
        .llseek = devkmsg_llseek,
        .poll = devkmsg_poll,
        .release = devkmsg_release,
};

#ifdef CONFIG_CRASH_CORE
/*
 * This appends the listed symbols to /proc/vmcore
 *
 * /proc/vmcore is used by various utilities, like crash and makedumpfile to
 * obtain access to symbols that are otherwise very difficult to locate.  These
 * symbols are specifically used so that utilities can access and extract the
 * dmesg log from a vmcore file after a crash.
 */
void log_buf_vmcoreinfo_setup(void)
{
        struct dev_printk_info *dev_info = NULL;

        VMCOREINFO_SYMBOL(prb);
        VMCOREINFO_SYMBOL(printk_rb_static);
        VMCOREINFO_SYMBOL(clear_seq);

        /*
         * Export struct size and field offsets. User space tools can
         * parse it and detect any changes to structure down the line.
         */

        VMCOREINFO_STRUCT_SIZE(printk_ringbuffer);
        VMCOREINFO_OFFSET(printk_ringbuffer, desc_ring);
        VMCOREINFO_OFFSET(printk_ringbuffer, text_data_ring);
        VMCOREINFO_OFFSET(printk_ringbuffer, fail);

        VMCOREINFO_STRUCT_SIZE(prb_desc_ring);
        VMCOREINFO_OFFSET(prb_desc_ring, count_bits);
        VMCOREINFO_OFFSET(prb_desc_ring, descs);
        VMCOREINFO_OFFSET(prb_desc_ring, infos);
        VMCOREINFO_OFFSET(prb_desc_ring, head_id);
        VMCOREINFO_OFFSET(prb_desc_ring, tail_id);

        VMCOREINFO_STRUCT_SIZE(prb_desc);
        VMCOREINFO_OFFSET(prb_desc, state_var);
        VMCOREINFO_OFFSET(prb_desc, text_blk_lpos);

        VMCOREINFO_STRUCT_SIZE(prb_data_blk_lpos);
        VMCOREINFO_OFFSET(prb_data_blk_lpos, begin);
        VMCOREINFO_OFFSET(prb_data_blk_lpos, next);

        VMCOREINFO_STRUCT_SIZE(printk_info);
        VMCOREINFO_OFFSET(printk_info, seq);
        VMCOREINFO_OFFSET(printk_info, ts_nsec);
        VMCOREINFO_OFFSET(printk_info, text_len);
        VMCOREINFO_OFFSET(printk_info, caller_id);
        VMCOREINFO_OFFSET(printk_info, dev_info);

        VMCOREINFO_STRUCT_SIZE(dev_printk_info);
        VMCOREINFO_OFFSET(dev_printk_info, subsystem);
        VMCOREINFO_LENGTH(printk_info_subsystem, sizeof(dev_info->subsystem));
        VMCOREINFO_OFFSET(dev_printk_info, device);
        VMCOREINFO_LENGTH(printk_info_device, sizeof(dev_info->device));

        VMCOREINFO_STRUCT_SIZE(prb_data_ring);
        VMCOREINFO_OFFSET(prb_data_ring, size_bits);
        VMCOREINFO_OFFSET(prb_data_ring, data);
        VMCOREINFO_OFFSET(prb_data_ring, head_lpos);
        VMCOREINFO_OFFSET(prb_data_ring, tail_lpos);

        VMCOREINFO_SIZE(atomic_long_t);
        VMCOREINFO_TYPE_OFFSET(atomic_long_t, counter);

        VMCOREINFO_STRUCT_SIZE(latched_seq);
        VMCOREINFO_OFFSET(latched_seq, val);
}
#endif

/* requested log_buf_len from kernel cmdline */
static unsigned long __initdata new_log_buf_len;

/* we practice scaling the ring buffer by powers of 2 */
static void __init log_buf_len_update(u64 size)
{
        if (size > (u64)LOG_BUF_LEN_MAX) {
                size = (u64)LOG_BUF_LEN_MAX;
                pr_err("log_buf over 2G is not supported.\n");
        }

        if (size)
                size = roundup_pow_of_two(size);
        if (size > log_buf_len)
                new_log_buf_len = (unsigned long)size;
}

/* save requested log_buf_len since it's too early to process it */
static int __init log_buf_len_setup(char *str)
{
        u64 size;

        if (!str)
                return -EINVAL;

        size = memparse(str, &str);

        log_buf_len_update(size);

        return 0;
}
early_param("log_buf_len", log_buf_len_setup);

#ifdef CONFIG_SMP
#define __LOG_CPU_MAX_BUF_LEN (1 << CONFIG_LOG_CPU_MAX_BUF_SHIFT)

static void __init log_buf_add_cpu(void)
{
        unsigned int cpu_extra;

        /*
         * archs should set up cpu_possible_bits properly with
         * set_cpu_possible() after setup_arch() but just in
         * case lets ensure this is valid.
         */
        if (num_possible_cpus() == 1)
                return;

        cpu_extra = (num_possible_cpus() - 1) * __LOG_CPU_MAX_BUF_LEN;

        /* by default this will only continue through for large > 64 CPUs */
        if (cpu_extra <= __LOG_BUF_LEN / 2)
                return;

        pr_info("log_buf_len individual max cpu contribution: %d bytes\n",
                __LOG_CPU_MAX_BUF_LEN);
        pr_info("log_buf_len total cpu_extra contributions: %d bytes\n",
                cpu_extra);
        pr_info("log_buf_len min size: %d bytes\n", __LOG_BUF_LEN);

        log_buf_len_update(cpu_extra + __LOG_BUF_LEN);
}
#else /* !CONFIG_SMP */
static inline void log_buf_add_cpu(void) {}
#endif /* CONFIG_SMP */

static void __init set_percpu_data_ready(void)
{
        __printk_percpu_data_ready = true;
}

static unsigned int __init add_to_rb(struct printk_ringbuffer *rb,
                                     struct printk_record *r)
{
        struct prb_reserved_entry e;
        struct printk_record dest_r;

        prb_rec_init_wr(&dest_r, r->info->text_len);

        if (!prb_reserve(&e, rb, &dest_r))
                return 0;

        memcpy(&dest_r.text_buf[0], &r->text_buf[0], r->info->text_len);
        dest_r.info->text_len = r->info->text_len;
        dest_r.info->facility = r->info->facility;
        dest_r.info->level = r->info->level;
        dest_r.info->flags = r->info->flags;
        dest_r.info->ts_nsec = r->info->ts_nsec;
        dest_r.info->caller_id = r->info->caller_id;
        memcpy(&dest_r.info->dev_info, &r->info->dev_info, sizeof(dest_r.info->dev_info));

        prb_final_commit(&e);

        return prb_record_text_space(&e);
}

static char setup_text_buf[PRINTKRB_RECORD_MAX] __initdata;

void __init setup_log_buf(int early)
{
        struct printk_info *new_infos;
        unsigned int new_descs_count;
        struct prb_desc *new_descs;
        struct printk_info info;
        struct printk_record r;
        unsigned int text_size;
        size_t new_descs_size;
        size_t new_infos_size;
        unsigned long flags;
        char *new_log_buf;
        unsigned int free;
        u64 seq;

        /*
         * Some archs call setup_log_buf() multiple times - first is very
         * early, e.g. from setup_arch(), and second - when percpu_areas
         * are initialised.
         */
        if (!early)
                set_percpu_data_ready();

        if (log_buf != __log_buf)
                return;

        if (!early && !new_log_buf_len)
                log_buf_add_cpu();

        if (!new_log_buf_len)
                return;

        new_descs_count = new_log_buf_len >> PRB_AVGBITS;
        if (new_descs_count == 0) {
                pr_err("new_log_buf_len: %lu too small\n", new_log_buf_len);
                return;
        }

        new_log_buf = memblock_alloc(new_log_buf_len, LOG_ALIGN);
        if (unlikely(!new_log_buf)) {
                pr_err("log_buf_len: %lu text bytes not available\n",
                       new_log_buf_len);
                return;
        }

        new_descs_size = new_descs_count * sizeof(struct prb_desc);
        new_descs = memblock_alloc(new_descs_size, LOG_ALIGN);
        if (unlikely(!new_descs)) {
                pr_err("log_buf_len: %zu desc bytes not available\n",
                       new_descs_size);
                goto err_free_log_buf;
        }

        new_infos_size = new_descs_count * sizeof(struct printk_info);
        new_infos = memblock_alloc(new_infos_size, LOG_ALIGN);
        if (unlikely(!new_infos)) {
                pr_err("log_buf_len: %zu info bytes not available\n",
                       new_infos_size);
                goto err_free_descs;
        }

        prb_rec_init_rd(&r, &info, &setup_text_buf[0], sizeof(setup_text_buf));

        prb_init(&printk_rb_dynamic,
                 new_log_buf, ilog2(new_log_buf_len),
                 new_descs, ilog2(new_descs_count),
                 new_infos);

        local_irq_save(flags);

        log_buf_len = new_log_buf_len;
        log_buf = new_log_buf;
        new_log_buf_len = 0;

        free = __LOG_BUF_LEN;
        prb_for_each_record(0, &printk_rb_static, seq, &r) {
                text_size = add_to_rb(&printk_rb_dynamic, &r);
                if (text_size > free)
                        free = 0;
                else
                        free -= text_size;
        }

        prb = &printk_rb_dynamic;

        local_irq_restore(flags);

        /*
         * Copy any remaining messages that might have appeared from
         * NMI context after copying but before switching to the
         * dynamic buffer.
         */
        prb_for_each_record(seq, &printk_rb_static, seq, &r) {
                text_size = add_to_rb(&printk_rb_dynamic, &r);
                if (text_size > free)
                        free = 0;
                else
                        free -= text_size;
        }

        if (seq != prb_next_seq(&printk_rb_static)) {
                pr_err("dropped %llu messages\n",
                       prb_next_seq(&printk_rb_static) - seq);
        }

        pr_info("log_buf_len: %u bytes\n", log_buf_len);
        pr_info("early log buf free: %u(%u%%)\n",
                free, (free * 100) / __LOG_BUF_LEN);
        return;

err_free_descs:
        memblock_free(new_descs, new_descs_size);
err_free_log_buf:
        memblock_free(new_log_buf, new_log_buf_len);
}

static bool __read_mostly ignore_loglevel;

static int __init ignore_loglevel_setup(char *str)
{
        ignore_loglevel = true;
        pr_info("debug: ignoring loglevel setting.\n");

        return 0;
}

early_param("ignore_loglevel", ignore_loglevel_setup);
module_param(ignore_loglevel, bool, S_IRUGO | S_IWUSR);
MODULE_PARM_DESC(ignore_loglevel,
                 "ignore loglevel setting (prints all kernel messages to the console)");

static bool suppress_message_printing(int level)
{
        return (level >= console_loglevel && !ignore_loglevel);
}

#ifdef CONFIG_BOOT_PRINTK_DELAY

static int boot_delay; /* msecs delay after each printk during bootup */
static unsigned long long loops_per_msec;       /* based on boot_delay */

static int __init boot_delay_setup(char *str)
{
        unsigned long lpj;

        lpj = preset_lpj ? preset_lpj : 1000000;        /* some guess */
        loops_per_msec = (unsigned long long)lpj / 1000 * HZ;

        get_option(&str, &boot_delay);
        if (boot_delay > 10 * 1000)
                boot_delay = 0;

        pr_debug("boot_delay: %u, preset_lpj: %ld, lpj: %lu, "
                "HZ: %d, loops_per_msec: %llu\n",
                boot_delay, preset_lpj, lpj, HZ, loops_per_msec);
        return 0;
}
early_param("boot_delay", boot_delay_setup);

static void boot_delay_msec(int level)
{
        unsigned long long k;
        unsigned long timeout;

        if ((boot_delay == 0 || system_state >= SYSTEM_RUNNING)
                || suppress_message_printing(level)) {
                return;
        }

        k = (unsigned long long)loops_per_msec * boot_delay;

        timeout = jiffies + msecs_to_jiffies(boot_delay);
        while (k) {
                k--;
                cpu_relax();
                /*
                 * use (volatile) jiffies to prevent
                 * compiler reduction; loop termination via jiffies
                 * is secondary and may or may not happen.
                 */
                if (time_after(jiffies, timeout))
                        break;
                touch_nmi_watchdog();
        }
}
#else
static inline void boot_delay_msec(int level)
{
}
#endif

static bool printk_time = IS_ENABLED(CONFIG_PRINTK_TIME);
module_param_named(time, printk_time, bool, S_IRUGO | S_IWUSR);

static size_t print_syslog(unsigned int level, char *buf)
{
        return sprintf(buf, "<%u>", level);
}

static size_t print_time(u64 ts, char *buf)
{
        unsigned long rem_nsec = do_div(ts, 1000000000);

        return sprintf(buf, "[%5lu.%06lu]",
                       (unsigned long)ts, rem_nsec / 1000);
}

