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 __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
43 atomic_set(&lock->count, 1);
44 spin_lock_init(&lock->wait_lock);
45 INIT_LIST_HEAD(&lock->wait_list);
46 mutex_clear_owner(lock);
48 debug_mutex_init(lock, name, key);
51 EXPORT_SYMBOL(__mutex_init);
53 #ifndef CONFIG_DEBUG_LOCK_ALLOC
55 * We split the mutex lock/unlock logic into separate fastpath and
56 * slowpath functions, to reduce the register pressure on the fastpath.
57 * We also put the fastpath first in the kernel image, to make sure the
58 * branch is predicted by the CPU as default-untaken.
60 static __used noinline void __sched
61 __mutex_lock_slowpath(atomic_t *lock_count);
64 * mutex_lock - acquire the mutex
65 * @lock: the mutex to be acquired
67 * Lock the mutex exclusively for this task. If the mutex is not
68 * available right now, it will sleep until it can get it.
70 * The mutex must later on be released by the same task that
71 * acquired it. Recursive locking is not allowed. The task
72 * may not exit without first unlocking the mutex. Also, kernel
73 * memory where the mutex resides mutex must not be freed with
74 * the mutex still locked. The mutex must first be initialized
75 * (or statically defined) before it can be locked. memset()-ing
76 * the mutex to 0 is not allowed.
78 * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging
79 * checks that will enforce the restrictions and will also do
80 * deadlock debugging. )
82 * This function is similar to (but not equivalent to) down().
84 void __sched mutex_lock(struct mutex *lock)
88 * The locking fastpath is the 1->0 transition from
89 * 'unlocked' into 'locked' state.
91 __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
92 mutex_set_owner(lock);
95 EXPORT_SYMBOL(mutex_lock);
98 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
100 * Mutex spinning code migrated from kernel/sched/core.c
103 static inline bool owner_running(struct mutex *lock, struct task_struct *owner)
105 if (lock->owner != owner)
109 * Ensure we emit the owner->on_cpu, dereference _after_ checking
110 * lock->owner still matches owner, if that fails, owner might
111 * point to free()d memory, if it still matches, the rcu_read_lock()
112 * ensures the memory stays valid.
116 return owner->on_cpu;
120 * Look out! "owner" is an entirely speculative pointer
121 * access and not reliable.
124 int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
127 while (owner_running(lock, owner)) {
131 arch_mutex_cpu_relax();
136 * We break out the loop above on need_resched() and when the
137 * owner changed, which is a sign for heavy contention. Return
138 * success only when lock->owner is NULL.
140 return lock->owner == NULL;
144 static __used noinline void __sched __mutex_unlock_slowpath(atomic_t *lock_count);
147 * mutex_unlock - release the mutex
148 * @lock: the mutex to be released
150 * Unlock a mutex that has been locked by this task previously.
152 * This function must not be used in interrupt context. Unlocking
153 * of a not locked mutex is not allowed.
155 * This function is similar to (but not equivalent to) up().
157 void __sched mutex_unlock(struct mutex *lock)
160 * The unlocking fastpath is the 0->1 transition from 'locked'
161 * into 'unlocked' state:
163 #ifndef CONFIG_DEBUG_MUTEXES
165 * When debugging is enabled we must not clear the owner before time,
166 * the slow path will always be taken, and that clears the owner field
167 * after verifying that it was indeed current.
169 mutex_clear_owner(lock);
171 __mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
174 EXPORT_SYMBOL(mutex_unlock);
177 * Lock a mutex (possibly interruptible), slowpath:
179 static inline int __sched
180 __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
181 struct lockdep_map *nest_lock, unsigned long ip)
183 struct task_struct *task = current;
184 struct mutex_waiter waiter;
188 mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
190 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
192 * Optimistic spinning.
194 * We try to spin for acquisition when we find that there are no
195 * pending waiters and the lock owner is currently running on a
198 * The rationale is that if the lock owner is running, it is likely to
199 * release the lock soon.
201 * Since this needs the lock owner, and this mutex implementation
202 * doesn't track the owner atomically in the lock field, we need to
203 * track it non-atomically.
205 * We can't do this for DEBUG_MUTEXES because that relies on wait_lock
206 * to serialize everything.
210 struct task_struct *owner;
213 * If there's an owner, wait for it to either
214 * release the lock or go to sleep.
