locking/ww_mutex: Re-check ww->ctx in the inner optimistic spin loop

[linux.git] / kernel / locking / mutex.c

/*
 * kernel/locking/mutex.c
 *
 * Mutexes: blocking mutual exclusion locks
 *
 * Started by Ingo Molnar:
 *
 *  Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
 *
 * Many thanks to Arjan van de Ven, Thomas Gleixner, Steven Rostedt and
 * David Howells for suggestions and improvements.
 *
 *  - Adaptive spinning for mutexes by Peter Zijlstra. (Ported to mainline
 *    from the -rt tree, where it was originally implemented for rtmutexes
 *    by Steven Rostedt, based on work by Gregory Haskins, Peter Morreale
 *    and Sven Dietrich.
 *
 * Also see Documentation/locking/mutex-design.txt.
 */
#include <linux/mutex.h>
#include <linux/ww_mutex.h>
#include <linux/sched.h>
#include <linux/sched/rt.h>
#include <linux/export.h>
#include <linux/spinlock.h>
#include <linux/interrupt.h>
#include <linux/debug_locks.h>
#include <linux/osq_lock.h>

#ifdef CONFIG_DEBUG_MUTEXES
# include "mutex-debug.h"
#else
# include "mutex.h"
#endif

void
__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
{
        atomic_long_set(&lock->owner, 0);
        spin_lock_init(&lock->wait_lock);
        INIT_LIST_HEAD(&lock->wait_list);
#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
        osq_lock_init(&lock->osq);
#endif

        debug_mutex_init(lock, name, key);
}
EXPORT_SYMBOL(__mutex_init);

/*
 * @owner: contains: 'struct task_struct *' to the current lock owner,
 * NULL means not owned. Since task_struct pointers are aligned at
 * at least L1_CACHE_BYTES, we have low bits to store extra state.
 *
 * Bit0 indicates a non-empty waiter list; unlock must issue a wakeup.
 * Bit1 indicates unlock needs to hand the lock to the top-waiter
 * Bit2 indicates handoff has been done and we're waiting for pickup.
 */
#define MUTEX_FLAG_WAITERS      0x01
#define MUTEX_FLAG_HANDOFF      0x02
#define MUTEX_FLAG_PICKUP       0x04

#define MUTEX_FLAGS             0x07

static inline struct task_struct *__owner_task(unsigned long owner)
{
        return (struct task_struct *)(owner & ~MUTEX_FLAGS);
}

static inline unsigned long __owner_flags(unsigned long owner)
{
        return owner & MUTEX_FLAGS;
}

/*
 * Trylock variant that retuns the owning task on failure.
 */
static inline struct task_struct *__mutex_trylock_or_owner(struct mutex *lock)
{
        unsigned long owner, curr = (unsigned long)current;

        owner = atomic_long_read(&lock->owner);
        for (;;) { /* must loop, can race against a flag */
                unsigned long old, flags = __owner_flags(owner);
                unsigned long task = owner & ~MUTEX_FLAGS;

                if (task) {
                        if (likely(task != curr))
                                break;

                        if (likely(!(flags & MUTEX_FLAG_PICKUP)))
                                break;

                        flags &= ~MUTEX_FLAG_PICKUP;
                } else {
#ifdef CONFIG_DEBUG_MUTEXES
                        DEBUG_LOCKS_WARN_ON(flags & MUTEX_FLAG_PICKUP);
#endif
                }

                /*
                 * We set the HANDOFF bit, we must make sure it doesn't live
                 * past the point where we acquire it. This would be possible
                 * if we (accidentally) set the bit on an unlocked mutex.
                 */
                flags &= ~MUTEX_FLAG_HANDOFF;

                old = atomic_long_cmpxchg_acquire(&lock->owner, owner, curr | flags);
                if (old == owner)
                        return NULL;

                owner = old;
        }

        return __owner_task(owner);
}

/*
 * Actual trylock that will work on any unlocked state.
 */
static inline bool __mutex_trylock(struct mutex *lock)
{
        return !__mutex_trylock_or_owner(lock);
}

#ifndef CONFIG_DEBUG_LOCK_ALLOC
/*
 * Lockdep annotations are contained to the slow paths for simplicity.
 * There is nothing that would stop spreading the lockdep annotations outwards
 * except more code.
 */