#ifdef CONFIG_PRINTK_CALLER
static size_t print_caller(u32 id, char *buf)
{
        char caller[12];

        snprintf(caller, sizeof(caller), "%c%u",
                 id & 0x80000000 ? 'C' : 'T', id & ~0x80000000);
        return sprintf(buf, "[%6s]", caller);
}
#else
#define print_caller(id, buf) 0
#endif

static size_t info_print_prefix(const struct printk_info  *info, bool syslog,
                                bool time, char *buf)
{
        size_t len = 0;

        if (syslog)
                len = print_syslog((info->facility << 3) | info->level, buf);

        if (time)
                len += print_time(info->ts_nsec, buf + len);

        len += print_caller(info->caller_id, buf + len);

        if (IS_ENABLED(CONFIG_PRINTK_CALLER) || time) {
                buf[len++] = ' ';
                buf[len] = '\0';
        }

        return len;
}

/*
 * Prepare the record for printing. The text is shifted within the given
 * buffer to avoid a need for another one. The following operations are
 * done:
 *
 *   - Add prefix for each line.
 *   - Drop truncated lines that no longer fit into the buffer.
 *   - Add the trailing newline that has been removed in vprintk_store().
 *   - Add a string terminator.
 *
 * Since the produced string is always terminated, the maximum possible
 * return value is @r->text_buf_size - 1;
 *
 * Return: The length of the updated/prepared text, including the added
 * prefixes and the newline. The terminator is not counted. The dropped
 * line(s) are not counted.
 */
static size_t record_print_text(struct printk_record *r, bool syslog,
                                bool time)
{
        size_t text_len = r->info->text_len;
        size_t buf_size = r->text_buf_size;
        char *text = r->text_buf;
        char prefix[PRINTK_PREFIX_MAX];
        bool truncated = false;
        size_t prefix_len;
        size_t line_len;
        size_t len = 0;
        char *next;

        /*
         * If the message was truncated because the buffer was not large
         * enough, treat the available text as if it were the full text.
         */
        if (text_len > buf_size)
                text_len = buf_size;

        prefix_len = info_print_prefix(r->info, syslog, time, prefix);

        /*
         * @text_len: bytes of unprocessed text
         * @line_len: bytes of current line _without_ newline
         * @text:     pointer to beginning of current line
         * @len:      number of bytes prepared in r->text_buf
         */
        for (;;) {
                next = memchr(text, '\n', text_len);
                if (next) {
                        line_len = next - text;
                } else {
                        /* Drop truncated line(s). */
                        if (truncated)
                                break;
                        line_len = text_len;
                }

                /*
                 * Truncate the text if there is not enough space to add the
                 * prefix and a trailing newline and a terminator.
                 */
                if (len + prefix_len + text_len + 1 + 1 > buf_size) {
                        /* Drop even the current line if no space. */
                        if (len + prefix_len + line_len + 1 + 1 > buf_size)
                                break;

                        text_len = buf_size - len - prefix_len - 1 - 1;
                        truncated = true;
                }

                memmove(text + prefix_len, text, text_len);
                memcpy(text, prefix, prefix_len);

                /*
                 * Increment the prepared length to include the text and
                 * prefix that were just moved+copied. Also increment for the
                 * newline at the end of this line. If this is the last line,
                 * there is no newline, but it will be added immediately below.
                 */
                len += prefix_len + line_len + 1;
                if (text_len == line_len) {
                        /*
                         * This is the last line. Add the trailing newline
                         * removed in vprintk_store().
                         */
                        text[prefix_len + line_len] = '\n';
                        break;
                }

                /*
                 * Advance beyond the added prefix and the related line with
                 * its newline.
                 */
                text += prefix_len + line_len + 1;

                /*
                 * The remaining text has only decreased by the line with its
                 * newline.
                 *
                 * Note that @text_len can become zero. It happens when @text
                 * ended with a newline (either due to truncation or the
                 * original string ending with "\n\n"). The loop is correctly
                 * repeated and (if not truncated) an empty line with a prefix
                 * will be prepared.
                 */
                text_len -= line_len + 1;
        }

        /*
         * If a buffer was provided, it will be terminated. Space for the
         * string terminator is guaranteed to be available. The terminator is
         * not counted in the return value.
         */
        if (buf_size > 0)
                r->text_buf[len] = 0;

        return len;
}

static size_t get_record_print_text_size(struct printk_info *info,
                                         unsigned int line_count,
                                         bool syslog, bool time)
{
        char prefix[PRINTK_PREFIX_MAX];
        size_t prefix_len;

        prefix_len = info_print_prefix(info, syslog, time, prefix);

        /*
         * Each line will be preceded with a prefix. The intermediate
         * newlines are already within the text, but a final trailing
         * newline will be added.
         */
        return ((prefix_len * line_count) + info->text_len + 1);
}

/*
 * Beginning with @start_seq, find the first record where it and all following
 * records up to (but not including) @max_seq fit into @size.
 *
 * @max_seq is simply an upper bound and does not need to exist. If the caller
 * does not require an upper bound, -1 can be used for @max_seq.
 */
static u64 find_first_fitting_seq(u64 start_seq, u64 max_seq, size_t size,
                                  bool syslog, bool time)
{
        struct printk_info info;
        unsigned int line_count;
        size_t len = 0;
        u64 seq;

        /* Determine the size of the records up to @max_seq. */
        prb_for_each_info(start_seq, prb, seq, &info, &line_count) {
                if (info.seq >= max_seq)
                        break;
                len += get_record_print_text_size(&info, line_count, syslog, time);
        }

        /*
         * Adjust the upper bound for the next loop to avoid subtracting
         * lengths that were never added.
         */
        if (seq < max_seq)
                max_seq = seq;

        /*
         * Move first record forward until length fits into the buffer. Ignore
         * newest messages that were not counted in the above cycle. Messages
         * might appear and get lost in the meantime. This is a best effort
         * that prevents an infinite loop that could occur with a retry.
         */
        prb_for_each_info(start_seq, prb, seq, &info, &line_count) {
                if (len <= size || info.seq >= max_seq)
                        break;
                len -= get_record_print_text_size(&info, line_count, syslog, time);
        }

        return seq;
}

/* The caller is responsible for making sure @size is greater than 0. */
static int syslog_print(char __user *buf, int size)
{
        struct printk_info info;
        struct printk_record r;
        char *text;
        int len = 0;
        u64 seq;

        text = kmalloc(PRINTK_MESSAGE_MAX, GFP_KERNEL);
        if (!text)
                return -ENOMEM;

        prb_rec_init_rd(&r, &info, text, PRINTK_MESSAGE_MAX);

        mutex_lock(&syslog_lock);

        /*
         * Wait for the @syslog_seq record to be available. @syslog_seq may
         * change while waiting.
         */
        do {
                seq = syslog_seq;

                mutex_unlock(&syslog_lock);
                /*
                 * Guarantee this task is visible on the waitqueue before
                 * checking the wake condition.
                 *
                 * The full memory barrier within set_current_state() of
                 * prepare_to_wait_event() pairs with the full memory barrier
                 * within wq_has_sleeper().
                 *
                 * This pairs with __wake_up_klogd:A.
                 */
                len = wait_event_interruptible(log_wait,
                                prb_read_valid(prb, seq, NULL)); /* LMM(syslog_print:A) */
                mutex_lock(&syslog_lock);

                if (len)
                        goto out;
        } while (syslog_seq != seq);

        /*
         * Copy records that fit into the buffer. The above cycle makes sure
         * that the first record is always available.
         */
        do {
                size_t n;
                size_t skip;
                int err;

                if (!prb_read_valid(prb, syslog_seq, &r))
                        break;

                if (r.info->seq != syslog_seq) {
                        /* message is gone, move to next valid one */
                        syslog_seq = r.info->seq;
                        syslog_partial = 0;
                }

                /*
                 * To keep reading/counting partial line consistent,
                 * use printk_time value as of the beginning of a line.
                 */
                if (!syslog_partial)
                        syslog_time = printk_time;

                skip = syslog_partial;
                n = record_print_text(&r, true, syslog_time);
                if (n - syslog_partial <= size) {
                        /* message fits into buffer, move forward */
                        syslog_seq = r.info->seq + 1;
                        n -= syslog_partial;
                        syslog_partial = 0;
                } else if (!len){
                        /* partial read(), remember position */
                        n = size;
                        syslog_partial += n;
                } else
                        n = 0;

                if (!n)
                        break;

                mutex_unlock(&syslog_lock);
                err = copy_to_user(buf, text + skip, n);
                mutex_lock(&syslog_lock);

                if (err) {
                        if (!len)
                                len = -EFAULT;
                        break;
                }

                len += n;
                size -= n;
                buf += n;
        } while (size);
out:
        mutex_unlock(&syslog_lock);
        kfree(text);
        return len;
}

static int syslog_print_all(char __user *buf, int size, bool clear)
{
        struct printk_info info;
        struct printk_record r;
        char *text;
        int len = 0;
        u64 seq;
        bool time;

        text = kmalloc(PRINTK_MESSAGE_MAX, GFP_KERNEL);
        if (!text)
                return -ENOMEM;

        time = printk_time;
        /*
         * Find first record that fits, including all following records,
         * into the user-provided buffer for this dump.
         */
        seq = find_first_fitting_seq(latched_seq_read_nolock(&clear_seq), -1,
                                     size, true, time);

        prb_rec_init_rd(&r, &info, text, PRINTK_MESSAGE_MAX);

        len = 0;
        prb_for_each_record(seq, prb, seq, &r) {
                int textlen;

                textlen = record_print_text(&r, true, time);

                if (len + textlen > size) {
                        seq--;
                        break;
                }

                if (copy_to_user(buf + len, text, textlen))
                        len = -EFAULT;
                else
                        len += textlen;

                if (len < 0)
                        break;
        }

        if (clear) {
                mutex_lock(&syslog_lock);
                latched_seq_write(&clear_seq, seq);
                mutex_unlock(&syslog_lock);
        }

        kfree(text);
        return len;
}

static void syslog_clear(void)
{
        mutex_lock(&syslog_lock);
        latched_seq_write(&clear_seq, prb_next_seq(prb));
        mutex_unlock(&syslog_lock);
}

int do_syslog(int type, char __user *buf, int len, int source)
{
        struct printk_info info;
        bool clear = false;
        static int saved_console_loglevel = LOGLEVEL_DEFAULT;
        int error;

        error = check_syslog_permissions(type, source);
        if (error)
                return error;

