4 * Mutexes: blocking mutual exclusion locks
6 * Started by Ingo Molnar:
8 * Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
10 * Many thanks to Arjan van de Ven, Thomas Gleixner, Steven Rostedt and
11 * David Howells for suggestions and improvements.
13 * - Adaptive spinning for mutexes by Peter Zijlstra. (Ported to mainline
14 * from the -rt tree, where it was originally implemented for rtmutexes
15 * by Steven Rostedt, based on work by Gregory Haskins, Peter Morreale
18 * Also see Documentation/mutex-design.txt.
20 #include <linux/mutex.h>
21 #include <linux/sched.h>
22 #include <linux/sched/rt.h>
23 #include <linux/export.h>
24 #include <linux/spinlock.h>
25 #include <linux/interrupt.h>
26 #include <linux/debug_locks.h>
29 * In the DEBUG case we are using the "NULL fastpath" for mutexes,
30 * which forces all calls into the slowpath:
32 #ifdef CONFIG_DEBUG_MUTEXES
33 # include "mutex-debug.h"
34 # include <asm-generic/mutex-null.h>
37 # include <asm/mutex.h>
41 * A mutex count of -1 indicates that waiters are sleeping waiting for the
42 * mutex. Some architectures can allow any negative number, not just -1, for
45 #ifdef __ARCH_ALLOW_ANY_NEGATIVE_MUTEX_COUNT
46 #define MUTEX_SHOW_NO_WAITER(mutex) (atomic_read(&(mutex)->count) >= 0)
48 #define MUTEX_SHOW_NO_WAITER(mutex) (atomic_read(&(mutex)->count) != -1)
52 __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
54 atomic_set(&lock->count, 1);
55 spin_lock_init(&lock->wait_lock);
56 INIT_LIST_HEAD(&lock->wait_list);
57 mutex_clear_owner(lock);
58 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
59 lock->spin_mlock = NULL;
62 debug_mutex_init(lock, name, key);
65 EXPORT_SYMBOL(__mutex_init);
67 #ifndef CONFIG_DEBUG_LOCK_ALLOC
69 * We split the mutex lock/unlock logic into separate fastpath and
70 * slowpath functions, to reduce the register pressure on the fastpath.
71 * We also put the fastpath first in the kernel image, to make sure the
72 * branch is predicted by the CPU as default-untaken.
74 static __used noinline void __sched
75 __mutex_lock_slowpath(atomic_t *lock_count);
78 * mutex_lock - acquire the mutex
79 * @lock: the mutex to be acquired
81 * Lock the mutex exclusively for this task. If the mutex is not
82 * available right now, it will sleep until it can get it.
84 * The mutex must later on be released by the same task that
85 * acquired it. Recursive locking is not allowed. The task
86 * may not exit without first unlocking the mutex. Also, kernel
87 * memory where the mutex resides mutex must not be freed with
88 * the mutex still locked. The mutex must first be initialized
89 * (or statically defined) before it can be locked. memset()-ing
90 * the mutex to 0 is not allowed.
92 * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging
93 * checks that will enforce the restrictions and will also do
94 * deadlock debugging. )
96 * This function is similar to (but not equivalent to) down().
98 void __sched mutex_lock(struct mutex *lock)
102 * The locking fastpath is the 1->0 transition from
103 * 'unlocked' into 'locked' state.
105 __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
106 mutex_set_owner(lock);
109 EXPORT_SYMBOL(mutex_lock);
112 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
114 * In order to avoid a stampede of mutex spinners from acquiring the mutex
115 * more or less simultaneously, the spinners need to acquire a MCS lock
116 * first before spinning on the owner field.
118 * We don't inline mspin_lock() so that perf can correctly account for the
119 * time spent in this lock function.