/*
 * Optimistic trylock that only works in the uncontended case. Make sure to
 * follow with a __mutex_trylock() before failing.
 */
static __always_inline bool __mutex_trylock_fast(struct mutex *lock)
{
        unsigned long curr = (unsigned long)current;

        if (!atomic_long_cmpxchg_acquire(&lock->owner, 0UL, curr))
                return true;

        return false;
}

static __always_inline bool __mutex_unlock_fast(struct mutex *lock)
{
        unsigned long curr = (unsigned long)current;

        if (atomic_long_cmpxchg_release(&lock->owner, curr, 0UL) == curr)
                return true;

        return false;
}
#endif

static inline void __mutex_set_flag(struct mutex *lock, unsigned long flag)
{
        atomic_long_or(flag, &lock->owner);
}

static inline void __mutex_clear_flag(struct mutex *lock, unsigned long flag)
{
        atomic_long_andnot(flag, &lock->owner);
}

static inline bool __mutex_waiter_is_first(struct mutex *lock, struct mutex_waiter *waiter)
{
        return list_first_entry(&lock->wait_list, struct mutex_waiter, list) == waiter;
}

/*
 * Give up ownership to a specific task, when @task = NULL, this is equivalent
 * to a regular unlock. Sets PICKUP on a handoff, clears HANDOF, preserves
 * WAITERS. Provides RELEASE semantics like a regular unlock, the
 * __mutex_trylock() provides a matching ACQUIRE semantics for the handoff.
 */
static void __mutex_handoff(struct mutex *lock, struct task_struct *task)
{
        unsigned long owner = atomic_long_read(&lock->owner);

        for (;;) {
                unsigned long old, new;

#ifdef CONFIG_DEBUG_MUTEXES
                DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current);
                DEBUG_LOCKS_WARN_ON(owner & MUTEX_FLAG_PICKUP);
#endif

                new = (owner & MUTEX_FLAG_WAITERS);
                new |= (unsigned long)task;
                if (task)
                        new |= MUTEX_FLAG_PICKUP;

                old = atomic_long_cmpxchg_release(&lock->owner, owner, new);
                if (old == owner)
                        break;

                owner = old;
        }
}

#ifndef CONFIG_DEBUG_LOCK_ALLOC
/*
 * We split the mutex lock/unlock logic into separate fastpath and
 * slowpath functions, to reduce the register pressure on the fastpath.
 * We also put the fastpath first in the kernel image, to make sure the
 * branch is predicted by the CPU as default-untaken.
 */
static void __sched __mutex_lock_slowpath(struct mutex *lock);

/**
 * mutex_lock - acquire the mutex
 * @lock: the mutex to be acquired
 *
 * Lock the mutex exclusively for this task. If the mutex is not
 * available right now, it will sleep until it can get it.
 *
 * The mutex must later on be released by the same task that
 * acquired it. Recursive locking is not allowed. The task
 * may not exit without first unlocking the mutex. Also, kernel
 * memory where the mutex resides must not be freed with
 * the mutex still locked. The mutex must first be initialized
 * (or statically defined) before it can be locked. memset()-ing
 * the mutex to 0 is not allowed.
 *
 * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging
 *   checks that will enforce the restrictions and will also do
 *   deadlock debugging. )
 *
 * This function is similar to (but not equivalent to) down().
 */
void __sched mutex_lock(struct mutex *lock)
{
        might_sleep();

        if (!__mutex_trylock_fast(lock))
                __mutex_lock_slowpath(lock);
}
EXPORT_SYMBOL(mutex_lock);
#endif

static __always_inline void
ww_mutex_lock_acquired(struct ww_mutex *ww, struct ww_acquire_ctx *ww_ctx)
{
#ifdef CONFIG_DEBUG_MUTEXES
        /*
         * If this WARN_ON triggers, you used ww_mutex_lock to acquire,
         * but released with a normal mutex_unlock in this call.
         *
         * This should never happen, always use ww_mutex_unlock.
         */
        DEBUG_LOCKS_WARN_ON(ww->ctx);

        /*
         * Not quite done after calling ww_acquire_done() ?
         */
        DEBUG_LOCKS_WARN_ON(ww_ctx->done_acquire);

        if (ww_ctx->contending_lock) {
                /*
                 * After -EDEADLK you tried to
                 * acquire a different ww_mutex? Bad!
                 */
                DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock != ww);

                /*
                 * You called ww_mutex_lock after receiving -EDEADLK,
                 * but 'forgot' to unlock everything else first?
                 */
                DEBUG_LOCKS_WARN_ON(ww_ctx->acquired > 0);
                ww_ctx->contending_lock = NULL;
        }