216 owner = ACCESS_ONCE(lock->owner);
217 if (owner && !mutex_spin_on_owner(lock, owner))
220 if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
221 lock_acquired(&lock->dep_map, ip);
222 mutex_set_owner(lock);
228 * When there's no owner, we might have preempted between the
229 * owner acquiring the lock and setting the owner field. If
230 * we're an RT task that will live-lock because we won't let
231 * the owner complete.
233 if (!owner && (need_resched() || rt_task(task)))
237 * The cpu_relax() call is a compiler barrier which forces
238 * everything in this loop to be re-loaded. We don't need
239 * memory barriers as we'll eventually observe the right
240 * values at the cost of a few extra spins.
242 arch_mutex_cpu_relax();
245 spin_lock_mutex(&lock->wait_lock, flags);
247 debug_mutex_lock_common(lock, &waiter);
248 debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));
250 /* add waiting tasks to the end of the waitqueue (FIFO): */
251 list_add_tail(&waiter.list, &lock->wait_list);
254 if (atomic_xchg(&lock->count, -1) == 1)
257 lock_contended(&lock->dep_map, ip);
261 * Lets try to take the lock again - this is needed even if
262 * we get here for the first time (shortly after failing to
263 * acquire the lock), to make sure that we get a wakeup once
264 * it's unlocked. Later on, if we sleep, this is the
265 * operation that gives us the lock. We xchg it to -1, so
266 * that when we release the lock, we properly wake up the
269 if (atomic_xchg(&lock->count, -1) == 1)
273 * got a signal? (This code gets eliminated in the
274 * TASK_UNINTERRUPTIBLE case.)
276 if (unlikely(signal_pending_state(state, task))) {
277 mutex_remove_waiter(lock, &waiter,
278 task_thread_info(task));
279 mutex_release(&lock->dep_map, 1, ip);
280 spin_unlock_mutex(&lock->wait_lock, flags);
282 debug_mutex_free_waiter(&waiter);
286 __set_task_state(task, state);
288 /* didn't get the lock, go to sleep: */
289 spin_unlock_mutex(&lock->wait_lock, flags);
290 schedule_preempt_disabled();
291 spin_lock_mutex(&lock->wait_lock, flags);
295 lock_acquired(&lock->dep_map, ip);
296 /* got the lock - rejoice! */
297 mutex_remove_waiter(lock, &waiter, current_thread_info());
298 mutex_set_owner(lock);
300 /* set it to 0 if there are no waiters left: */
301 if (likely(list_empty(&lock->wait_list)))
302 atomic_set(&lock->count, 0);
304 spin_unlock_mutex(&lock->wait_lock, flags);
306 debug_mutex_free_waiter(&waiter);
312 #ifdef CONFIG_DEBUG_LOCK_ALLOC
314 mutex_lock_nested(struct mutex *lock, unsigned int subclass)
317 __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, subclass, NULL, _RET_IP_);
320 EXPORT_SYMBOL_GPL(mutex_lock_nested);
323 _mutex_lock_nest_lock(struct mutex *lock, struct lockdep_map *nest)
326 __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, nest, _RET_IP_);
329 EXPORT_SYMBOL_GPL(_mutex_lock_nest_lock);
332 mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass)
335 return __mutex_lock_common(lock, TASK_KILLABLE, subclass, NULL, _RET_IP_);
337 EXPORT_SYMBOL_GPL(mutex_lock_killable_nested);
340 mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
343 return __mutex_lock_common(lock, TASK_INTERRUPTIBLE,
344 subclass, NULL, _RET_IP_);
347 EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);
351 * Release the lock, slowpath:
354 __mutex_unlock_common_slowpath(atomic_t *lock_count, int nested)
356 struct mutex *lock = container_of(lock_count, struct mutex, count);
359 spin_lock_mutex(&lock->wait_lock, flags);
360 mutex_release(&lock->dep_map, nested, _RET_IP_);
361 debug_mutex_unlock(lock);
364 * some architectures leave the lock unlocked in the fastpath failure
365 * case, others need to leave it locked. In the later case we have to
368 if (__mutex_slowpath_needs_to_unlock())
369 atomic_set(&lock->count, 1);
371 if (!list_empty(&lock->wait_list)) {
372 /* get the first entry from the wait-list: */
373 struct mutex_waiter *waiter =
374 list_entry(lock->wait_list.next,
375 struct mutex_waiter, list);
377 debug_mutex_wake_waiter(lock, waiter);
379 wake_up_process(waiter->task);
382 spin_unlock_mutex(&lock->wait_lock, flags);
386 * Release the lock, slowpath:
388 static __used noinline void
389 __mutex_unlock_slowpath(atomic_t *lock_count)
391 __mutex_unlock_common_slowpath(lock_count, 1);
394 #ifndef CONFIG_DEBUG_LOCK_ALLOC
396 * Here come the less common (and hence less performance-critical) APIs:
397 * mutex_lock_interruptible() and mutex_trylock().
