mutex.c

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/*
 * kernel/mutex.c
 *
 * Mutexes: blocking mutual exclusion locks
 *
 * Started by Ingo Molnar:
 *
 *  Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <[email protected]>
 *
 * 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/mutex-design.txt.
 */
#include <linux/mutex.h>
#include <linux/sched.h>
#include <linux/export.h>
#include <linux/spinlock.h>
#include <linux/interrupt.h>
#include <linux/debug_locks.h>

/*
 * In the DEBUG case we are using the "NULL fastpath" for mutexes,
 * which forces all calls into the slowpath:
 */
#ifdef CONFIG_DEBUG_MUTEXES
# include "mutex-debug.h"
# include <asm-generic/mutex-null.h>
#else
# include "mutex.h"
# include <asm/mutex.h>
#endif

void
__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
{
    atomic_set(&lock->count, 1);
    spin_lock_init(&lock->wait_lock);
    INIT_LIST_HEAD(&lock->wait_list);
    mutex_clear_owner(lock);

    debug_mutex_init(lock, name, key);
}

EXPORT_SYMBOL(__mutex_init);

#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 __used noinline void __sched
__mutex_lock_slowpath(atomic_t *lock_count);

void __sched mutex_lock(struct mutex *lock)
{
    might_sleep();
    /*
     * The locking fastpath is the 1->0 transition from
     * 'unlocked' into 'locked' state.
     */
    __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
    mutex_set_owner(lock);
}

EXPORT_SYMBOL(mutex_lock);
#endif

static __used noinline void __sched __mutex_unlock_slowpath(atomic_t *lock_count);

void __sched mutex_unlock(struct mutex *lock)
{
    /*
     * The unlocking fastpath is the 0->1 transition from 'locked'
     * into 'unlocked' state:
     */
#ifndef CONFIG_DEBUG_MUTEXES
    /*
     * When debugging is enabled we must not clear the owner before time,
     * the slow path will always be taken, and that clears the owner field
     * after verifying that it was indeed current.
     */
    mutex_clear_owner(lock);
#endif
    __mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
}

EXPORT_SYMBOL(mutex_unlock);

/*
 * Lock a mutex (possibly interruptible), slowpath:
 */
static inline int __sched
__mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
            struct lockdep_map *nest_lock, unsigned long ip)
{
    struct task_struct *task = current;
    struct mutex_waiter waiter;
    unsigned long flags;

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

#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
    /*
     * Optimistic spinning.
     *
     * We try to spin for acquisition when we find that there are no
     * pending waiters and the lock owner is currently running on a
     * (different) CPU.
     *
     * The rationale is that if the lock owner is running, it is likely to
     * release the lock soon.
     *
     * Since this needs the lock owner, and this mutex implementation
     * doesn't track the owner atomically in the lock field, we need to
     * track it non-atomically.
     *
     * We can't do this for DEBUG_MUTEXES because that relies on wait_lock
     * to serialize everything.
     */

    for (;;) {
        struct task_struct *owner;

        /*
         * If there's an owner, wait for it to either
         * release the lock or go to sleep.
         */
        owner = ACCESS_ONCE(lock->owner);
        if (owner && !mutex_spin_on_owner(lock, owner))
            break;

        if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
            lock_acquired(&lock->dep_map, ip);
            mutex_set_owner(lock);
            preempt_enable();
            return 0;
        }

        /*
         * When there's no owner, we might have preempted between the
         * owner acquiring the lock and setting the owner field. If
         * we're an RT task that will live-lock because we won't let
         * the owner complete.
         */
        if (!owner && (need_resched() || rt_task(task)))
            break;

        /*
         * 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.
         */
        arch_mutex_cpu_relax();
    }
#endif
    spin_lock_mutex(&lock->wait_lock, flags);

    debug_mutex_lock_common(lock, &waiter);
    debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));

    /* add waiting tasks to the end of the waitqueue (FIFO): */
    list_add_tail(&waiter.list, &lock->wait_list);
    waiter.task = task;

    if (atomic_xchg(&lock->count, -1) == 1)
        goto done;

    lock_contended(&lock->dep_map, ip);

    for (;;) {
        /*
         * Lets try to take the lock again - this is needed even if
         * we get here for the first time (shortly after failing to
         * acquire the lock), to make sure that we get a wakeup once
         * it's unlocked. Later on, if we sleep, this is the
         * operation that gives us the lock. We xchg it to -1, so
         * that when we release the lock, we properly wake up the
         * other waiters:
         */
        if (atomic_xchg(&lock->count, -1) == 1)
            break;

        /*
         * got a signal? (This code gets eliminated in the
         * TASK_UNINTERRUPTIBLE case.)
         */
        if (unlikely(signal_pending_state(state, task))) {
            mutex_remove_waiter(lock, &waiter,
                        task_thread_info(task));
            mutex_release(&lock->dep_map, 1, ip);
            spin_unlock_mutex(&lock->wait_lock, flags);

            debug_mutex_free_waiter(&waiter);
            preempt_enable();
            return -EINTR;
        }
        __set_task_state(task, state);