        /*
         * Naughty, using a different class will lead to undefined behavior!
         */
        DEBUG_LOCKS_WARN_ON(ww_ctx->ww_class != ww->ww_class);
#endif
        ww_ctx->acquired++;
}

static inline bool __sched
__ww_ctx_stamp_after(struct ww_acquire_ctx *a, struct ww_acquire_ctx *b)
{
        return a->stamp - b->stamp <= LONG_MAX &&
               (a->stamp != b->stamp || a > b);
}

/*
 * Wake up any waiters that may have to back off when the lock is held by the
 * given context.
 *
 * Due to the invariants on the wait list, this can only affect the first
 * waiter with a context.
 *
 * The current task must not be on the wait list.
 */
static void __sched
__ww_mutex_wakeup_for_backoff(struct mutex *lock, struct ww_acquire_ctx *ww_ctx)
{
        struct mutex_waiter *cur;

        lockdep_assert_held(&lock->wait_lock);

        list_for_each_entry(cur, &lock->wait_list, list) {
                if (!cur->ww_ctx)
                        continue;

                if (cur->ww_ctx->acquired > 0 &&
                    __ww_ctx_stamp_after(cur->ww_ctx, ww_ctx)) {
                        debug_mutex_wake_waiter(lock, cur);
                        wake_up_process(cur->task);
                }

                break;
        }
}

/*
 * After acquiring lock with fastpath or when we lost out in contested
 * slowpath, set ctx and wake up any waiters so they can recheck.
 */
static __always_inline void
ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
{
        unsigned long flags;

        ww_mutex_lock_acquired(lock, ctx);

        lock->ctx = ctx;

        /*
         * The lock->ctx update should be visible on all cores before
         * the atomic read is done, otherwise contended waiters might be
         * missed. The contended waiters will either see ww_ctx == NULL
         * and keep spinning, or it will acquire wait_lock, add itself
         * to waiter list and sleep.
         */
        smp_mb(); /* ^^^ */

        /*
         * Check if lock is contended, if not there is nobody to wake up
         */
        if (likely(!(atomic_long_read(&lock->base.owner) & MUTEX_FLAG_WAITERS)))
                return;

        /*
         * Uh oh, we raced in fastpath, wake up everyone in this case,
         * so they can see the new lock->ctx.
         */
        spin_lock_mutex(&lock->base.wait_lock, flags);
        __ww_mutex_wakeup_for_backoff(&lock->base, ctx);
        spin_unlock_mutex(&lock->base.wait_lock, flags);
}

/*
 * After acquiring lock in the slowpath set ctx.
 *
 * Unlike for the fast path, the caller ensures that waiters are woken up where
 * necessary.
 *
 * Callers must hold the mutex wait_lock.
 */
static __always_inline void
ww_mutex_set_context_slowpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
{
        ww_mutex_lock_acquired(lock, ctx);
        lock->ctx = ctx;
}

#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
/*
 * Look out! "owner" is an entirely speculative pointer access and not
 * reliable.
 *
 * "noinline" so that this function shows up on perf profiles.
 */
static noinline
bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
                         struct ww_acquire_ctx *ww_ctx)
{
        bool ret = true;

        rcu_read_lock();
        while (__mutex_owner(lock) == owner) {
                /*
                 * Ensure we emit the owner->on_cpu, dereference _after_
                 * checking lock->owner still matches owner. If that fails,
                 * owner might point to freed memory. If it still matches,
                 * the rcu_read_lock() ensures the memory stays valid.
                 */
                barrier();

                /*
                 * Use vcpu_is_preempted to detect lock holder preemption issue.
                 */
                if (!owner->on_cpu || need_resched() ||
                                vcpu_is_preempted(task_cpu(owner))) {
                        ret = false;
                        break;
                }

                if (ww_ctx && ww_ctx->acquired > 0) {
                        struct ww_mutex *ww;

                        ww = container_of(lock, struct ww_mutex, base);

                        /*
                         * If ww->ctx is set the contents are undefined, only
                         * by acquiring wait_lock there is a guarantee that
                         * they are not invalid when reading.
                         *
                         * As such, when deadlock detection needs to be
                         * performed the optimistic spinning cannot be done.
                         *
                         * Check this in every inner iteration because we may
                         * be racing against another thread's ww_mutex_lock.
                         */
                        if (READ_ONCE(ww->ctx)) {
                                ret = false;
                                break;
                        }
                }

                cpu_relax();
        }
        rcu_read_unlock();

        return ret;
}

/*
 * Initial check for entering the mutex spinning loop
 */
static inline int mutex_can_spin_on_owner(struct mutex *lock)
{
        struct task_struct *owner;
        int retval = 1;

        if (need_resched())
                return 0;

        rcu_read_lock();
        owner = __mutex_owner(lock);