        switch (type) {
        case SYSLOG_ACTION_CLOSE:       /* Close log */
                break;
        case SYSLOG_ACTION_OPEN:        /* Open log */
                break;
        case SYSLOG_ACTION_READ:        /* Read from log */
                if (!buf || len < 0)
                        return -EINVAL;
                if (!len)
                        return 0;
                if (!access_ok(buf, len))
                        return -EFAULT;
                error = syslog_print(buf, len);
                break;
        /* Read/clear last kernel messages */
        case SYSLOG_ACTION_READ_CLEAR:
                clear = true;
                fallthrough;
        /* Read last kernel messages */
        case SYSLOG_ACTION_READ_ALL:
                if (!buf || len < 0)
                        return -EINVAL;
                if (!len)
                        return 0;
                if (!access_ok(buf, len))
                        return -EFAULT;
                error = syslog_print_all(buf, len, clear);
                break;
        /* Clear ring buffer */
        case SYSLOG_ACTION_CLEAR:
                syslog_clear();
                break;
        /* Disable logging to console */
        case SYSLOG_ACTION_CONSOLE_OFF:
                if (saved_console_loglevel == LOGLEVEL_DEFAULT)
                        saved_console_loglevel = console_loglevel;
                console_loglevel = minimum_console_loglevel;
                break;
        /* Enable logging to console */
        case SYSLOG_ACTION_CONSOLE_ON:
                if (saved_console_loglevel != LOGLEVEL_DEFAULT) {
                        console_loglevel = saved_console_loglevel;
                        saved_console_loglevel = LOGLEVEL_DEFAULT;
                }
                break;
        /* Set level of messages printed to console */
        case SYSLOG_ACTION_CONSOLE_LEVEL:
                if (len < 1 || len > 8)
                        return -EINVAL;
                if (len < minimum_console_loglevel)
                        len = minimum_console_loglevel;
                console_loglevel = len;
                /* Implicitly re-enable logging to console */
                saved_console_loglevel = LOGLEVEL_DEFAULT;
                break;
        /* Number of chars in the log buffer */
        case SYSLOG_ACTION_SIZE_UNREAD:
                mutex_lock(&syslog_lock);
                if (!prb_read_valid_info(prb, syslog_seq, &info, NULL)) {
                        /* No unread messages. */
                        mutex_unlock(&syslog_lock);
                        return 0;
                }
                if (info.seq != syslog_seq) {
                        /* messages are gone, move to first one */
                        syslog_seq = info.seq;
                        syslog_partial = 0;
                }
                if (source == SYSLOG_FROM_PROC) {
                        /*
                         * Short-cut for poll(/"proc/kmsg") which simply checks
                         * for pending data, not the size; return the count of
                         * records, not the length.
                         */
                        error = prb_next_seq(prb) - syslog_seq;
                } else {
                        bool time = syslog_partial ? syslog_time : printk_time;
                        unsigned int line_count;
                        u64 seq;

                        prb_for_each_info(syslog_seq, prb, seq, &info,
                                          &line_count) {
                                error += get_record_print_text_size(&info, line_count,
                                                                    true, time);
                                time = printk_time;
                        }
                        error -= syslog_partial;
                }
                mutex_unlock(&syslog_lock);
                break;
        /* Size of the log buffer */
        case SYSLOG_ACTION_SIZE_BUFFER:
                error = log_buf_len;
                break;
        default:
                error = -EINVAL;
                break;
        }

        return error;
}

SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
{
        return do_syslog(type, buf, len, SYSLOG_FROM_READER);
}

/*
 * Special console_lock variants that help to reduce the risk of soft-lockups.
 * They allow to pass console_lock to another printk() call using a busy wait.
 */

#ifdef CONFIG_LOCKDEP
static struct lockdep_map console_owner_dep_map = {
        .name = "console_owner"
};
#endif

static DEFINE_RAW_SPINLOCK(console_owner_lock);
static struct task_struct *console_owner;
static bool console_waiter;

/**
 * console_lock_spinning_enable - mark beginning of code where another
 *      thread might safely busy wait
 *
 * This basically converts console_lock into a spinlock. This marks
 * the section where the console_lock owner can not sleep, because
 * there may be a waiter spinning (like a spinlock). Also it must be
 * ready to hand over the lock at the end of the section.
 */
static void console_lock_spinning_enable(void)
{
        raw_spin_lock(&console_owner_lock);
        console_owner = current;
        raw_spin_unlock(&console_owner_lock);

        /* The waiter may spin on us after setting console_owner */
        spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
}

/**
 * console_lock_spinning_disable_and_check - mark end of code where another
 *      thread was able to busy wait and check if there is a waiter
 * @cookie: cookie returned from console_srcu_read_lock()
 *
 * This is called at the end of the section where spinning is allowed.
 * It has two functions. First, it is a signal that it is no longer
 * safe to start busy waiting for the lock. Second, it checks if
 * there is a busy waiter and passes the lock rights to her.
 *
 * Important: Callers lose both the console_lock and the SRCU read lock if
 *      there was a busy waiter. They must not touch items synchronized by
 *      console_lock or SRCU read lock in this case.
 *
 * Return: 1 if the lock rights were passed, 0 otherwise.
 */
static int console_lock_spinning_disable_and_check(int cookie)
{
        int waiter;

        raw_spin_lock(&console_owner_lock);
        waiter = READ_ONCE(console_waiter);
        console_owner = NULL;
        raw_spin_unlock(&console_owner_lock);

        if (!waiter) {
                spin_release(&console_owner_dep_map, _THIS_IP_);
                return 0;
        }

        /* The waiter is now free to continue */
        WRITE_ONCE(console_waiter, false);

        spin_release(&console_owner_dep_map, _THIS_IP_);

        /*
         * Preserve lockdep lock ordering. Release the SRCU read lock before
         * releasing the console_lock.
         */
        console_srcu_read_unlock(cookie);

        /*
         * Hand off console_lock to waiter. The waiter will perform
         * the up(). After this, the waiter is the console_lock owner.
         */
        mutex_release(&console_lock_dep_map, _THIS_IP_);
        return 1;
}

/**
 * console_trylock_spinning - try to get console_lock by busy waiting
 *
 * This allows to busy wait for the console_lock when the current
 * owner is running in specially marked sections. It means that
 * the current owner is running and cannot reschedule until it
 * is ready to lose the lock.
 *
 * Return: 1 if we got the lock, 0 othrewise
 */
static int console_trylock_spinning(void)
{
        struct task_struct *owner = NULL;
        bool waiter;
        bool spin = false;
        unsigned long flags;

        if (console_trylock())
                return 1;

        /*
         * It's unsafe to spin once a panic has begun. If we are the
         * panic CPU, we may have already halted the owner of the
         * console_sem. If we are not the panic CPU, then we should
         * avoid taking console_sem, so the panic CPU has a better
         * chance of cleanly acquiring it later.
         */
        if (panic_in_progress())
                return 0;

        printk_safe_enter_irqsave(flags);

        raw_spin_lock(&console_owner_lock);
        owner = READ_ONCE(console_owner);
        waiter = READ_ONCE(console_waiter);
        if (!waiter && owner && owner != current) {
                WRITE_ONCE(console_waiter, true);
                spin = true;
        }
        raw_spin_unlock(&console_owner_lock);

        /*
         * If there is an active printk() writing to the
         * consoles, instead of having it write our data too,
         * see if we can offload that load from the active
         * printer, and do some printing ourselves.
         * Go into a spin only if there isn't already a waiter
         * spinning, and there is an active printer, and
         * that active printer isn't us (recursive printk?).
         */
        if (!spin) {
                printk_safe_exit_irqrestore(flags);
                return 0;
        }

        /* We spin waiting for the owner to release us */
        spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
        /* Owner will clear console_waiter on hand off */
        while (READ_ONCE(console_waiter))
                cpu_relax();
        spin_release(&console_owner_dep_map, _THIS_IP_);

        printk_safe_exit_irqrestore(flags);
        /*
         * The owner passed the console lock to us.
         * Since we did not spin on console lock, annotate
         * this as a trylock. Otherwise lockdep will
         * complain.
         */
        mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);

        return 1;
}

/*
 * Recursion is tracked separately on each CPU. If NMIs are supported, an
 * additional NMI context per CPU is also separately tracked. Until per-CPU
 * is available, a separate "early tracking" is performed.
 */
static DEFINE_PER_CPU(u8, printk_count);
static u8 printk_count_early;
#ifdef CONFIG_HAVE_NMI
static DEFINE_PER_CPU(u8, printk_count_nmi);
static u8 printk_count_nmi_early;
#endif

/*
 * Recursion is limited to keep the output sane. printk() should not require
 * more than 1 level of recursion (allowing, for example, printk() to trigger
 * a WARN), but a higher value is used in case some printk-internal errors
 * exist, such as the ringbuffer validation checks failing.
 */
#define PRINTK_MAX_RECURSION 3

/*
 * Return a pointer to the dedicated counter for the CPU+context of the
 * caller.
 */
static u8 *__printk_recursion_counter(void)
{
#ifdef CONFIG_HAVE_NMI
        if (in_nmi()) {
                if (printk_percpu_data_ready())
                        return this_cpu_ptr(&printk_count_nmi);
                return &printk_count_nmi_early;
        }
#endif
        if (printk_percpu_data_ready())
                return this_cpu_ptr(&printk_count);
        return &printk_count_early;
}

/*
 * Enter recursion tracking. Interrupts are disabled to simplify tracking.
 * The caller must check the boolean return value to see if the recursion is
 * allowed. On failure, interrupts are not disabled.
 *
 * @recursion_ptr must be a variable of type (u8 *) and is the same variable
 * that is passed to printk_exit_irqrestore().
 */
#define printk_enter_irqsave(recursion_ptr, flags)      \
({                                                      \
        bool success = true;                            \
                                                        \
        typecheck(u8 *, recursion_ptr);                 \
        local_irq_save(flags);                          \
        (recursion_ptr) = __printk_recursion_counter(); \
        if (*(recursion_ptr) > PRINTK_MAX_RECURSION) {  \
                local_irq_restore(flags);               \
                success = false;                        \
        } else {                                        \
                (*(recursion_ptr))++;                   \
        }                                               \
        success;                                        \
})

/* Exit recursion tracking, restoring interrupts. */
#define printk_exit_irqrestore(recursion_ptr, flags)    \
        do {                                            \
                typecheck(u8 *, recursion_ptr);         \
                (*(recursion_ptr))--;                   \
                local_irq_restore(flags);               \
        } while (0)

int printk_delay_msec __read_mostly;

static inline void printk_delay(int level)
{
        boot_delay_msec(level);

        if (unlikely(printk_delay_msec)) {
                int m = printk_delay_msec;

                while (m--) {
                        mdelay(1);
                        touch_nmi_watchdog();
                }
        }
}

static inline u32 printk_caller_id(void)
{
        return in_task() ? task_pid_nr(current) :
                0x80000000 + smp_processor_id();
}

/**
 * printk_parse_prefix - Parse level and control flags.
 *
 * @text:     The terminated text message.
 * @level:    A pointer to the current level value, will be updated.
 * @flags:    A pointer to the current printk_info flags, will be updated.
 *
 * @level may be NULL if the caller is not interested in the parsed value.
 * Otherwise the variable pointed to by @level must be set to
 * LOGLEVEL_DEFAULT in order to be updated with the parsed value.
 *
 * @flags may be NULL if the caller is not interested in the parsed value.
 * Otherwise the variable pointed to by @flags will be OR'd with the parsed
 * value.
 *
 * Return: The length of the parsed level and control flags.
 */
u16 printk_parse_prefix(const char *text, int *level,
                        enum printk_info_flags *flags)
{
        u16 prefix_len = 0;
        int kern_level;

        while (*text) {
                kern_level = printk_get_level(text);
                if (!kern_level)
                        break;

                switch (kern_level) {
                case '0' ... '7':
                        if (level && *level == LOGLEVEL_DEFAULT)
                                *level = kern_level - '0';
                        break;
                case 'c':       /* KERN_CONT */
                        if (flags)
                                *flags |= LOG_CONT;
                }

                prefix_len += 2;
                text += 2;
        }

        return prefix_len;
}

__printf(5, 0)
static u16 printk_sprint(char *text, u16 size, int facility,
                         enum printk_info_flags *flags, const char *fmt,
                         va_list args)
{
        u16 text_len;

        text_len = vscnprintf(text, size, fmt, args);

        /* Mark and strip a trailing newline. */
        if (text_len && text[text_len - 1] == '\n') {
                text_len--;
                *flags |= LOG_NEWLINE;
        }

        /* Strip log level and control flags. */
        if (facility == 0) {
                u16 prefix_len;

                prefix_len = printk_parse_prefix(text, NULL, NULL);
                if (prefix_len) {
                        text_len -= prefix_len;
                        memmove(text, text + prefix_len, text_len);
                }
        }

        trace_console(text, text_len);

        return text_len;
}

__printf(4, 0)
int vprintk_store(int facility, int level,
                  const struct dev_printk_info *dev_info,
                  const char *fmt, va_list args)
{
        struct prb_reserved_entry e;
        enum printk_info_flags flags = 0;
        struct printk_record r;
        unsigned long irqflags;
        u16 trunc_msg_len = 0;
        char prefix_buf[8];
        u8 *recursion_ptr;
        u16 reserve_size;
        va_list args2;
        u32 caller_id;
        u16 text_len;
        int ret = 0;
        u64 ts_nsec;

        if (!printk_enter_irqsave(recursion_ptr, irqflags))
                return 0;