        /* didn't get the lock, go to sleep: */
        spin_unlock_mutex(&lock->wait_lock, flags);
        schedule_preempt_disabled();
        spin_lock_mutex(&lock->wait_lock, flags);
    }

done:
    lock_acquired(&lock->dep_map, ip);
    /* got the lock - rejoice! */
    mutex_remove_waiter(lock, &waiter, current_thread_info());
    mutex_set_owner(lock);

    /* set it to 0 if there are no waiters left: */
    if (likely(list_empty(&lock->wait_list)))
        atomic_set(&lock->count, 0);

    spin_unlock_mutex(&lock->wait_lock, flags);

    debug_mutex_free_waiter(&waiter);
    preempt_enable();

    return 0;
}

#ifdef CONFIG_DEBUG_LOCK_ALLOC
void __sched
mutex_lock_nested(struct mutex *lock, unsigned int subclass)
{
    might_sleep();
    __mutex_lock_common(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)
{
    might_sleep();
    __mutex_lock_common(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)
{
    might_sleep();
    return __mutex_lock_common(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)
{
    might_sleep();
    return __mutex_lock_common(lock, TASK_INTERRUPTIBLE,
                   subclass, NULL, _RET_IP_);
}

EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);
#endif

/*
 * Release the lock, slowpath:
 */
static inline void
__mutex_unlock_common_slowpath(atomic_t *lock_count, int nested)
{
    struct mutex *lock = container_of(lock_count, struct mutex, count);
    unsigned long flags;

    spin_lock_mutex(&lock->wait_lock, flags);
    mutex_release(&lock->dep_map, nested, _RET_IP_);
    debug_mutex_unlock(lock);

    /*
     * some architectures leave the lock unlocked in the fastpath failure
     * case, others need to leave it locked. In the later case we have to
     * unlock it here
     */
    if (__mutex_slowpath_needs_to_unlock())
        atomic_set(&lock->count, 1);

    if (!list_empty(&lock->wait_list)) {
        /* get the first entry from the wait-list: */
        struct mutex_waiter *waiter =
                list_entry(lock->wait_list.next,
                       struct mutex_waiter, list);

        debug_mutex_wake_waiter(lock, waiter);

        wake_up_process(waiter->task);
    }

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

/*
 * Release the lock, slowpath:
 */
static __used noinline void
__mutex_unlock_slowpath(atomic_t *lock_count)
{
    __mutex_unlock_common_slowpath(lock_count, 1);
}

#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(atomic_t *lock_count);

static noinline int __sched
__mutex_lock_interruptible_slowpath(atomic_t *lock_count);

int __sched mutex_lock_interruptible(struct mutex *lock)
{
    int ret;

    might_sleep();
    ret =  __mutex_fastpath_lock_retval
            (&lock->count, __mutex_lock_interruptible_slowpath);
    if (!ret)
        mutex_set_owner(lock);

    return ret;
}

EXPORT_SYMBOL(mutex_lock_interruptible);

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

    might_sleep();
    ret = __mutex_fastpath_lock_retval
            (&lock->count, __mutex_lock_killable_slowpath);
    if (!ret)
        mutex_set_owner(lock);

    return ret;
}
EXPORT_SYMBOL(mutex_lock_killable);

static __used noinline void __sched
__mutex_lock_slowpath(atomic_t *lock_count)
{
    struct mutex *lock = container_of(lock_count, struct mutex, count);

    __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, NULL, _RET_IP_);
}

static noinline int __sched
__mutex_lock_killable_slowpath(atomic_t *lock_count)
{
    struct mutex *lock = container_of(lock_count, struct mutex, count);

    return __mutex_lock_common(lock, TASK_KILLABLE, 0, NULL, _RET_IP_);
}

static noinline int __sched
__mutex_lock_interruptible_slowpath(atomic_t *lock_count)
{
    struct mutex *lock = container_of(lock_count, struct mutex, count);

    return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0, NULL, _RET_IP_);
}
#endif

/*
 * Spinlock based trylock, we take the spinlock and check whether we
 * can get the lock:
 */
static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
{
    struct mutex *lock = container_of(lock_count, struct mutex, count);
    unsigned long flags;
    int prev;

    spin_lock_mutex(&lock->wait_lock, flags);

    prev = atomic_xchg(&lock->count, -1);
    if (likely(prev == 1)) {
        mutex_set_owner(lock);
        mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
    }

    /* Set it back to 0 if there are no waiters: */
    if (likely(list_empty(&lock->wait_list)))
        atomic_set(&lock->count, 0);

    spin_unlock_mutex(&lock->wait_lock, flags);

    return prev == 1;
}

int __sched mutex_trylock(struct mutex *lock)
{
    int ret;

    ret = __mutex_fastpath_trylock(&lock->count, __mutex_trylock_slowpath);
    if (ret)
        mutex_set_owner(lock);

    return ret;
}
EXPORT_SYMBOL(mutex_trylock);

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);