        /*
         * As lock holder preemption issue, we both skip spinning if task is not
         * on cpu or its cpu is preempted
         */
        if (owner)
                retval = owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
        rcu_read_unlock();

        /*
         * If lock->owner is not set, the mutex has been released. Return true
         * such that we'll trylock in the spin path, which is a faster option
         * than the blocking slow path.
         */
        return retval;
}

/*
 * Optimistic spinning.
 *
 * We try to spin for acquisition when we find that the lock owner
 * is currently running on a (different) CPU and while we don't
 * need to reschedule. The rationale is that if the lock owner is
 * running, it is likely to release the lock soon.
 *
 * The mutex spinners are queued up using MCS lock so that only one
 * spinner can compete for the mutex. However, if mutex spinning isn't
 * going to happen, there is no point in going through the lock/unlock
 * overhead.
 *
 * Returns true when the lock was taken, otherwise false, indicating
 * that we need to jump to the slowpath and sleep.
 *
 * The waiter flag is set to true if the spinner is a waiter in the wait
 * queue. The waiter-spinner will spin on the lock directly and concurrently
 * with the spinner at the head of the OSQ, if present, until the owner is
 * changed to itself.
 */
static __always_inline bool
mutex_optimistic_spin(struct mutex *lock, struct ww_acquire_ctx *ww_ctx,
                      const bool use_ww_ctx, const bool waiter)
{
        if (!waiter) {
                /*
                 * The purpose of the mutex_can_spin_on_owner() function is
                 * to eliminate the overhead of osq_lock() and osq_unlock()
                 * in case spinning isn't possible. As a waiter-spinner
                 * is not going to take OSQ lock anyway, there is no need
                 * to call mutex_can_spin_on_owner().
                 */
                if (!mutex_can_spin_on_owner(lock))
                        goto fail;

                /*
                 * In order to avoid a stampede of mutex spinners trying to
                 * acquire the mutex all at once, the spinners need to take a
                 * MCS (queued) lock first before spinning on the owner field.
                 */
                if (!osq_lock(&lock->osq))
                        goto fail;
        }

        for (;;) {
                struct task_struct *owner;

                /* Try to acquire the mutex... */
                owner = __mutex_trylock_or_owner(lock);
                if (!owner)
                        break;

                /*
                 * There's an owner, wait for it to either
                 * release the lock or go to sleep.
                 */
                if (!mutex_spin_on_owner(lock, owner, ww_ctx))
                        goto fail_unlock;

                /*
                 * The cpu_relax() call is a compiler barrier which forces
                 * everything in this loop to be re-loaded. We don't need
                 * memory barriers as we'll eventually observe the right
                 * values at the cost of a few extra spins.
                 */
                cpu_relax();
        }

        if (!waiter)
                osq_unlock(&lock->osq);

        return true;


fail_unlock:
        if (!waiter)
                osq_unlock(&lock->osq);

fail:
        /*
         * If we fell out of the spin path because of need_resched(),
         * reschedule now, before we try-lock the mutex. This avoids getting
         * scheduled out right after we obtained the mutex.
         */
        if (need_resched()) {
                /*
                 * We _should_ have TASK_RUNNING here, but just in case
                 * we do not, make it so, otherwise we might get stuck.
                 */
                __set_current_state(TASK_RUNNING);
                schedule_preempt_disabled();
        }

        return false;
}
#else
static __always_inline bool
mutex_optimistic_spin(struct mutex *lock, struct ww_acquire_ctx *ww_ctx,
                      const bool use_ww_ctx, const bool waiter)
{
        return false;
}
#endif

static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigned long ip);

/**
 * mutex_unlock - release the mutex
 * @lock: the mutex to be released
 *
 * Unlock a mutex that has been locked by this task previously.
 *
 * This function must not be used in interrupt context. Unlocking
 * of a not locked mutex is not allowed.
 *
 * This function is similar to (but not equivalent to) up().
 */
void __sched mutex_unlock(struct mutex *lock)
{
#ifndef CONFIG_DEBUG_LOCK_ALLOC
        if (__mutex_unlock_fast(lock))
                return;
#endif
        __mutex_unlock_slowpath(lock, _RET_IP_);
}
EXPORT_SYMBOL(mutex_unlock);