        /*
         * Since the duration of printk() can vary depending on the message
         * and state of the ringbuffer, grab the timestamp now so that it is
         * close to the call of printk(). This provides a more deterministic
         * timestamp with respect to the caller.
         */
        ts_nsec = local_clock();

        caller_id = printk_caller_id();

        /*
         * The sprintf needs to come first since the syslog prefix might be
         * passed in as a parameter. An extra byte must be reserved so that
         * later the vscnprintf() into the reserved buffer has room for the
         * terminating '\0', which is not counted by vsnprintf().
         */
        va_copy(args2, args);
        reserve_size = vsnprintf(&prefix_buf[0], sizeof(prefix_buf), fmt, args2) + 1;
        va_end(args2);

        if (reserve_size > PRINTKRB_RECORD_MAX)
                reserve_size = PRINTKRB_RECORD_MAX;

        /* Extract log level or control flags. */
        if (facility == 0)
                printk_parse_prefix(&prefix_buf[0], &level, &flags);

        if (level == LOGLEVEL_DEFAULT)
                level = default_message_loglevel;

        if (dev_info)
                flags |= LOG_NEWLINE;

        if (flags & LOG_CONT) {
                prb_rec_init_wr(&r, reserve_size);
                if (prb_reserve_in_last(&e, prb, &r, caller_id, PRINTKRB_RECORD_MAX)) {
                        text_len = printk_sprint(&r.text_buf[r.info->text_len], reserve_size,
                                                 facility, &flags, fmt, args);
                        r.info->text_len += text_len;

                        if (flags & LOG_NEWLINE) {
                                r.info->flags |= LOG_NEWLINE;
                                prb_final_commit(&e);
                        } else {
                                prb_commit(&e);
                        }

                        ret = text_len;
                        goto out;
                }
        }

        /*
         * Explicitly initialize the record before every prb_reserve() call.
         * prb_reserve_in_last() and prb_reserve() purposely invalidate the
         * structure when they fail.
         */
        prb_rec_init_wr(&r, reserve_size);
        if (!prb_reserve(&e, prb, &r)) {
                /* truncate the message if it is too long for empty buffer */
                truncate_msg(&reserve_size, &trunc_msg_len);

                prb_rec_init_wr(&r, reserve_size + trunc_msg_len);
                if (!prb_reserve(&e, prb, &r))
                        goto out;
        }

        /* fill message */
        text_len = printk_sprint(&r.text_buf[0], reserve_size, facility, &flags, fmt, args);
        if (trunc_msg_len)
                memcpy(&r.text_buf[text_len], trunc_msg, trunc_msg_len);
        r.info->text_len = text_len + trunc_msg_len;
        r.info->facility = facility;
        r.info->level = level & 7;
        r.info->flags = flags & 0x1f;
        r.info->ts_nsec = ts_nsec;
        r.info->caller_id = caller_id;
        if (dev_info)
                memcpy(&r.info->dev_info, dev_info, sizeof(r.info->dev_info));

        /* A message without a trailing newline can be continued. */
        if (!(flags & LOG_NEWLINE))
                prb_commit(&e);
        else
                prb_final_commit(&e);

        ret = text_len + trunc_msg_len;
out:
        printk_exit_irqrestore(recursion_ptr, irqflags);
        return ret;
}

asmlinkage int vprintk_emit(int facility, int level,
                            const struct dev_printk_info *dev_info,
                            const char *fmt, va_list args)
{
        int printed_len;
        bool in_sched = false;

        /* Suppress unimportant messages after panic happens */
        if (unlikely(suppress_printk))
                return 0;

        if (unlikely(suppress_panic_printk) &&
            atomic_read(&panic_cpu) != raw_smp_processor_id())
                return 0;

        if (level == LOGLEVEL_SCHED) {
                level = LOGLEVEL_DEFAULT;
                in_sched = true;
        }

        printk_delay(level);

        printed_len = vprintk_store(facility, level, dev_info, fmt, args);

        /* If called from the scheduler, we can not call up(). */
        if (!in_sched) {
                /*
                 * The caller may be holding system-critical or
                 * timing-sensitive locks. Disable preemption during
                 * printing of all remaining records to all consoles so that
                 * this context can return as soon as possible. Hopefully
                 * another printk() caller will take over the printing.
                 */
                preempt_disable();
                /*
                 * Try to acquire and then immediately release the console
                 * semaphore. The release will print out buffers. With the
                 * spinning variant, this context tries to take over the
                 * printing from another printing context.
                 */
                if (console_trylock_spinning())
                        console_unlock();
                preempt_enable();
        }

        wake_up_klogd();
        return printed_len;
}
EXPORT_SYMBOL(vprintk_emit);

int vprintk_default(const char *fmt, va_list args)
{
        return vprintk_emit(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
}
EXPORT_SYMBOL_GPL(vprintk_default);

asmlinkage __visible int _printk(const char *fmt, ...)
{
        va_list args;
        int r;

        va_start(args, fmt);
        r = vprintk(fmt, args);
        va_end(args);

        return r;
}
EXPORT_SYMBOL(_printk);

static bool pr_flush(int timeout_ms, bool reset_on_progress);
static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress);

#else /* CONFIG_PRINTK */

#define printk_time             false

#define prb_read_valid(rb, seq, r)      false
#define prb_first_valid_seq(rb)         0
#define prb_next_seq(rb)                0

static u64 syslog_seq;

static size_t record_print_text(const struct printk_record *r,
                                bool syslog, bool time)
{
        return 0;
}
static ssize_t info_print_ext_header(char *buf, size_t size,
                                     struct printk_info *info)
{
        return 0;
}
static ssize_t msg_print_ext_body(char *buf, size_t size,
                                  char *text, size_t text_len,
                                  struct dev_printk_info *dev_info) { return 0; }
static void console_lock_spinning_enable(void) { }
static int console_lock_spinning_disable_and_check(int cookie) { return 0; }
static bool suppress_message_printing(int level) { return false; }
static bool pr_flush(int timeout_ms, bool reset_on_progress) { return true; }
static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress) { return true; }

#endif /* CONFIG_PRINTK */

#ifdef CONFIG_EARLY_PRINTK
struct console *early_console;

asmlinkage __visible void early_printk(const char *fmt, ...)
{
        va_list ap;
        char buf[512];
        int n;

        if (!early_console)
                return;

        va_start(ap, fmt);
        n = vscnprintf(buf, sizeof(buf), fmt, ap);
        va_end(ap);

        early_console->write(early_console, buf, n);
}
#endif

static void set_user_specified(struct console_cmdline *c, bool user_specified)
{
        if (!user_specified)
                return;

        /*
         * @c console was defined by the user on the command line.
         * Do not clear when added twice also by SPCR or the device tree.
         */
        c->user_specified = true;
        /* At least one console defined by the user on the command line. */
        console_set_on_cmdline = 1;
}

static int __add_preferred_console(char *name, int idx, char *options,
                                   char *brl_options, bool user_specified)
{
        struct console_cmdline *c;
        int i;

        /*
         *      See if this tty is not yet registered, and
         *      if we have a slot free.
         */
        for (i = 0, c = console_cmdline;
             i < MAX_CMDLINECONSOLES && c->name[0];
             i++, c++) {
                if (strcmp(c->name, name) == 0 && c->index == idx) {
                        if (!brl_options)
                                preferred_console = i;
                        set_user_specified(c, user_specified);
                        return 0;
                }
        }
        if (i == MAX_CMDLINECONSOLES)
                return -E2BIG;
        if (!brl_options)
                preferred_console = i;
        strscpy(c->name, name, sizeof(c->name));
        c->options = options;
        set_user_specified(c, user_specified);
        braille_set_options(c, brl_options);

        c->index = idx;
        return 0;
}

static int __init console_msg_format_setup(char *str)
{
        if (!strcmp(str, "syslog"))
                console_msg_format = MSG_FORMAT_SYSLOG;
        if (!strcmp(str, "default"))
                console_msg_format = MSG_FORMAT_DEFAULT;
        return 1;
}
__setup("console_msg_format=", console_msg_format_setup);

/*
 * Set up a console.  Called via do_early_param() in init/main.c
 * for each "console=" parameter in the boot command line.
 */
static int __init console_setup(char *str)
{
        char buf[sizeof(console_cmdline[0].name) + 4]; /* 4 for "ttyS" */
        char *s, *options, *brl_options = NULL;
        int idx;

        /*
         * console="" or console=null have been suggested as a way to
         * disable console output. Use ttynull that has been created
         * for exactly this purpose.
         */
        if (str[0] == 0 || strcmp(str, "null") == 0) {
                __add_preferred_console("ttynull", 0, NULL, NULL, true);
                return 1;
        }

        if (_braille_console_setup(&str, &brl_options))
                return 1;

        /*
         * Decode str into name, index, options.
         */
        if (str[0] >= '0' && str[0] <= '9') {
                strcpy(buf, "ttyS");
                strncpy(buf + 4, str, sizeof(buf) - 5);
        } else {
                strncpy(buf, str, sizeof(buf) - 1);
        }
        buf[sizeof(buf) - 1] = 0;
        options = strchr(str, ',');
        if (options)
                *(options++) = 0;
#ifdef __sparc__
        if (!strcmp(str, "ttya"))
                strcpy(buf, "ttyS0");
        if (!strcmp(str, "ttyb"))
                strcpy(buf, "ttyS1");
#endif
        for (s = buf; *s; s++)
                if (isdigit(*s) || *s == ',')
                        break;
        idx = simple_strtoul(s, NULL, 10);
        *s = 0;

        __add_preferred_console(buf, idx, options, brl_options, true);
        return 1;
}
__setup("console=", console_setup);

/**
 * add_preferred_console - add a device to the list of preferred consoles.
 * @name: device name
 * @idx: device index
 * @options: options for this console
 *
 * The last preferred console added will be used for kernel messages
 * and stdin/out/err for init.  Normally this is used by console_setup
 * above to handle user-supplied console arguments; however it can also
 * be used by arch-specific code either to override the user or more
 * commonly to provide a default console (ie from PROM variables) when
 * the user has not supplied one.
 */
int add_preferred_console(char *name, int idx, char *options)
{
        return __add_preferred_console(name, idx, options, NULL, false);
}

bool console_suspend_enabled = true;
EXPORT_SYMBOL(console_suspend_enabled);

static int __init console_suspend_disable(char *str)
{
        console_suspend_enabled = false;
        return 1;
}
__setup("no_console_suspend", console_suspend_disable);
module_param_named(console_suspend, console_suspend_enabled,
                bool, S_IRUGO | S_IWUSR);
MODULE_PARM_DESC(console_suspend, "suspend console during suspend"
        " and hibernate operations");

static bool printk_console_no_auto_verbose;

void console_verbose(void)
{
        if (console_loglevel && !printk_console_no_auto_verbose)
                console_loglevel = CONSOLE_LOGLEVEL_MOTORMOUTH;
}
EXPORT_SYMBOL_GPL(console_verbose);

module_param_named(console_no_auto_verbose, printk_console_no_auto_verbose, bool, 0644);
MODULE_PARM_DESC(console_no_auto_verbose, "Disable console loglevel raise to highest on oops/panic/etc");

/**
 * suspend_console - suspend the console subsystem
 *
 * This disables printk() while we go into suspend states
 */
void suspend_console(void)
{
        if (!console_suspend_enabled)
                return;
        pr_info("Suspending console(s) (use no_console_suspend to debug)\n");
        pr_flush(1000, true);
        console_lock();
        console_suspended = 1;
        up_console_sem();
}

void resume_console(void)
{
        if (!console_suspend_enabled)
                return;
        down_console_sem();
        console_suspended = 0;
        console_unlock();
        pr_flush(1000, true);
}