399 static noinline int __sched
400 __mutex_lock_killable_slowpath(atomic_t *lock_count);
402 static noinline int __sched
403 __mutex_lock_interruptible_slowpath(atomic_t *lock_count);
406 * mutex_lock_interruptible - acquire the mutex, interruptible
407 * @lock: the mutex to be acquired
409 * Lock the mutex like mutex_lock(), and return 0 if the mutex has
410 * been acquired or sleep until the mutex becomes available. If a
411 * signal arrives while waiting for the lock then this function
414 * This function is similar to (but not equivalent to) down_interruptible().
416 int __sched mutex_lock_interruptible(struct mutex *lock)
421 ret = __mutex_fastpath_lock_retval
422 (&lock->count, __mutex_lock_interruptible_slowpath);
424 mutex_set_owner(lock);
429 EXPORT_SYMBOL(mutex_lock_interruptible);
431 int __sched mutex_lock_killable(struct mutex *lock)
436 ret = __mutex_fastpath_lock_retval
437 (&lock->count, __mutex_lock_killable_slowpath);
439 mutex_set_owner(lock);
443 EXPORT_SYMBOL(mutex_lock_killable);
445 static __used noinline void __sched
446 __mutex_lock_slowpath(atomic_t *lock_count)
448 struct mutex *lock = container_of(lock_count, struct mutex, count);
450 __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, NULL, _RET_IP_);
453 static noinline int __sched
454 __mutex_lock_killable_slowpath(atomic_t *lock_count)
456 struct mutex *lock = container_of(lock_count, struct mutex, count);
458 return __mutex_lock_common(lock, TASK_KILLABLE, 0, NULL, _RET_IP_);
461 static noinline int __sched
462 __mutex_lock_interruptible_slowpath(atomic_t *lock_count)
464 struct mutex *lock = container_of(lock_count, struct mutex, count);
466 return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0, NULL, _RET_IP_);
471 * Spinlock based trylock, we take the spinlock and check whether we
474 static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
476 struct mutex *lock = container_of(lock_count, struct mutex, count);
480 spin_lock_mutex(&lock->wait_lock, flags);
482 prev = atomic_xchg(&lock->count, -1);
483 if (likely(prev == 1)) {
484 mutex_set_owner(lock);
485 mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
488 /* Set it back to 0 if there are no waiters: */
489 if (likely(list_empty(&lock->wait_list)))
490 atomic_set(&lock->count, 0);
492 spin_unlock_mutex(&lock->wait_lock, flags);
498 * mutex_trylock - try to acquire the mutex, without waiting
499 * @lock: the mutex to be acquired
501 * Try to acquire the mutex atomically. Returns 1 if the mutex
502 * has been acquired successfully, and 0 on contention.
504 * NOTE: this function follows the spin_trylock() convention, so
505 * it is negated from the down_trylock() return values! Be careful
506 * about this when converting semaphore users to mutexes.
508 * This function must not be used in interrupt context. The
509 * mutex must be released by the same task that acquired it.
511 int __sched mutex_trylock(struct mutex *lock)
515 ret = __mutex_fastpath_trylock(&lock->count, __mutex_trylock_slowpath);
517 mutex_set_owner(lock);
521 EXPORT_SYMBOL(mutex_trylock);
524 * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
525 * @cnt: the atomic which we are to dec
526 * @lock: the mutex to return holding if we dec to 0
528 * return true and hold lock if we dec to 0, return false otherwise
530 int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
532 /* dec if we can't possibly hit 0 */
533 if (atomic_add_unless(cnt, -1, 1))
535 /* we might hit 0, so take the lock */
537 if (!atomic_dec_and_test(cnt)) {
538 /* when we actually did the dec, we didn't hit 0 */
542 /* we hit 0, and we hold the lock */
545 EXPORT_SYMBOL(atomic_dec_and_mutex_lock);