/**
 * ww_mutex_unlock - release the w/w mutex
 * @lock: the mutex to be released
 *
 * Unlock a mutex that has been locked by this task previously with any of the
 * ww_mutex_lock* functions (with or without an acquire context). It is
 * forbidden to release the locks after releasing the acquire context.
 *
 * This function must not be used in interrupt context. Unlocking
 * of a unlocked mutex is not allowed.
 */
void __sched ww_mutex_unlock(struct ww_mutex *lock)
{
        /*
         * The unlocking fastpath is the 0->1 transition from 'locked'
         * into 'unlocked' state:
         */
        if (lock->ctx) {
#ifdef CONFIG_DEBUG_MUTEXES
                DEBUG_LOCKS_WARN_ON(!lock->ctx->acquired);
#endif
                if (lock->ctx->acquired > 0)
                        lock->ctx->acquired--;
                lock->ctx = NULL;
        }

        mutex_unlock(&lock->base);
}
EXPORT_SYMBOL(ww_mutex_unlock);

static inline int __sched
__ww_mutex_lock_check_stamp(struct mutex *lock, struct mutex_waiter *waiter,
                            struct ww_acquire_ctx *ctx)
{
        struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
        struct ww_acquire_ctx *hold_ctx = READ_ONCE(ww->ctx);
        struct mutex_waiter *cur;

        if (hold_ctx && __ww_ctx_stamp_after(ctx, hold_ctx))
                goto deadlock;

        /*
         * If there is a waiter in front of us that has a context, then its
         * stamp is earlier than ours and we must back off.
         */
        cur = waiter;
        list_for_each_entry_continue_reverse(cur, &lock->wait_list, list) {
                if (cur->ww_ctx)
                        goto deadlock;
        }

        return 0;

deadlock:
#ifdef CONFIG_DEBUG_MUTEXES
        DEBUG_LOCKS_WARN_ON(ctx->contending_lock);
        ctx->contending_lock = ww;
#endif
        return -EDEADLK;
}

static inline int __sched
__ww_mutex_add_waiter(struct mutex_waiter *waiter,
                      struct mutex *lock,
                      struct ww_acquire_ctx *ww_ctx)
{
        struct mutex_waiter *cur;
        struct list_head *pos;

        if (!ww_ctx) {
                list_add_tail(&waiter->list, &lock->wait_list);
                return 0;
        }

        /*
         * Add the waiter before the first waiter with a higher stamp.
         * Waiters without a context are skipped to avoid starving
         * them.
         */
        pos = &lock->wait_list;
        list_for_each_entry_reverse(cur, &lock->wait_list, list) {
                if (!cur->ww_ctx)
                        continue;

                if (__ww_ctx_stamp_after(ww_ctx, cur->ww_ctx)) {
                        /* Back off immediately if necessary. */
                        if (ww_ctx->acquired > 0) {
#ifdef CONFIG_DEBUG_MUTEXES
                                struct ww_mutex *ww;

                                ww = container_of(lock, struct ww_mutex, base);
                                DEBUG_LOCKS_WARN_ON(ww_ctx->contending_lock);
                                ww_ctx->contending_lock = ww;
#endif
                                return -EDEADLK;
                        }

                        break;
                }

                pos = &cur->list;

                /*
                 * Wake up the waiter so that it gets a chance to back
                 * off.
                 */
                if (cur->ww_ctx->acquired > 0) {
                        debug_mutex_wake_waiter(lock, cur);
                        wake_up_process(cur->task);
                }
        }

        list_add_tail(&waiter->list, pos);
        return 0;
}

/*
 * Lock a mutex (possibly interruptible), slowpath:
 */
static __always_inline int __sched
__mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
                    struct lockdep_map *nest_lock, unsigned long ip,
                    struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
{
        struct mutex_waiter waiter;
        unsigned long flags;
        bool first = false;
        struct ww_mutex *ww;
        int ret;

        might_sleep();

        ww = container_of(lock, struct ww_mutex, base);
        if (use_ww_ctx && ww_ctx) {
                if (unlikely(ww_ctx == READ_ONCE(ww->ctx)))
                        return -EALREADY;
        }

        preempt_disable();
        mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);

        if (__mutex_trylock(lock) ||
            mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, false)) {
                /* got the lock, yay! */
                lock_acquired(&lock->dep_map, ip);
                if (use_ww_ctx && ww_ctx)
                        ww_mutex_set_context_fastpath(ww, ww_ctx);
                preempt_enable();
                return 0;
        }