/**
 * console_cpu_notify - print deferred console messages after CPU hotplug
 * @cpu: unused
 *
 * If printk() is called from a CPU that is not online yet, the messages
 * will be printed on the console only if there are CON_ANYTIME consoles.
 * This function is called when a new CPU comes online (or fails to come
 * up) or goes offline.
 */
static int console_cpu_notify(unsigned int cpu)
{
        if (!cpuhp_tasks_frozen) {
                /* If trylock fails, someone else is doing the printing */
                if (console_trylock())
                        console_unlock();
        }
        return 0;
}

/**
 * console_lock - block the console subsystem from printing
 *
 * Acquires a lock which guarantees that no consoles will
 * be in or enter their write() callback.
 *
 * Can sleep, returns nothing.
 */
void console_lock(void)
{
        might_sleep();

        down_console_sem();
        if (console_suspended)
                return;
        console_locked = 1;
        console_may_schedule = 1;
}
EXPORT_SYMBOL(console_lock);

/**
 * console_trylock - try to block the console subsystem from printing
 *
 * Try to acquire a lock which guarantees that no consoles will
 * be in or enter their write() callback.
 *
 * returns 1 on success, and 0 on failure to acquire the lock.
 */
int console_trylock(void)
{
        if (down_trylock_console_sem())
                return 0;
        if (console_suspended) {
                up_console_sem();
                return 0;
        }
        console_locked = 1;
        console_may_schedule = 0;
        return 1;
}
EXPORT_SYMBOL(console_trylock);

int is_console_locked(void)
{
        return console_locked;
}
EXPORT_SYMBOL(is_console_locked);

/*
 * Return true when this CPU should unlock console_sem without pushing all
 * messages to the console. This reduces the chance that the console is
 * locked when the panic CPU tries to use it.
 */
static bool abandon_console_lock_in_panic(void)
{
        if (!panic_in_progress())
                return false;

        /*
         * We can use raw_smp_processor_id() here because it is impossible for
         * the task to be migrated to the panic_cpu, or away from it. If
         * panic_cpu has already been set, and we're not currently executing on
         * that CPU, then we never will be.
         */
        return atomic_read(&panic_cpu) != raw_smp_processor_id();
}

/*
 * Check if the given console is currently capable and allowed to print
 * records.
 *
 * Requires the console_srcu_read_lock.
 */
static inline bool console_is_usable(struct console *con)
{
        short flags = console_srcu_read_flags(con);

        if (!(flags & CON_ENABLED))
                return false;

        if (!con->write)
                return false;

        /*
         * Console drivers may assume that per-cpu resources have been
         * allocated. So unless they're explicitly marked as being able to
         * cope (CON_ANYTIME) don't call them until this CPU is officially up.
         */
        if (!cpu_online(raw_smp_processor_id()) && !(flags & CON_ANYTIME))
                return false;

        return true;
}

static void __console_unlock(void)
{
        console_locked = 0;
        up_console_sem();
}

/*
 * Prepend the message in @pmsg->pbufs->outbuf with a "dropped message". This
 * is achieved by shifting the existing message over and inserting the dropped
 * message.
 *
 * @pmsg is the printk message to prepend.
 *
 * @dropped is the dropped count to report in the dropped message.
 *
 * If the message text in @pmsg->pbufs->outbuf does not have enough space for
 * the dropped message, the message text will be sufficiently truncated.
 *
 * If @pmsg->pbufs->outbuf is modified, @pmsg->outbuf_len is updated.
 */
#ifdef CONFIG_PRINTK
static void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped)
{
        struct printk_buffers *pbufs = pmsg->pbufs;
        const size_t scratchbuf_sz = sizeof(pbufs->scratchbuf);
        const size_t outbuf_sz = sizeof(pbufs->outbuf);
        char *scratchbuf = &pbufs->scratchbuf[0];
        char *outbuf = &pbufs->outbuf[0];
        size_t len;

        len = scnprintf(scratchbuf, scratchbuf_sz,
                       "** %lu printk messages dropped **\n", dropped);

        /*
         * Make sure outbuf is sufficiently large before prepending.
         * Keep at least the prefix when the message must be truncated.
         * It is a rather theoretical problem when someone tries to
         * use a minimalist buffer.
         */
        if (WARN_ON_ONCE(len + PRINTK_PREFIX_MAX >= outbuf_sz))
                return;

        if (pmsg->outbuf_len + len >= outbuf_sz) {
                /* Truncate the message, but keep it terminated. */
                pmsg->outbuf_len = outbuf_sz - (len + 1);
                outbuf[pmsg->outbuf_len] = 0;
        }

        memmove(outbuf + len, outbuf, pmsg->outbuf_len + 1);
        memcpy(outbuf, scratchbuf, len);
        pmsg->outbuf_len += len;
}
#else
#define console_prepend_dropped(pmsg, dropped)
#endif /* CONFIG_PRINTK */

/*
 * Read and format the specified record (or a later record if the specified
 * record is not available).
 *
 * @pmsg will contain the formatted result. @pmsg->pbufs must point to a
 * struct printk_buffers.
 *
 * @seq is the record to read and format. If it is not available, the next
 * valid record is read.
 *
 * @is_extended specifies if the message should be formatted for extended
 * console output.
 *
 * @may_supress specifies if records may be skipped based on loglevel.
 *
 * Returns false if no record is available. Otherwise true and all fields
 * of @pmsg are valid. (See the documentation of struct printk_message
 * for information about the @pmsg fields.)
 */
static bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
                                    bool is_extended, bool may_suppress)
{
        static int panic_console_dropped;

        struct printk_buffers *pbufs = pmsg->pbufs;
        const size_t scratchbuf_sz = sizeof(pbufs->scratchbuf);
        const size_t outbuf_sz = sizeof(pbufs->outbuf);
        char *scratchbuf = &pbufs->scratchbuf[0];
        char *outbuf = &pbufs->outbuf[0];
        struct printk_info info;
        struct printk_record r;
        size_t len = 0;

        /*
         * Formatting extended messages requires a separate buffer, so use the
         * scratch buffer to read in the ringbuffer text.
         *
         * Formatting normal messages is done in-place, so read the ringbuffer
         * text directly into the output buffer.
         */
        if (is_extended)
                prb_rec_init_rd(&r, &info, scratchbuf, scratchbuf_sz);
        else
                prb_rec_init_rd(&r, &info, outbuf, outbuf_sz);

        if (!prb_read_valid(prb, seq, &r))
                return false;

        pmsg->seq = r.info->seq;
        pmsg->dropped = r.info->seq - seq;

        /*
         * Check for dropped messages in panic here so that printk
         * suppression can occur as early as possible if necessary.
         */
        if (pmsg->dropped &&
            panic_in_progress() &&
            panic_console_dropped++ > 10) {
                suppress_panic_printk = 1;
                pr_warn_once("Too many dropped messages. Suppress messages on non-panic CPUs to prevent livelock.\n");
        }

        /* Skip record that has level above the console loglevel. */
        if (may_suppress && suppress_message_printing(r.info->level))
                goto out;

        if (is_extended) {
                len = info_print_ext_header(outbuf, outbuf_sz, r.info);
                len += msg_print_ext_body(outbuf + len, outbuf_sz - len,
                                          &r.text_buf[0], r.info->text_len, &r.info->dev_info);
        } else {
                len = record_print_text(&r, console_msg_format & MSG_FORMAT_SYSLOG, printk_time);
        }
out:
        pmsg->outbuf_len = len;
        return true;
}

/*
 * Print one record for the given console. The record printed is whatever
 * record is the next available record for the given console.
 *
 * @handover will be set to true if a printk waiter has taken over the
 * console_lock, in which case the caller is no longer holding both the
 * console_lock and the SRCU read lock. Otherwise it is set to false.
 *
 * @cookie is the cookie from the SRCU read lock.
 *
 * Returns false if the given console has no next record to print, otherwise
 * true.
 *
 * Requires the console_lock and the SRCU read lock.
 */
static bool console_emit_next_record(struct console *con, bool *handover, int cookie)
{
        static struct printk_buffers pbufs;

        bool is_extended = console_srcu_read_flags(con) & CON_EXTENDED;
        char *outbuf = &pbufs.outbuf[0];
        struct printk_message pmsg = {
                .pbufs = &pbufs,
        };
        unsigned long flags;

        *handover = false;

        if (!printk_get_next_message(&pmsg, con->seq, is_extended, true))
                return false;

        con->dropped += pmsg.dropped;

        /* Skip messages of formatted length 0. */
        if (pmsg.outbuf_len == 0) {
                con->seq = pmsg.seq + 1;
                goto skip;
        }

        if (con->dropped && !is_extended) {
                console_prepend_dropped(&pmsg, con->dropped);
                con->dropped = 0;
        }

        /*
         * While actively printing out messages, if another printk()
         * were to occur on another CPU, it may wait for this one to
         * finish. This task can not be preempted if there is a
         * waiter waiting to take over.
         *
         * Interrupts are disabled because the hand over to a waiter
         * must not be interrupted until the hand over is completed
         * (@console_waiter is cleared).
         */
        printk_safe_enter_irqsave(flags);
        console_lock_spinning_enable();

        /* Do not trace print latency. */
        stop_critical_timings();

        /* Write everything out to the hardware. */
        con->write(con, outbuf, pmsg.outbuf_len);

        start_critical_timings();

        con->seq = pmsg.seq + 1;

        *handover = console_lock_spinning_disable_and_check(cookie);
        printk_safe_exit_irqrestore(flags);
skip:
        return true;
}

/*
 * Print out all remaining records to all consoles.
 *
 * @do_cond_resched is set by the caller. It can be true only in schedulable
 * context.
 *
 * @next_seq is set to the sequence number after the last available record.
 * The value is valid only when this function returns true. It means that all
 * usable consoles are completely flushed.
 *
 * @handover will be set to true if a printk waiter has taken over the
 * console_lock, in which case the caller is no longer holding the
 * console_lock. Otherwise it is set to false.
 *
 * Returns true when there was at least one usable console and all messages
 * were flushed to all usable consoles. A returned false informs the caller
 * that everything was not flushed (either there were no usable consoles or
 * another context has taken over printing or it is a panic situation and this
 * is not the panic CPU). Regardless the reason, the caller should assume it
 * is not useful to immediately try again.
 *
 * Requires the console_lock.
 */
static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handover)
{
        bool any_usable = false;
        struct console *con;
        bool any_progress;
        int cookie;

        *next_seq = 0;
        *handover = false;

        do {
                any_progress = false;

                cookie = console_srcu_read_lock();
                for_each_console_srcu(con) {
                        bool progress;

                        if (!console_is_usable(con))
                                continue;
                        any_usable = true;

                        progress = console_emit_next_record(con, handover, cookie);

                        /*
                         * If a handover has occurred, the SRCU read lock
                         * is already released.
                         */
                        if (*handover)
                                return false;

                        /* Track the next of the highest seq flushed. */
                        if (con->seq > *next_seq)
                                *next_seq = con->seq;

                        if (!progress)
                                continue;
                        any_progress = true;

                        /* Allow panic_cpu to take over the consoles safely. */
                        if (abandon_console_lock_in_panic())
                                goto abandon;

                        if (do_cond_resched)
                                cond_resched();
                }
                console_srcu_read_unlock(cookie);
        } while (any_progress);

        return any_usable;

abandon:
        console_srcu_read_unlock(cookie);
        return false;
}

/**
 * console_unlock - unblock the console subsystem from printing
 *
 * Releases the console_lock which the caller holds to block printing of
 * the console subsystem.
 *
 * While the console_lock was held, console output may have been buffered
 * by printk().  If this is the case, console_unlock(); emits
 * the output prior to releasing the lock.
 *
 * console_unlock(); may be called from any context.
 */
void console_unlock(void)
{
        bool do_cond_resched;
        bool handover;
        bool flushed;
        u64 next_seq;

        if (console_suspended) {
                up_console_sem();
                return;
        }

        /*
         * Console drivers are called with interrupts disabled, so
         * @console_may_schedule should be cleared before; however, we may
         * end up dumping a lot of lines, for example, if called from
         * console registration path, and should invoke cond_resched()
         * between lines if allowable.  Not doing so can cause a very long
         * scheduling stall on a slow console leading to RCU stall and
         * softlockup warnings which exacerbate the issue with more
         * messages practically incapacitating the system. Therefore, create
         * a local to use for the printing loop.
         */
        do_cond_resched = console_may_schedule;

        do {
                console_may_schedule = 0;

                flushed = console_flush_all(do_cond_resched, &next_seq, &handover);
                if (!handover)
                        __console_unlock();