122 struct mspin_node *next ;
123 int locked; /* 1 if lock acquired */
125 #define MLOCK(mutex) ((struct mspin_node **)&((mutex)->spin_mlock))
128 void mspin_lock(struct mspin_node **lock, struct mspin_node *node)
130 struct mspin_node *prev;
136 prev = xchg(lock, node);
137 if (likely(prev == NULL)) {
142 ACCESS_ONCE(prev->next) = node;
144 /* Wait until the lock holder passes the lock down */
145 while (!ACCESS_ONCE(node->locked))
146 arch_mutex_cpu_relax();
149 static void mspin_unlock(struct mspin_node **lock, struct mspin_node *node)
151 struct mspin_node *next = ACCESS_ONCE(node->next);
155 * Release the lock by setting it to NULL
157 if (cmpxchg(lock, node, NULL) == node)
159 /* Wait until the next pointer is set */
160 while (!(next = ACCESS_ONCE(node->next)))
161 arch_mutex_cpu_relax();
163 ACCESS_ONCE(next->locked) = 1;
168 * Mutex spinning code migrated from kernel/sched/core.c
171 static inline bool owner_running(struct mutex *lock, struct task_struct *owner)
173 if (lock->owner != owner)
177 * Ensure we emit the owner->on_cpu, dereference _after_ checking
178 * lock->owner still matches owner, if that fails, owner might
179 * point to free()d memory, if it still matches, the rcu_read_lock()
180 * ensures the memory stays valid.
184 return owner->on_cpu;
188 * Look out! "owner" is an entirely speculative pointer
189 * access and not reliable.
192 int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
195 while (owner_running(lock, owner)) {
199 arch_mutex_cpu_relax();
204 * We break out the loop above on need_resched() and when the
205 * owner changed, which is a sign for heavy contention. Return
206 * success only when lock->owner is NULL.
208 return lock->owner == NULL;
212 * Initial check for entering the mutex spinning loop
214 static inline int mutex_can_spin_on_owner(struct mutex *lock)
220 retval = lock->owner->on_cpu;
223 * if lock->owner is not set, the mutex owner may have just acquired
224 * it and not set the owner yet or the mutex has been released.
230 static __used noinline void __sched __mutex_unlock_slowpath(atomic_t *lock_count);
233 * mutex_unlock - release the mutex
234 * @lock: the mutex to be released
236 * Unlock a mutex that has been locked by this task previously.
238 * This function must not be used in interrupt context. Unlocking
239 * of a not locked mutex is not allowed.
241 * This function is similar to (but not equivalent to) up().
243 void __sched mutex_unlock(struct mutex *lock)
246 * The unlocking fastpath is the 0->1 transition from 'locked'
247 * into 'unlocked' state:
249 #ifndef CONFIG_DEBUG_MUTEXES
251 * When debugging is enabled we must not clear the owner before time,
252 * the slow path will always be taken, and that clears the owner field
253 * after verifying that it was indeed current.
255 mutex_clear_owner(lock);
257 __mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
260 EXPORT_SYMBOL(mutex_unlock);
263 * Lock a mutex (possibly interruptible), slowpath:
265 static inline int __sched
266 __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
267 struct lockdep_map *nest_lock, unsigned long ip)
269 struct task_struct *task = current;
270 struct mutex_waiter waiter;
274 mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
276 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
278 * Optimistic spinning.
280 * We try to spin for acquisition when we find that there are no
281 * pending waiters and the lock owner is currently running on a
284 * The rationale is that if the lock owner is running, it is likely to
285 * release the lock soon.
287 * Since this needs the lock owner, and this mutex implementation
288 * doesn't track the owner atomically in the lock field, we need to
289 * track it non-atomically.
291 * We can't do this for DEBUG_MUTEXES because that relies on wait_lock
292 * to serialize everything.
294 * The mutex spinners are queued up using MCS lock so that only one
295 * spinner can compete for the mutex. However, if mutex spinning isn't
296 * going to happen, there is no point in going through the lock/unlock
299 if (!mutex_can_spin_on_owner(lock))
303 struct task_struct *owner;
304 struct mspin_node node;
307 * If there's an owner, wait for it to either
308 * release the lock or go to sleep.
310 mspin_lock(MLOCK(lock), &node);
311 owner = ACCESS_ONCE(lock->owner);
312 if (owner && !mutex_spin_on_owner(lock, owner)) {
313 mspin_unlock(MLOCK(lock), &node);
317 if ((atomic_read(&lock->count) == 1) &&
318 (atomic_cmpxchg(&lock->count, 1, 0) == 1)) {
319 lock_acquired(&lock->dep_map, ip);
320 mutex_set_owner(lock);
321 mspin_unlock(MLOCK(lock), &node);
325 mspin_unlock(MLOCK(lock), &node);
328 * When there's no owner, we might have preempted between the
329 * owner acquiring the lock and setting the owner field. If
330 * we're an RT task that will live-lock because we won't let
331 * the owner complete.