        spin_lock_mutex(&lock->wait_lock, flags);
        /*
         * After waiting to acquire the wait_lock, try again.
         */
        if (__mutex_trylock(lock)) {
                if (use_ww_ctx && ww_ctx)
                        __ww_mutex_wakeup_for_backoff(lock, ww_ctx);

                goto skip_wait;
        }

        debug_mutex_lock_common(lock, &waiter);
        debug_mutex_add_waiter(lock, &waiter, current);

        lock_contended(&lock->dep_map, ip);

        if (!use_ww_ctx) {
                /* add waiting tasks to the end of the waitqueue (FIFO): */
                list_add_tail(&waiter.list, &lock->wait_list);
        } else {
                /* Add in stamp order, waking up waiters that must back off. */
                ret = __ww_mutex_add_waiter(&waiter, lock, ww_ctx);
                if (ret)
                        goto err_early_backoff;

                waiter.ww_ctx = ww_ctx;
        }

        waiter.task = current;

        if (__mutex_waiter_is_first(lock, &waiter))
                __mutex_set_flag(lock, MUTEX_FLAG_WAITERS);

        set_current_state(state);
        for (;;) {
                /*
                 * Once we hold wait_lock, we're serialized against
                 * mutex_unlock() handing the lock off to us, do a trylock
                 * before testing the error conditions to make sure we pick up
                 * the handoff.
                 */
                if (__mutex_trylock(lock))
                        goto acquired;

                /*
                 * Check for signals and wound conditions while holding
                 * wait_lock. This ensures the lock cancellation is ordered
                 * against mutex_unlock() and wake-ups do not go missing.
                 */
                if (unlikely(signal_pending_state(state, current))) {
                        ret = -EINTR;
                        goto err;
                }

                if (use_ww_ctx && ww_ctx && ww_ctx->acquired > 0) {
                        ret = __ww_mutex_lock_check_stamp(lock, &waiter, ww_ctx);
                        if (ret)
                                goto err;
                }

                spin_unlock_mutex(&lock->wait_lock, flags);
                schedule_preempt_disabled();

                /*
                 * ww_mutex needs to always recheck its position since its waiter
                 * list is not FIFO ordered.
                 */
                if ((use_ww_ctx && ww_ctx) || !first) {
                        first = __mutex_waiter_is_first(lock, &waiter);
                        if (first)
                                __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
                }

                set_current_state(state);
                /*
                 * Here we order against unlock; we must either see it change
                 * state back to RUNNING and fall through the next schedule(),
                 * or we must see its unlock and acquire.
                 */
                if (__mutex_trylock(lock) ||
                    (first && mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, true)))
                        break;

                spin_lock_mutex(&lock->wait_lock, flags);
        }
        spin_lock_mutex(&lock->wait_lock, flags);
acquired:
        __set_current_state(TASK_RUNNING);

        mutex_remove_waiter(lock, &waiter, current);
        if (likely(list_empty(&lock->wait_list)))
                __mutex_clear_flag(lock, MUTEX_FLAGS);

        debug_mutex_free_waiter(&waiter);

skip_wait:
        /* got the lock - cleanup and rejoice! */
        lock_acquired(&lock->dep_map, ip);

        if (use_ww_ctx && ww_ctx)
                ww_mutex_set_context_slowpath(ww, ww_ctx);

        spin_unlock_mutex(&lock->wait_lock, flags);
        preempt_enable();
        return 0;

err:
        __set_current_state(TASK_RUNNING);
        mutex_remove_waiter(lock, &waiter, current);
err_early_backoff:
        spin_unlock_mutex(&lock->wait_lock, flags);
        debug_mutex_free_waiter(&waiter);
        mutex_release(&lock->dep_map, 1, ip);
        preempt_enable();
        return ret;
}

static int __sched
__mutex_lock(struct mutex *lock, long state, unsigned int subclass,
             struct lockdep_map *nest_lock, unsigned long ip)
{
        return __mutex_lock_common(lock, state, subclass, nest_lock, ip, NULL, false);
}

static int __sched
__ww_mutex_lock(struct mutex *lock, long state, unsigned int subclass,
                struct lockdep_map *nest_lock, unsigned long ip,
                struct ww_acquire_ctx *ww_ctx)
{
        return __mutex_lock_common(lock, state, subclass, nest_lock, ip, ww_ctx, true);
}

#ifdef CONFIG_DEBUG_LOCK_ALLOC
void __sched
mutex_lock_nested(struct mutex *lock, unsigned int subclass)
{
        __mutex_lock(lock, TASK_UNINTERRUPTIBLE, subclass, NULL, _RET_IP_);
}