                /*
                 * Abort if there was a failure to flush all messages to all
                 * usable consoles. Either it is not possible to flush (in
                 * which case it would be an infinite loop of retrying) or
                 * another context has taken over printing.
                 */
                if (!flushed)
                        break;

                /*
                 * Some context may have added new records after
                 * console_flush_all() but before unlocking the console.
                 * Re-check if there is a new record to flush. If the trylock
                 * fails, another context is already handling the printing.
                 */
        } while (prb_read_valid(prb, next_seq, NULL) && console_trylock());
}
EXPORT_SYMBOL(console_unlock);

/**
 * console_conditional_schedule - yield the CPU if required
 *
 * If the console code is currently allowed to sleep, and
 * if this CPU should yield the CPU to another task, do
 * so here.
 *
 * Must be called within console_lock();.
 */
void __sched console_conditional_schedule(void)
{
        if (console_may_schedule)
                cond_resched();
}
EXPORT_SYMBOL(console_conditional_schedule);

void console_unblank(void)
{
        struct console *c;
        int cookie;

        /*
         * Stop console printing because the unblank() callback may
         * assume the console is not within its write() callback.
         *
         * If @oops_in_progress is set, this may be an atomic context.
         * In that case, attempt a trylock as best-effort.
         */
        if (oops_in_progress) {
                if (down_trylock_console_sem() != 0)
                        return;
        } else
                console_lock();

        console_locked = 1;
        console_may_schedule = 0;

        cookie = console_srcu_read_lock();
        for_each_console_srcu(c) {
                if ((console_srcu_read_flags(c) & CON_ENABLED) && c->unblank)
                        c->unblank();
        }
        console_srcu_read_unlock(cookie);

        console_unlock();

        if (!oops_in_progress)
                pr_flush(1000, true);
}

/**
 * console_flush_on_panic - flush console content on panic
 * @mode: flush all messages in buffer or just the pending ones
 *
 * Immediately output all pending messages no matter what.
 */
void console_flush_on_panic(enum con_flush_mode mode)
{
        /*
         * If someone else is holding the console lock, trylock will fail
         * and may_schedule may be set.  Ignore and proceed to unlock so
         * that messages are flushed out.  As this can be called from any
         * context and we don't want to get preempted while flushing,
         * ensure may_schedule is cleared.
         */
        console_trylock();
        console_may_schedule = 0;

        if (mode == CONSOLE_REPLAY_ALL) {
                struct console *c;
                int cookie;
                u64 seq;

                seq = prb_first_valid_seq(prb);

                cookie = console_srcu_read_lock();
                for_each_console_srcu(c) {
                        /*
                         * If the above console_trylock() failed, this is an
                         * unsynchronized assignment. But in that case, the
                         * kernel is in "hope and pray" mode anyway.
                         */
                        c->seq = seq;
                }
                console_srcu_read_unlock(cookie);
        }
        console_unlock();
}

/*
 * Return the console tty driver structure and its associated index
 */
struct tty_driver *console_device(int *index)
{
        struct console *c;
        struct tty_driver *driver = NULL;
        int cookie;

        /*
         * Take console_lock to serialize device() callback with
         * other console operations. For example, fg_console is
         * modified under console_lock when switching vt.
         */
        console_lock();

        cookie = console_srcu_read_lock();
        for_each_console_srcu(c) {
                if (!c->device)
                        continue;
                driver = c->device(c, index);
                if (driver)
                        break;
        }
        console_srcu_read_unlock(cookie);

        console_unlock();
        return driver;
}

/*
 * Prevent further output on the passed console device so that (for example)
 * serial drivers can disable console output before suspending a port, and can
 * re-enable output afterwards.
 */
void console_stop(struct console *console)
{
        __pr_flush(console, 1000, true);
        console_list_lock();
        console_srcu_write_flags(console, console->flags & ~CON_ENABLED);
        console_list_unlock();

        /*
         * Ensure that all SRCU list walks have completed. All contexts must
         * be able to see that this console is disabled so that (for example)
         * the caller can suspend the port without risk of another context
         * using the port.
         */
        synchronize_srcu(&console_srcu);
}
EXPORT_SYMBOL(console_stop);

void console_start(struct console *console)
{
        console_list_lock();
        console_srcu_write_flags(console, console->flags | CON_ENABLED);
        console_list_unlock();
        __pr_flush(console, 1000, true);
}
EXPORT_SYMBOL(console_start);

static int __read_mostly keep_bootcon;

static int __init keep_bootcon_setup(char *str)
{
        keep_bootcon = 1;
        pr_info("debug: skip boot console de-registration.\n");

        return 0;
}

early_param("keep_bootcon", keep_bootcon_setup);

/*
 * This is called by register_console() to try to match
 * the newly registered console with any of the ones selected
 * by either the command line or add_preferred_console() and
 * setup/enable it.
 *
 * Care need to be taken with consoles that are statically
 * enabled such as netconsole
 */
static int try_enable_preferred_console(struct console *newcon,
                                        bool user_specified)
{
        struct console_cmdline *c;
        int i, err;

        for (i = 0, c = console_cmdline;
             i < MAX_CMDLINECONSOLES && c->name[0];
             i++, c++) {
                if (c->user_specified != user_specified)
                        continue;
                if (!newcon->match ||
                    newcon->match(newcon, c->name, c->index, c->options) != 0) {
                        /* default matching */
                        BUILD_BUG_ON(sizeof(c->name) != sizeof(newcon->name));
                        if (strcmp(c->name, newcon->name) != 0)
                                continue;
                        if (newcon->index >= 0 &&
                            newcon->index != c->index)
                                continue;
                        if (newcon->index < 0)
                                newcon->index = c->index;

                        if (_braille_register_console(newcon, c))
                                return 0;

                        if (newcon->setup &&
                            (err = newcon->setup(newcon, c->options)) != 0)
                                return err;
                }
                newcon->flags |= CON_ENABLED;
                if (i == preferred_console)
                        newcon->flags |= CON_CONSDEV;
                return 0;
        }

        /*
         * Some consoles, such as pstore and netconsole, can be enabled even
         * without matching. Accept the pre-enabled consoles only when match()
         * and setup() had a chance to be called.
         */
        if (newcon->flags & CON_ENABLED && c->user_specified == user_specified)
                return 0;

        return -ENOENT;
}

/* Try to enable the console unconditionally */
static void try_enable_default_console(struct console *newcon)
{
        if (newcon->index < 0)
                newcon->index = 0;

        if (newcon->setup && newcon->setup(newcon, NULL) != 0)
                return;

        newcon->flags |= CON_ENABLED;

        if (newcon->device)
                newcon->flags |= CON_CONSDEV;
}

#define con_printk(lvl, con, fmt, ...)                  \
        printk(lvl pr_fmt("%sconsole [%s%d] " fmt),     \
               (con->flags & CON_BOOT) ? "boot" : "",   \
               con->name, con->index, ##__VA_ARGS__)

static void console_init_seq(struct console *newcon, bool bootcon_registered)
{
        struct console *con;
        bool handover;

        if (newcon->flags & (CON_PRINTBUFFER | CON_BOOT)) {
                /* Get a consistent copy of @syslog_seq. */
                mutex_lock(&syslog_lock);
                newcon->seq = syslog_seq;
                mutex_unlock(&syslog_lock);
        } else {
                /* Begin with next message added to ringbuffer. */
                newcon->seq = prb_next_seq(prb);

                /*
                 * If any enabled boot consoles are due to be unregistered
                 * shortly, some may not be caught up and may be the same
                 * device as @newcon. Since it is not known which boot console
                 * is the same device, flush all consoles and, if necessary,
                 * start with the message of the enabled boot console that is
                 * the furthest behind.
                 */
                if (bootcon_registered && !keep_bootcon) {
                        /*
                         * Hold the console_lock to stop console printing and
                         * guarantee safe access to console->seq.
                         */
                        console_lock();

                        /*
                         * Flush all consoles and set the console to start at
                         * the next unprinted sequence number.
                         */
                        if (!console_flush_all(true, &newcon->seq, &handover)) {
                                /*
                                 * Flushing failed. Just choose the lowest
                                 * sequence of the enabled boot consoles.
                                 */

                                /*
                                 * If there was a handover, this context no
                                 * longer holds the console_lock.
                                 */
                                if (handover)
                                        console_lock();

                                newcon->seq = prb_next_seq(prb);
                                for_each_console(con) {
                                        if ((con->flags & CON_BOOT) &&
                                            (con->flags & CON_ENABLED) &&
                                            con->seq < newcon->seq) {
                                                newcon->seq = con->seq;
                                        }
                                }
                        }

                        console_unlock();
                }
        }
}

#define console_first()                         \
        hlist_entry(console_list.first, struct console, node)

static int unregister_console_locked(struct console *console);

/*
 * The console driver calls this routine during kernel initialization
 * to register the console printing procedure with printk() and to
 * print any messages that were printed by the kernel before the
 * console driver was initialized.
 *
 * This can happen pretty early during the boot process (because of
 * early_printk) - sometimes before setup_arch() completes - be careful
 * of what kernel features are used - they may not be initialised yet.
 *
 * There are two types of consoles - bootconsoles (early_printk) and
 * "real" consoles (everything which is not a bootconsole) which are
 * handled differently.
 *  - Any number of bootconsoles can be registered at any time.
 *  - As soon as a "real" console is registered, all bootconsoles
 *    will be unregistered automatically.
 *  - Once a "real" console is registered, any attempt to register a
 *    bootconsoles will be rejected
 */
void register_console(struct console *newcon)
{
        struct console *con;
        bool bootcon_registered = false;
        bool realcon_registered = false;
        int err;

        console_list_lock();

        for_each_console(con) {
                if (WARN(con == newcon, "console '%s%d' already registered\n",
                                         con->name, con->index)) {
                        goto unlock;
                }

                if (con->flags & CON_BOOT)
                        bootcon_registered = true;
                else
                        realcon_registered = true;
        }

        /* Do not register boot consoles when there already is a real one. */
        if ((newcon->flags & CON_BOOT) && realcon_registered) {
                pr_info("Too late to register bootconsole %s%d\n",
                        newcon->name, newcon->index);
                goto unlock;
        }

        /*
         * See if we want to enable this console driver by default.
         *
         * Nope when a console is preferred by the command line, device
         * tree, or SPCR.
         *
         * The first real console with tty binding (driver) wins. More
         * consoles might get enabled before the right one is found.
         *
         * Note that a console with tty binding will have CON_CONSDEV
         * flag set and will be first in the list.
         */
        if (preferred_console < 0) {
                if (hlist_empty(&console_list) || !console_first()->device ||
                    console_first()->flags & CON_BOOT) {
                        try_enable_default_console(newcon);
                }
        }

        /* See if this console matches one we selected on the command line */
        err = try_enable_preferred_console(newcon, true);

        /* If not, try to match against the platform default(s) */
        if (err == -ENOENT)
                err = try_enable_preferred_console(newcon, false);

        /* printk() messages are not printed to the Braille console. */
        if (err || newcon->flags & CON_BRL)
                goto unlock;

        /*
         * If we have a bootconsole, and are switching to a real console,
         * don't print everything out again, since when the boot console, and
         * the real console are the same physical device, it's annoying to
         * see the beginning boot messages twice
         */
        if (bootcon_registered &&
            ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV)) {
                newcon->flags &= ~CON_PRINTBUFFER;
        }

        newcon->dropped = 0;
        console_init_seq(newcon, bootcon_registered);

        /*
         * Put this console in the list - keep the
         * preferred driver at the head of the list.
         */
        if (hlist_empty(&console_list)) {
                /* Ensure CON_CONSDEV is always set for the head. */
                newcon->flags |= CON_CONSDEV;
                hlist_add_head_rcu(&newcon->node, &console_list);

        } else if (newcon->flags & CON_CONSDEV) {
                /* Only the new head can have CON_CONSDEV set. */
                console_srcu_write_flags(console_first(), console_first()->flags & ~CON_CONSDEV);
                hlist_add_head_rcu(&newcon->node, &console_list);