333 if (!owner && (need_resched() || rt_task(task)))
337 * The cpu_relax() call is a compiler barrier which forces
338 * everything in this loop to be re-loaded. We don't need
339 * memory barriers as we'll eventually observe the right
340 * values at the cost of a few extra spins.
342 arch_mutex_cpu_relax();
346 spin_lock_mutex(&lock->wait_lock, flags);
348 debug_mutex_lock_common(lock, &waiter);
349 debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));
351 /* add waiting tasks to the end of the waitqueue (FIFO): */
352 list_add_tail(&waiter.list, &lock->wait_list);
355 if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, -1) == 1))
358 lock_contended(&lock->dep_map, ip);
362 * Lets try to take the lock again - this is needed even if
363 * we get here for the first time (shortly after failing to
364 * acquire the lock), to make sure that we get a wakeup once
365 * it's unlocked. Later on, if we sleep, this is the
366 * operation that gives us the lock. We xchg it to -1, so
367 * that when we release the lock, we properly wake up the
370 if (MUTEX_SHOW_NO_WAITER(lock) &&
371 (atomic_xchg(&lock->count, -1) == 1))
375 * got a signal? (This code gets eliminated in the
376 * TASK_UNINTERRUPTIBLE case.)
378 if (unlikely(signal_pending_state(state, task))) {
379 mutex_remove_waiter(lock, &waiter,
380 task_thread_info(task));
381 mutex_release(&lock->dep_map, 1, ip);
382 spin_unlock_mutex(&lock->wait_lock, flags);
384 debug_mutex_free_waiter(&waiter);
388 __set_task_state(task, state);
390 /* didn't get the lock, go to sleep: */
391 spin_unlock_mutex(&lock->wait_lock, flags);
392 schedule_preempt_disabled();
393 spin_lock_mutex(&lock->wait_lock, flags);
397 lock_acquired(&lock->dep_map, ip);
398 /* got the lock - rejoice! */
399 mutex_remove_waiter(lock, &waiter, current_thread_info());
400 mutex_set_owner(lock);
402 /* set it to 0 if there are no waiters left: */
403 if (likely(list_empty(&lock->wait_list)))
404 atomic_set(&lock->count, 0);
406 spin_unlock_mutex(&lock->wait_lock, flags);
408 debug_mutex_free_waiter(&waiter);
414 #ifdef CONFIG_DEBUG_LOCK_ALLOC
416 mutex_lock_nested(struct mutex *lock, unsigned int subclass)
419 __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, subclass, NULL, _RET_IP_);
422 EXPORT_SYMBOL_GPL(mutex_lock_nested);
425 _mutex_lock_nest_lock(struct mutex *lock, struct lockdep_map *nest)
428 __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, nest, _RET_IP_);
431 EXPORT_SYMBOL_GPL(_mutex_lock_nest_lock);
434 mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass)
437 return __mutex_lock_common(lock, TASK_KILLABLE, subclass, NULL, _RET_IP_);
439 EXPORT_SYMBOL_GPL(mutex_lock_killable_nested);
442 mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
445 return __mutex_lock_common(lock, TASK_INTERRUPTIBLE,
446 subclass, NULL, _RET_IP_);
449 EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);
453 * Release the lock, slowpath:
456 __mutex_unlock_common_slowpath(atomic_t *lock_count, int nested)
458 struct mutex *lock = container_of(lock_count, struct mutex, count);
461 spin_lock_mutex(&lock->wait_lock, flags);
462 mutex_release(&lock->dep_map, nested, _RET_IP_);
463 debug_mutex_unlock(lock);
466 * some architectures leave the lock unlocked in the fastpath failure
467 * case, others need to leave it locked. In the later case we have to
470 if (__mutex_slowpath_needs_to_unlock())
471 atomic_set(&lock->count, 1);
473 if (!list_empty(&lock->wait_list)) {
474 /* get the first entry from the wait-list: */
475 struct mutex_waiter *waiter =
476 list_entry(lock->wait_list.next,
477 struct mutex_waiter, list);
479 debug_mutex_wake_waiter(lock, waiter);
481 wake_up_process(waiter->task);
484 spin_unlock_mutex(&lock->wait_lock, flags);
488 * Release the lock, slowpath:
490 static __used noinline void
491 __mutex_unlock_slowpath(atomic_t *lock_count)
493 __mutex_unlock_common_slowpath(lock_count, 1);
496 #ifndef CONFIG_DEBUG_LOCK_ALLOC
498 * Here come the less common (and hence less performance-critical) APIs:
499 * mutex_lock_interruptible() and mutex_trylock().