EXPORT_SYMBOL_GPL(mutex_lock_nested);

void __sched
_mutex_lock_nest_lock(struct mutex *lock, struct lockdep_map *nest)
{
        __mutex_lock(lock, TASK_UNINTERRUPTIBLE, 0, nest, _RET_IP_);
}
EXPORT_SYMBOL_GPL(_mutex_lock_nest_lock);

int __sched
mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass)
{
        return __mutex_lock(lock, TASK_KILLABLE, subclass, NULL, _RET_IP_);
}
EXPORT_SYMBOL_GPL(mutex_lock_killable_nested);

int __sched
mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
{
        return __mutex_lock(lock, TASK_INTERRUPTIBLE, subclass, NULL, _RET_IP_);
}
EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);

static inline int
ww_mutex_deadlock_injection(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
{
#ifdef CONFIG_DEBUG_WW_MUTEX_SLOWPATH
        unsigned tmp;

        if (ctx->deadlock_inject_countdown-- == 0) {
                tmp = ctx->deadlock_inject_interval;
                if (tmp > UINT_MAX/4)
                        tmp = UINT_MAX;
                else
                        tmp = tmp*2 + tmp + tmp/2;

                ctx->deadlock_inject_interval = tmp;
                ctx->deadlock_inject_countdown = tmp;
                ctx->contending_lock = lock;

                ww_mutex_unlock(lock);

                return -EDEADLK;
        }
#endif

        return 0;
}

int __sched
ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
{
        int ret;

        might_sleep();
        ret =  __ww_mutex_lock(&lock->base, TASK_UNINTERRUPTIBLE,
                               0, ctx ? &ctx->dep_map : NULL, _RET_IP_,
                               ctx);
        if (!ret && ctx && ctx->acquired > 1)
                return ww_mutex_deadlock_injection(lock, ctx);

        return ret;
}
EXPORT_SYMBOL_GPL(ww_mutex_lock);

int __sched
ww_mutex_lock_interruptible(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
{
        int ret;

        might_sleep();
        ret = __ww_mutex_lock(&lock->base, TASK_INTERRUPTIBLE,
                              0, ctx ? &ctx->dep_map : NULL, _RET_IP_,
                              ctx);

        if (!ret && ctx && ctx->acquired > 1)
                return ww_mutex_deadlock_injection(lock, ctx);

        return ret;
}
EXPORT_SYMBOL_GPL(ww_mutex_lock_interruptible);

#endif

/*
 * Release the lock, slowpath:
 */
static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigned long ip)
{
        struct task_struct *next = NULL;
        unsigned long owner, flags;
        DEFINE_WAKE_Q(wake_q);

        mutex_release(&lock->dep_map, 1, ip);

        /*
         * Release the lock before (potentially) taking the spinlock such that
         * other contenders can get on with things ASAP.
         *
         * Except when HANDOFF, in that case we must not clear the owner field,
         * but instead set it to the top waiter.
         */
        owner = atomic_long_read(&lock->owner);
        for (;;) {
                unsigned long old;

#ifdef CONFIG_DEBUG_MUTEXES
                DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current);
                DEBUG_LOCKS_WARN_ON(owner & MUTEX_FLAG_PICKUP);
#endif

                if (owner & MUTEX_FLAG_HANDOFF)
                        break;

                old = atomic_long_cmpxchg_release(&lock->owner, owner,
                                                  __owner_flags(owner));
                if (old == owner) {
                        if (owner & MUTEX_FLAG_WAITERS)
                                break;

                        return;
                }

                owner = old;
        }

        spin_lock_mutex(&lock->wait_lock, flags);
        debug_mutex_unlock(lock);
        if (!list_empty(&lock->wait_list)) {
                /* get the first entry from the wait-list: */
                struct mutex_waiter *waiter =
                        list_first_entry(&lock->wait_list,
                                         struct mutex_waiter, list);

                next = waiter->task;

                debug_mutex_wake_waiter(lock, waiter);
                wake_q_add(&wake_q, next);
        }

        if (owner & MUTEX_FLAG_HANDOFF)
                __mutex_handoff(lock, next);

        spin_unlock_mutex(&lock->wait_lock, flags);

        wake_up_q(&wake_q);
}

#ifndef CONFIG_DEBUG_LOCK_ALLOC
/*
 * Here come the less common (and hence less performance-critical) APIs:
 * mutex_lock_interruptible() and mutex_trylock().
 */
static noinline int __sched
__mutex_lock_killable_slowpath(struct mutex *lock);

static noinline int __sched
__mutex_lock_interruptible_slowpath(struct mutex *lock);