        } else {
                hlist_add_behind_rcu(&newcon->node, console_list.first);
        }

        /*
         * No need to synchronize SRCU here! The caller does not rely
         * on all contexts being able to see the new console before
         * register_console() completes.
         */

        console_sysfs_notify();

        /*
         * By unregistering the bootconsoles after we enable the real console
         * we get the "console xxx enabled" message on all the consoles -
         * boot consoles, real consoles, etc - this is to ensure that end
         * users know there might be something in the kernel's log buffer that
         * went to the bootconsole (that they do not see on the real console)
         */
        con_printk(KERN_INFO, newcon, "enabled\n");
        if (bootcon_registered &&
            ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV) &&
            !keep_bootcon) {
                struct hlist_node *tmp;

                hlist_for_each_entry_safe(con, tmp, &console_list, node) {
                        if (con->flags & CON_BOOT)
                                unregister_console_locked(con);
                }
        }
unlock:
        console_list_unlock();
}
EXPORT_SYMBOL(register_console);

/* Must be called under console_list_lock(). */
static int unregister_console_locked(struct console *console)
{
        int res;

        lockdep_assert_console_list_lock_held();

        con_printk(KERN_INFO, console, "disabled\n");

        res = _braille_unregister_console(console);
        if (res < 0)
                return res;
        if (res > 0)
                return 0;

        /* Disable it unconditionally */
        console_srcu_write_flags(console, console->flags & ~CON_ENABLED);

        if (!console_is_registered_locked(console))
                return -ENODEV;

        hlist_del_init_rcu(&console->node);

        /*
         * <HISTORICAL>
         * If this isn't the last console and it has CON_CONSDEV set, we
         * need to set it on the next preferred console.
         * </HISTORICAL>
         *
         * The above makes no sense as there is no guarantee that the next
         * console has any device attached. Oh well....
         */
        if (!hlist_empty(&console_list) && console->flags & CON_CONSDEV)
                console_srcu_write_flags(console_first(), console_first()->flags | CON_CONSDEV);

        /*
         * Ensure that all SRCU list walks have completed. All contexts
         * must not be able to see this console in the list so that any
         * exit/cleanup routines can be performed safely.
         */
        synchronize_srcu(&console_srcu);

        console_sysfs_notify();

        if (console->exit)
                res = console->exit(console);

        return res;
}

int unregister_console(struct console *console)
{
        int res;

        console_list_lock();
        res = unregister_console_locked(console);
        console_list_unlock();
        return res;
}
EXPORT_SYMBOL(unregister_console);

/**
 * console_force_preferred_locked - force a registered console preferred
 * @con: The registered console to force preferred.
 *
 * Must be called under console_list_lock().
 */
void console_force_preferred_locked(struct console *con)
{
        struct console *cur_pref_con;

        if (!console_is_registered_locked(con))
                return;

        cur_pref_con = console_first();

        /* Already preferred? */
        if (cur_pref_con == con)
                return;

        /*
         * Delete, but do not re-initialize the entry. This allows the console
         * to continue to appear registered (via any hlist_unhashed_lockless()
         * checks), even though it was briefly removed from the console list.
         */
        hlist_del_rcu(&con->node);

        /*
         * Ensure that all SRCU list walks have completed so that the console
         * can be added to the beginning of the console list and its forward
         * list pointer can be re-initialized.
         */
        synchronize_srcu(&console_srcu);

        con->flags |= CON_CONSDEV;
        WARN_ON(!con->device);

        /* Only the new head can have CON_CONSDEV set. */
        console_srcu_write_flags(cur_pref_con, cur_pref_con->flags & ~CON_CONSDEV);
        hlist_add_head_rcu(&con->node, &console_list);
}
EXPORT_SYMBOL(console_force_preferred_locked);

/*
 * Initialize the console device. This is called *early*, so
 * we can't necessarily depend on lots of kernel help here.
 * Just do some early initializations, and do the complex setup
 * later.
 */
void __init console_init(void)
{
        int ret;
        initcall_t call;
        initcall_entry_t *ce;

        /* Setup the default TTY line discipline. */
        n_tty_init();

        /*
         * set up the console device so that later boot sequences can
         * inform about problems etc..
         */
        ce = __con_initcall_start;
        trace_initcall_level("console");
        while (ce < __con_initcall_end) {
                call = initcall_from_entry(ce);
                trace_initcall_start(call);
                ret = call();
                trace_initcall_finish(call, ret);
                ce++;
        }
}

/*
 * Some boot consoles access data that is in the init section and which will
 * be discarded after the initcalls have been run. To make sure that no code
 * will access this data, unregister the boot consoles in a late initcall.
 *
 * If for some reason, such as deferred probe or the driver being a loadable
 * module, the real console hasn't registered yet at this point, there will
 * be a brief interval in which no messages are logged to the console, which
 * makes it difficult to diagnose problems that occur during this time.
 *
 * To mitigate this problem somewhat, only unregister consoles whose memory
 * intersects with the init section. Note that all other boot consoles will
 * get unregistered when the real preferred console is registered.
 */
static int __init printk_late_init(void)
{
        struct hlist_node *tmp;
        struct console *con;
        int ret;

        console_list_lock();
        hlist_for_each_entry_safe(con, tmp, &console_list, node) {
                if (!(con->flags & CON_BOOT))
                        continue;

                /* Check addresses that might be used for enabled consoles. */
                if (init_section_intersects(con, sizeof(*con)) ||
                    init_section_contains(con->write, 0) ||
                    init_section_contains(con->read, 0) ||
                    init_section_contains(con->device, 0) ||
                    init_section_contains(con->unblank, 0) ||
                    init_section_contains(con->data, 0)) {
                        /*
                         * Please, consider moving the reported consoles out
                         * of the init section.
                         */
                        pr_warn("bootconsole [%s%d] uses init memory and must be disabled even before the real one is ready\n",
                                con->name, con->index);
                        unregister_console_locked(con);
                }
        }
        console_list_unlock();

        ret = cpuhp_setup_state_nocalls(CPUHP_PRINTK_DEAD, "printk:dead", NULL,
                                        console_cpu_notify);
        WARN_ON(ret < 0);
        ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN, "printk:online",
                                        console_cpu_notify, NULL);
        WARN_ON(ret < 0);
        printk_sysctl_init();
        return 0;
}
late_initcall(printk_late_init);

#if defined CONFIG_PRINTK
/* If @con is specified, only wait for that console. Otherwise wait for all. */
static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress)
{
        int remaining = timeout_ms;
        struct console *c;
        u64 last_diff = 0;
        u64 printk_seq;
        int cookie;
        u64 diff;
        u64 seq;

        might_sleep();

        seq = prb_next_seq(prb);

        for (;;) {
                diff = 0;

                /*
                 * Hold the console_lock to guarantee safe access to
                 * console->seq and to prevent changes to @console_suspended
                 * until all consoles have been processed.
                 */
                console_lock();

                cookie = console_srcu_read_lock();
                for_each_console_srcu(c) {
                        if (con && con != c)
                                continue;
                        if (!console_is_usable(c))
                                continue;
                        printk_seq = c->seq;
                        if (printk_seq < seq)
                                diff += seq - printk_seq;
                }
                console_srcu_read_unlock(cookie);

                /*
                 * If consoles are suspended, it cannot be expected that they
                 * make forward progress, so timeout immediately. @diff is
                 * still used to return a valid flush status.
                 */
                if (console_suspended)
                        remaining = 0;
                else if (diff != last_diff && reset_on_progress)
                        remaining = timeout_ms;

                console_unlock();

                if (diff == 0 || remaining == 0)
                        break;

                if (remaining < 0) {
                        /* no timeout limit */
                        msleep(100);
                } else if (remaining < 100) {
                        msleep(remaining);
                        remaining = 0;
                } else {
                        msleep(100);
                        remaining -= 100;
                }

                last_diff = diff;
        }

        return (diff == 0);
}

/**
 * pr_flush() - Wait for printing threads to catch up.
 *
 * @timeout_ms:        The maximum time (in ms) to wait.
 * @reset_on_progress: Reset the timeout if forward progress is seen.
 *
 * A value of 0 for @timeout_ms means no waiting will occur. A value of -1
 * represents infinite waiting.
 *
 * If @reset_on_progress is true, the timeout will be reset whenever any
 * printer has been seen to make some forward progress.
 *
 * Context: Process context. May sleep while acquiring console lock.
 * Return: true if all enabled printers are caught up.
 */
static bool pr_flush(int timeout_ms, bool reset_on_progress)
{
        return __pr_flush(NULL, timeout_ms, reset_on_progress);
}

/*
 * Delayed printk version, for scheduler-internal messages:
 */
#define PRINTK_PENDING_WAKEUP   0x01
#define PRINTK_PENDING_OUTPUT   0x02

static DEFINE_PER_CPU(int, printk_pending);

static void wake_up_klogd_work_func(struct irq_work *irq_work)
{
        int pending = this_cpu_xchg(printk_pending, 0);

        if (pending & PRINTK_PENDING_OUTPUT) {
                /* If trylock fails, someone else is doing the printing */
                if (console_trylock())
                        console_unlock();
        }

        if (pending & PRINTK_PENDING_WAKEUP)
                wake_up_interruptible(&log_wait);
}

static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) =
        IRQ_WORK_INIT_LAZY(wake_up_klogd_work_func);

static void __wake_up_klogd(int val)
{
        if (!printk_percpu_data_ready())
                return;

        preempt_disable();
        /*
         * Guarantee any new records can be seen by tasks preparing to wait
         * before this context checks if the wait queue is empty.
         *
         * The full memory barrier within wq_has_sleeper() pairs with the full
         * memory barrier within set_current_state() of
         * prepare_to_wait_event(), which is called after ___wait_event() adds
         * the waiter but before it has checked the wait condition.
         *
         * This pairs with devkmsg_read:A and syslog_print:A.
         */
        if (wq_has_sleeper(&log_wait) || /* LMM(__wake_up_klogd:A) */
            (val & PRINTK_PENDING_OUTPUT)) {
                this_cpu_or(printk_pending, val);
                irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
        }
        preempt_enable();
}

void wake_up_klogd(void)
{
        __wake_up_klogd(PRINTK_PENDING_WAKEUP);
}

void defer_console_output(void)
{
        /*
         * New messages may have been added directly to the ringbuffer
         * using vprintk_store(), so wake any waiters as well.
         */
        __wake_up_klogd(PRINTK_PENDING_WAKEUP | PRINTK_PENDING_OUTPUT);
}

void printk_trigger_flush(void)
{
        defer_console_output();
}

int vprintk_deferred(const char *fmt, va_list args)
{
        int r;

        r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, fmt, args);
        defer_console_output();

        return r;
}

int _printk_deferred(const char *fmt, ...)
{
        va_list args;
        int r;

        va_start(args, fmt);
        r = vprintk_deferred(fmt, args);
        va_end(args);

        return r;
}

/*
 * printk rate limiting, lifted from the networking subsystem.
 *
 * This enforces a rate limit: not more than 10 kernel messages
 * every 5s to make a denial-of-service attack impossible.
 */
DEFINE_RATELIMIT_STATE(printk_ratelimit_state, 5 * HZ, 10);

int __printk_ratelimit(const char *func)
{
        return ___ratelimit(&printk_ratelimit_state, func);
}
EXPORT_SYMBOL(__printk_ratelimit);

/**
 * printk_timed_ratelimit - caller-controlled printk ratelimiting
 * @caller_jiffies: pointer to caller's state
 * @interval_msecs: minimum interval between prints
 *
 * printk_timed_ratelimit() returns true if more than @interval_msecs
 * milliseconds have elapsed since the last time printk_timed_ratelimit()
 * returned true.
 */
bool printk_timed_ratelimit(unsigned long *caller_jiffies,
                        unsigned int interval_msecs)
{
        unsigned long elapsed = jiffies - *caller_jiffies;

        if (*caller_jiffies && elapsed <= msecs_to_jiffies(interval_msecs))
                return false;

