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/module.h>
23 #include <linux/spinlock.h>
24 #include <linux/interrupt.h>
25 #include <linux/debug_locks.h>
28 * In the DEBUG case we are using the "NULL fastpath" for mutexes,
29 * which forces all calls into the slowpath:
31 #ifdef CONFIG_DEBUG_MUTEXES
32 # include "mutex-debug.h"
33 # include <asm-generic/mutex-null.h>
36 # include <asm/mutex.h>
40 __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
42 atomic_set(&lock->count, 1);
43 spin_lock_init(&lock->wait_lock);
44 INIT_LIST_HEAD(&lock->wait_list);
45 mutex_clear_owner(lock);
47 debug_mutex_init(lock, name, key);
50 EXPORT_SYMBOL(__mutex_init);
52 #ifndef CONFIG_DEBUG_LOCK_ALLOC
54 * We split the mutex lock/unlock logic into separate fastpath and
55 * slowpath functions, to reduce the register pressure on the fastpath.
56 * We also put the fastpath first in the kernel image, to make sure the
57 * branch is predicted by the CPU as default-untaken.
59 static __used noinline void __sched
60 __mutex_lock_slowpath(atomic_t *lock_count);
63 * mutex_lock - acquire the mutex
64 * @lock: the mutex to be acquired
66 * Lock the mutex exclusively for this task. If the mutex is not
67 * available right now, it will sleep until it can get it.
69 * The mutex must later on be released by the same task that
70 * acquired it. Recursive locking is not allowed. The task
71 * may not exit without first unlocking the mutex. Also, kernel
72 * memory where the mutex resides mutex must not be freed with
73 * the mutex still locked. The mutex must first be initialized
74 * (or statically defined) before it can be locked. memset()-ing
75 * the mutex to 0 is not allowed.
77 * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging
78 * checks that will enforce the restrictions and will also do
79 * deadlock debugging. )
81 * This function is similar to (but not equivalent to) down().
83 void __sched mutex_lock(struct mutex *lock)
87 * The locking fastpath is the 1->0 transition from
88 * 'unlocked' into 'locked' state.
90 __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
91 mutex_set_owner(lock);
94 EXPORT_SYMBOL(mutex_lock);
97 static __used noinline void __sched __mutex_unlock_slowpath(atomic_t *lock_count);
100 * mutex_unlock - release the mutex
101 * @lock: the mutex to be released
103 * Unlock a mutex that has been locked by this task previously.
105 * This function must not be used in interrupt context. Unlocking
106 * of a not locked mutex is not allowed.
108 * This function is similar to (but not equivalent to) up().
110 void __sched mutex_unlock(struct mutex *lock)
113 * The unlocking fastpath is the 0->1 transition from 'locked'
114 * into 'unlocked' state:
116 #ifndef CONFIG_DEBUG_MUTEXES
118 * When debugging is enabled we must not clear the owner before time,
119 * the slow path will always be taken, and that clears the owner field
120 * after verifying that it was indeed current.
122 mutex_clear_owner(lock);
124 __mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
127 EXPORT_SYMBOL(mutex_unlock);
130 * Lock a mutex (possibly interruptible), slowpath:
132 static inline int __sched
133 __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
134 struct lockdep_map *nest_lock, unsigned long ip)
136 struct task_struct *task = current;
137 struct mutex_waiter waiter;
141 mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
143 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
145 * Optimistic spinning.
147 * We try to spin for acquisition when we find that there are no
148 * pending waiters and the lock owner is currently running on a
151 * The rationale is that if the lock owner is running, it is likely to
152 * release the lock soon.
154 * Since this needs the lock owner, and this mutex implementation
155 * doesn't track the owner atomically in the lock field, we need to
156 * track it non-atomically.
158 * We can't do this for DEBUG_MUTEXES because that relies on wait_lock
159 * to serialize everything.
163 struct task_struct *owner;
166 * If there's an owner, wait for it to either
167 * release the lock or go to sleep.
169 owner = ACCESS_ONCE(lock->owner);
170 if (owner && !mutex_spin_on_owner(lock, owner))
173 if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
174 lock_acquired(&lock->dep_map, ip);
175 mutex_set_owner(lock);
181 * When there's no owner, we might have preempted between the
182 * owner acquiring the lock and setting the owner field. If
183 * we're an RT task that will live-lock because we won't let
184 * the owner complete.
186 if (!owner && (need_resched() || rt_task(task)))
190 * The cpu_relax() call is a compiler barrier which forces
191 * everything in this loop to be re-loaded. We don't need
192 * memory barriers as we'll eventually observe the right
193 * values at the cost of a few extra spins.
195 arch_mutex_cpu_relax();
198 spin_lock_mutex(&lock->wait_lock, flags);
200 debug_mutex_lock_common(lock, &waiter);
201 debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));
203 /* add waiting tasks to the end of the waitqueue (FIFO): */
204 list_add_tail(&waiter.list, &lock->wait_list);
207 if (atomic_xchg(&lock->count, -1) == 1)
210 lock_contended(&lock->dep_map, ip);
214 * Lets try to take the lock again - this is needed even if
215 * we get here for the first time (shortly after failing to
216 * acquire the lock), to make sure that we get a wakeup once
217 * it's unlocked. Later on, if we sleep, this is the
218 * operation that gives us the lock. We xchg it to -1, so
219 * that when we release the lock, we properly wake up the
222 if (atomic_xchg(&lock->count, -1) == 1)
226 * got a signal? (This code gets eliminated in the
227 * TASK_UNINTERRUPTIBLE case.)