/**
 * mutex_lock_interruptible - acquire the mutex, interruptible
 * @lock: the mutex to be acquired
 *
 * Lock the mutex like mutex_lock(), and return 0 if the mutex has
 * been acquired or sleep until the mutex becomes available. If a
 * signal arrives while waiting for the lock then this function
 * returns -EINTR.
 *
 * This function is similar to (but not equivalent to) down_interruptible().
 */
int __sched mutex_lock_interruptible(struct mutex *lock)
{
        might_sleep();

        if (__mutex_trylock_fast(lock))
                return 0;

        return __mutex_lock_interruptible_slowpath(lock);
}

EXPORT_SYMBOL(mutex_lock_interruptible);

int __sched mutex_lock_killable(struct mutex *lock)
{
        might_sleep();

        if (__mutex_trylock_fast(lock))
                return 0;

        return __mutex_lock_killable_slowpath(lock);
}
EXPORT_SYMBOL(mutex_lock_killable);

static noinline void __sched
__mutex_lock_slowpath(struct mutex *lock)
{
        __mutex_lock(lock, TASK_UNINTERRUPTIBLE, 0, NULL, _RET_IP_);
}

static noinline int __sched
__mutex_lock_killable_slowpath(struct mutex *lock)
{
        return __mutex_lock(lock, TASK_KILLABLE, 0, NULL, _RET_IP_);
}

static noinline int __sched
__mutex_lock_interruptible_slowpath(struct mutex *lock)
{
        return __mutex_lock(lock, TASK_INTERRUPTIBLE, 0, NULL, _RET_IP_);
}

static noinline int __sched
__ww_mutex_lock_slowpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
{
        return __ww_mutex_lock(&lock->base, TASK_UNINTERRUPTIBLE, 0, NULL,
                               _RET_IP_, ctx);
}

static noinline int __sched
__ww_mutex_lock_interruptible_slowpath(struct ww_mutex *lock,
                                            struct ww_acquire_ctx *ctx)
{
        return __ww_mutex_lock(&lock->base, TASK_INTERRUPTIBLE, 0, NULL,
                               _RET_IP_, ctx);
}

#endif

/**
 * mutex_trylock - try to acquire the mutex, without waiting
 * @lock: the mutex to be acquired
 *
 * Try to acquire the mutex atomically. Returns 1 if the mutex
 * has been acquired successfully, and 0 on contention.
 *
 * NOTE: this function follows the spin_trylock() convention, so
 * it is negated from the down_trylock() return values! Be careful
 * about this when converting semaphore users to mutexes.
 *
 * This function must not be used in interrupt context. The
 * mutex must be released by the same task that acquired it.
 */
int __sched mutex_trylock(struct mutex *lock)
{
        bool locked = __mutex_trylock(lock);

        if (locked)
                mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);

        return locked;
}
EXPORT_SYMBOL(mutex_trylock);

#ifndef CONFIG_DEBUG_LOCK_ALLOC
int __sched
ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
{
        might_sleep();

        if (__mutex_trylock_fast(&lock->base)) {
                if (ctx)
                        ww_mutex_set_context_fastpath(lock, ctx);
                return 0;
        }

        return __ww_mutex_lock_slowpath(lock, ctx);
}
EXPORT_SYMBOL(ww_mutex_lock);

int __sched
ww_mutex_lock_interruptible(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
{
        might_sleep();

        if (__mutex_trylock_fast(&lock->base)) {
                if (ctx)
                        ww_mutex_set_context_fastpath(lock, ctx);
                return 0;
        }

        return __ww_mutex_lock_interruptible_slowpath(lock, ctx);
}
EXPORT_SYMBOL(ww_mutex_lock_interruptible);

#endif

/**
 * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
 * @cnt: the atomic which we are to dec
 * @lock: the mutex to return holding if we dec to 0
 *
 * return true and hold lock if we dec to 0, return false otherwise
 */
int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
{
        /* dec if we can't possibly hit 0 */
        if (atomic_add_unless(cnt, -1, 1))
                return 0;
        /* we might hit 0, so take the lock */
        mutex_lock(lock);
        if (!atomic_dec_and_test(cnt)) {
                /* when we actually did the dec, we didn't hit 0 */
                mutex_unlock(lock);
                return 0;
        }
        /* we hit 0, and we hold the lock */
        return 1;
}
EXPORT_SYMBOL(atomic_dec_and_mutex_lock);