        *caller_jiffies = jiffies;
        return true;
}
EXPORT_SYMBOL(printk_timed_ratelimit);

static DEFINE_SPINLOCK(dump_list_lock);
static LIST_HEAD(dump_list);

/**
 * kmsg_dump_register - register a kernel log dumper.
 * @dumper: pointer to the kmsg_dumper structure
 *
 * Adds a kernel log dumper to the system. The dump callback in the
 * structure will be called when the kernel oopses or panics and must be
 * set. Returns zero on success and %-EINVAL or %-EBUSY otherwise.
 */
int kmsg_dump_register(struct kmsg_dumper *dumper)
{
        unsigned long flags;
        int err = -EBUSY;

        /* The dump callback needs to be set */
        if (!dumper->dump)
                return -EINVAL;

        spin_lock_irqsave(&dump_list_lock, flags);
        /* Don't allow registering multiple times */
        if (!dumper->registered) {
                dumper->registered = 1;
                list_add_tail_rcu(&dumper->list, &dump_list);
                err = 0;
        }
        spin_unlock_irqrestore(&dump_list_lock, flags);

        return err;
}
EXPORT_SYMBOL_GPL(kmsg_dump_register);

/**
 * kmsg_dump_unregister - unregister a kmsg dumper.
 * @dumper: pointer to the kmsg_dumper structure
 *
 * Removes a dump device from the system. Returns zero on success and
 * %-EINVAL otherwise.
 */
int kmsg_dump_unregister(struct kmsg_dumper *dumper)
{
        unsigned long flags;
        int err = -EINVAL;

        spin_lock_irqsave(&dump_list_lock, flags);
        if (dumper->registered) {
                dumper->registered = 0;
                list_del_rcu(&dumper->list);
                err = 0;
        }
        spin_unlock_irqrestore(&dump_list_lock, flags);
        synchronize_rcu();

        return err;
}
EXPORT_SYMBOL_GPL(kmsg_dump_unregister);

static bool always_kmsg_dump;
module_param_named(always_kmsg_dump, always_kmsg_dump, bool, S_IRUGO | S_IWUSR);

const char *kmsg_dump_reason_str(enum kmsg_dump_reason reason)
{
        switch (reason) {
        case KMSG_DUMP_PANIC:
                return "Panic";
        case KMSG_DUMP_OOPS:
                return "Oops";
        case KMSG_DUMP_EMERG:
                return "Emergency";
        case KMSG_DUMP_SHUTDOWN:
                return "Shutdown";
        default:
                return "Unknown";
        }
}
EXPORT_SYMBOL_GPL(kmsg_dump_reason_str);

/**
 * kmsg_dump - dump kernel log to kernel message dumpers.
 * @reason: the reason (oops, panic etc) for dumping
 *
 * Call each of the registered dumper's dump() callback, which can
 * retrieve the kmsg records with kmsg_dump_get_line() or
 * kmsg_dump_get_buffer().
 */
void kmsg_dump(enum kmsg_dump_reason reason)
{
        struct kmsg_dumper *dumper;

        rcu_read_lock();
        list_for_each_entry_rcu(dumper, &dump_list, list) {
                enum kmsg_dump_reason max_reason = dumper->max_reason;

                /*
                 * If client has not provided a specific max_reason, default
                 * to KMSG_DUMP_OOPS, unless always_kmsg_dump was set.
                 */
                if (max_reason == KMSG_DUMP_UNDEF) {
                        max_reason = always_kmsg_dump ? KMSG_DUMP_MAX :
                                                        KMSG_DUMP_OOPS;
                }
                if (reason > max_reason)
                        continue;

                /* invoke dumper which will iterate over records */
                dumper->dump(dumper, reason);
        }
        rcu_read_unlock();
}

/**
 * kmsg_dump_get_line - retrieve one kmsg log line
 * @iter: kmsg dump iterator
 * @syslog: include the "<4>" prefixes
 * @line: buffer to copy the line to
 * @size: maximum size of the buffer
 * @len: length of line placed into buffer
 *
 * Start at the beginning of the kmsg buffer, with the oldest kmsg
 * record, and copy one record into the provided buffer.
 *
 * Consecutive calls will return the next available record moving
 * towards the end of the buffer with the youngest messages.
 *
 * A return value of FALSE indicates that there are no more records to
 * read.
 */
bool kmsg_dump_get_line(struct kmsg_dump_iter *iter, bool syslog,
                        char *line, size_t size, size_t *len)
{
        u64 min_seq = latched_seq_read_nolock(&clear_seq);
        struct printk_info info;
        unsigned int line_count;
        struct printk_record r;
        size_t l = 0;
        bool ret = false;

        if (iter->cur_seq < min_seq)
                iter->cur_seq = min_seq;

        prb_rec_init_rd(&r, &info, line, size);

        /* Read text or count text lines? */
        if (line) {
                if (!prb_read_valid(prb, iter->cur_seq, &r))
                        goto out;
                l = record_print_text(&r, syslog, printk_time);
        } else {
                if (!prb_read_valid_info(prb, iter->cur_seq,
                                         &info, &line_count)) {
                        goto out;
                }
                l = get_record_print_text_size(&info, line_count, syslog,
                                               printk_time);

        }

        iter->cur_seq = r.info->seq + 1;
        ret = true;
out:
        if (len)
                *len = l;
        return ret;
}
EXPORT_SYMBOL_GPL(kmsg_dump_get_line);

/**
 * kmsg_dump_get_buffer - copy kmsg log lines
 * @iter: kmsg dump iterator
 * @syslog: include the "<4>" prefixes
 * @buf: buffer to copy the line to
 * @size: maximum size of the buffer
 * @len_out: length of line placed into buffer
 *
 * Start at the end of the kmsg buffer and fill the provided buffer
 * with as many of the *youngest* kmsg records that fit into it.
 * If the buffer is large enough, all available kmsg records will be
 * copied with a single call.
 *
 * Consecutive calls will fill the buffer with the next block of
 * available older records, not including the earlier retrieved ones.
 *
 * A return value of FALSE indicates that there are no more records to
 * read.
 */
bool kmsg_dump_get_buffer(struct kmsg_dump_iter *iter, bool syslog,
                          char *buf, size_t size, size_t *len_out)
{
        u64 min_seq = latched_seq_read_nolock(&clear_seq);
        struct printk_info info;
        struct printk_record r;
        u64 seq;
        u64 next_seq;
        size_t len = 0;
        bool ret = false;
        bool time = printk_time;

        if (!buf || !size)
                goto out;

        if (iter->cur_seq < min_seq)
                iter->cur_seq = min_seq;

        if (prb_read_valid_info(prb, iter->cur_seq, &info, NULL)) {
                if (info.seq != iter->cur_seq) {
                        /* messages are gone, move to first available one */
                        iter->cur_seq = info.seq;
                }
        }

        /* last entry */
        if (iter->cur_seq >= iter->next_seq)
                goto out;

        /*
         * Find first record that fits, including all following records,
         * into the user-provided buffer for this dump. Pass in size-1
         * because this function (by way of record_print_text()) will
         * not write more than size-1 bytes of text into @buf.
         */
        seq = find_first_fitting_seq(iter->cur_seq, iter->next_seq,
                                     size - 1, syslog, time);

        /*
         * Next kmsg_dump_get_buffer() invocation will dump block of
         * older records stored right before this one.
         */
        next_seq = seq;

        prb_rec_init_rd(&r, &info, buf, size);

        len = 0;
        prb_for_each_record(seq, prb, seq, &r) {
                if (r.info->seq >= iter->next_seq)
                        break;

                len += record_print_text(&r, syslog, time);

                /* Adjust record to store to remaining buffer space. */
                prb_rec_init_rd(&r, &info, buf + len, size - len);
        }

        iter->next_seq = next_seq;
        ret = true;
out:
        if (len_out)
                *len_out = len;
        return ret;
}
EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer);

/**
 * kmsg_dump_rewind - reset the iterator
 * @iter: kmsg dump iterator
 *
 * Reset the dumper's iterator so that kmsg_dump_get_line() and
 * kmsg_dump_get_buffer() can be called again and used multiple
 * times within the same dumper.dump() callback.
 */
void kmsg_dump_rewind(struct kmsg_dump_iter *iter)
{
        iter->cur_seq = latched_seq_read_nolock(&clear_seq);
        iter->next_seq = prb_next_seq(prb);
}
EXPORT_SYMBOL_GPL(kmsg_dump_rewind);

#endif

#ifdef CONFIG_SMP
static atomic_t printk_cpu_sync_owner = ATOMIC_INIT(-1);
static atomic_t printk_cpu_sync_nested = ATOMIC_INIT(0);

/**
 * __printk_cpu_sync_wait() - Busy wait until the printk cpu-reentrant
 *                            spinning lock is not owned by any CPU.
 *
 * Context: Any context.
 */
void __printk_cpu_sync_wait(void)
{
        do {
                cpu_relax();
        } while (atomic_read(&printk_cpu_sync_owner) != -1);
}
EXPORT_SYMBOL(__printk_cpu_sync_wait);

/**
 * __printk_cpu_sync_try_get() - Try to acquire the printk cpu-reentrant
 *                               spinning lock.
 *
 * If no processor has the lock, the calling processor takes the lock and
 * becomes the owner. If the calling processor is already the owner of the
 * lock, this function succeeds immediately.
 *
 * Context: Any context. Expects interrupts to be disabled.
 * Return: 1 on success, otherwise 0.
 */
int __printk_cpu_sync_try_get(void)
{
        int cpu;
        int old;

        cpu = smp_processor_id();

        /*
         * Guarantee loads and stores from this CPU when it is the lock owner
         * are _not_ visible to the previous lock owner. This pairs with
         * __printk_cpu_sync_put:B.
         *
         * Memory barrier involvement:
         *
         * If __printk_cpu_sync_try_get:A reads from __printk_cpu_sync_put:B,
         * then __printk_cpu_sync_put:A can never read from
         * __printk_cpu_sync_try_get:B.
         *
         * Relies on:
         *
         * RELEASE from __printk_cpu_sync_put:A to __printk_cpu_sync_put:B
         * of the previous CPU
         *    matching
         * ACQUIRE from __printk_cpu_sync_try_get:A to
         * __printk_cpu_sync_try_get:B of this CPU
         */
        old = atomic_cmpxchg_acquire(&printk_cpu_sync_owner, -1,
                                     cpu); /* LMM(__printk_cpu_sync_try_get:A) */
        if (old == -1) {
                /*
                 * This CPU is now the owner and begins loading/storing
                 * data: LMM(__printk_cpu_sync_try_get:B)
                 */
                return 1;

        } else if (old == cpu) {
                /* This CPU is already the owner. */
                atomic_inc(&printk_cpu_sync_nested);
                return 1;
        }

        return 0;
}
EXPORT_SYMBOL(__printk_cpu_sync_try_get);

/**
 * __printk_cpu_sync_put() - Release the printk cpu-reentrant spinning lock.
 *
 * The calling processor must be the owner of the lock.
 *
 * Context: Any context. Expects interrupts to be disabled.
 */
void __printk_cpu_sync_put(void)
{
        if (atomic_read(&printk_cpu_sync_nested)) {
                atomic_dec(&printk_cpu_sync_nested);
                return;
        }

        /*
         * This CPU is finished loading/storing data:
         * LMM(__printk_cpu_sync_put:A)
         */

        /*
         * Guarantee loads and stores from this CPU when it was the
         * lock owner are visible to the next lock owner. This pairs
         * with __printk_cpu_sync_try_get:A.
         *
         * Memory barrier involvement:
         *
         * If __printk_cpu_sync_try_get:A reads from __printk_cpu_sync_put:B,
         * then __printk_cpu_sync_try_get:B reads from __printk_cpu_sync_put:A.
         *
         * Relies on:
         *
         * RELEASE from __printk_cpu_sync_put:A to __printk_cpu_sync_put:B
         * of this CPU
         *    matching
         * ACQUIRE from __printk_cpu_sync_try_get:A to
         * __printk_cpu_sync_try_get:B of the next CPU
         */
        atomic_set_release(&printk_cpu_sync_owner,
                           -1); /* LMM(__printk_cpu_sync_put:B) */
}
EXPORT_SYMBOL(__printk_cpu_sync_put);
#endif /* CONFIG_SMP */