501 static noinline int __sched
502 __mutex_lock_killable_slowpath(atomic_t *lock_count);
504 static noinline int __sched
505 __mutex_lock_interruptible_slowpath(atomic_t *lock_count);
508 * mutex_lock_interruptible - acquire the mutex, interruptible
509 * @lock: the mutex to be acquired
511 * Lock the mutex like mutex_lock(), and return 0 if the mutex has
512 * been acquired or sleep until the mutex becomes available. If a
513 * signal arrives while waiting for the lock then this function
516 * This function is similar to (but not equivalent to) down_interruptible().
518 int __sched mutex_lock_interruptible(struct mutex *lock)
523 ret = __mutex_fastpath_lock_retval
524 (&lock->count, __mutex_lock_interruptible_slowpath);
526 mutex_set_owner(lock);
531 EXPORT_SYMBOL(mutex_lock_interruptible);
533 int __sched mutex_lock_killable(struct mutex *lock)
538 ret = __mutex_fastpath_lock_retval
539 (&lock->count, __mutex_lock_killable_slowpath);
541 mutex_set_owner(lock);
545 EXPORT_SYMBOL(mutex_lock_killable);
547 static __used noinline void __sched
548 __mutex_lock_slowpath(atomic_t *lock_count)
550 struct mutex *lock = container_of(lock_count, struct mutex, count);
552 __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, NULL, _RET_IP_);
555 static noinline int __sched
556 __mutex_lock_killable_slowpath(atomic_t *lock_count)
558 struct mutex *lock = container_of(lock_count, struct mutex, count);
560 return __mutex_lock_common(lock, TASK_KILLABLE, 0, NULL, _RET_IP_);
563 static noinline int __sched
564 __mutex_lock_interruptible_slowpath(atomic_t *lock_count)
566 struct mutex *lock = container_of(lock_count, struct mutex, count);
568 return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0, NULL, _RET_IP_);
573 * Spinlock based trylock, we take the spinlock and check whether we
576 static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
578 struct mutex *lock = container_of(lock_count, struct mutex, count);
582 spin_lock_mutex(&lock->wait_lock, flags);
584 prev = atomic_xchg(&lock->count, -1);
585 if (likely(prev == 1)) {
586 mutex_set_owner(lock);
587 mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
590 /* Set it back to 0 if there are no waiters: */
591 if (likely(list_empty(&lock->wait_list)))
592 atomic_set(&lock->count, 0);
594 spin_unlock_mutex(&lock->wait_lock, flags);
600 * mutex_trylock - try to acquire the mutex, without waiting
601 * @lock: the mutex to be acquired
603 * Try to acquire the mutex atomically. Returns 1 if the mutex
604 * has been acquired successfully, and 0 on contention.
606 * NOTE: this function follows the spin_trylock() convention, so
607 * it is negated from the down_trylock() return values! Be careful
608 * about this when converting semaphore users to mutexes.
610 * This function must not be used in interrupt context. The
611 * mutex must be released by the same task that acquired it.
613 int __sched mutex_trylock(struct mutex *lock)
617 ret = __mutex_fastpath_trylock(&lock->count, __mutex_trylock_slowpath);
619 mutex_set_owner(lock);
623 EXPORT_SYMBOL(mutex_trylock);
626 * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
627 * @cnt: the atomic which we are to dec
628 * @lock: the mutex to return holding if we dec to 0
630 * return true and hold lock if we dec to 0, return false otherwise
632 int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
634 /* dec if we can't possibly hit 0 */
635 if (atomic_add_unless(cnt, -1, 1))
637 /* we might hit 0, so take the lock */
639 if (!atomic_dec_and_test(cnt)) {
640 /* when we actually did the dec, we didn't hit 0 */
644 /* we hit 0, and we hold the lock */
647 EXPORT_SYMBOL(atomic_dec_and_mutex_lock);