229 if (unlikely(signal_pending_state(state, task))) {
230 mutex_remove_waiter(lock, &waiter,
231 task_thread_info(task));
232 mutex_release(&lock->dep_map, 1, ip);
233 spin_unlock_mutex(&lock->wait_lock, flags);
235 debug_mutex_free_waiter(&waiter);
239 __set_task_state(task, state);
241 /* didn't get the lock, go to sleep: */
242 spin_unlock_mutex(&lock->wait_lock, flags);
243 preempt_enable_no_resched();
246 spin_lock_mutex(&lock->wait_lock, flags);
250 lock_acquired(&lock->dep_map, ip);
251 /* got the lock - rejoice! */
252 mutex_remove_waiter(lock, &waiter, current_thread_info());
253 mutex_set_owner(lock);
255 /* set it to 0 if there are no waiters left: */
256 if (likely(list_empty(&lock->wait_list)))
257 atomic_set(&lock->count, 0);
259 spin_unlock_mutex(&lock->wait_lock, flags);
261 debug_mutex_free_waiter(&waiter);
267 #ifdef CONFIG_DEBUG_LOCK_ALLOC
269 mutex_lock_nested(struct mutex *lock, unsigned int subclass)
272 __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, subclass, NULL, _RET_IP_);
275 EXPORT_SYMBOL_GPL(mutex_lock_nested);
278 _mutex_lock_nest_lock(struct mutex *lock, struct lockdep_map *nest)
281 __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, nest, _RET_IP_);
284 EXPORT_SYMBOL_GPL(_mutex_lock_nest_lock);
287 mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass)
290 return __mutex_lock_common(lock, TASK_KILLABLE, subclass, NULL, _RET_IP_);
292 EXPORT_SYMBOL_GPL(mutex_lock_killable_nested);
295 mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
298 return __mutex_lock_common(lock, TASK_INTERRUPTIBLE,
299 subclass, NULL, _RET_IP_);
302 EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);
306 * Release the lock, slowpath:
309 __mutex_unlock_common_slowpath(atomic_t *lock_count, int nested)
311 struct mutex *lock = container_of(lock_count, struct mutex, count);
314 spin_lock_mutex(&lock->wait_lock, flags);
315 mutex_release(&lock->dep_map, nested, _RET_IP_);
316 debug_mutex_unlock(lock);
319 * some architectures leave the lock unlocked in the fastpath failure
320 * case, others need to leave it locked. In the later case we have to
323 if (__mutex_slowpath_needs_to_unlock())
324 atomic_set(&lock->count, 1);
326 if (!list_empty(&lock->wait_list)) {
327 /* get the first entry from the wait-list: */
328 struct mutex_waiter *waiter =
329 list_entry(lock->wait_list.next,
330 struct mutex_waiter, list);
332 debug_mutex_wake_waiter(lock, waiter);
334 wake_up_process(waiter->task);
337 spin_unlock_mutex(&lock->wait_lock, flags);
341 * Release the lock, slowpath:
343 static __used noinline void
344 __mutex_unlock_slowpath(atomic_t *lock_count)
346 __mutex_unlock_common_slowpath(lock_count, 1);
349 #ifndef CONFIG_DEBUG_LOCK_ALLOC
351 * Here come the less common (and hence less performance-critical) APIs:
352 * mutex_lock_interruptible() and mutex_trylock().
354 static noinline int __sched
355 __mutex_lock_killable_slowpath(atomic_t *lock_count);
357 static noinline int __sched
358 __mutex_lock_interruptible_slowpath(atomic_t *lock_count);
361 * mutex_lock_interruptible - acquire the mutex, interruptible
362 * @lock: the mutex to be acquired
364 * Lock the mutex like mutex_lock(), and return 0 if the mutex has
365 * been acquired or sleep until the mutex becomes available. If a
366 * signal arrives while waiting for the lock then this function
369 * This function is similar to (but not equivalent to) down_interruptible().
371 int __sched mutex_lock_interruptible(struct mutex *lock)
376 ret = __mutex_fastpath_lock_retval
377 (&lock->count, __mutex_lock_interruptible_slowpath);
379 mutex_set_owner(lock);
384 EXPORT_SYMBOL(mutex_lock_interruptible);
386 int __sched mutex_lock_killable(struct mutex *lock)
391 ret = __mutex_fastpath_lock_retval
392 (&lock->count, __mutex_lock_killable_slowpath);
394 mutex_set_owner(lock);
398 EXPORT_SYMBOL(mutex_lock_killable);
400 static __used noinline void __sched
401 __mutex_lock_slowpath(atomic_t *lock_count)
403 struct mutex *lock = container_of(lock_count, struct mutex, count);
405 __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, NULL, _RET_IP_);
408 static noinline int __sched
409 __mutex_lock_killable_slowpath(atomic_t *lock_count)
411 struct mutex *lock = container_of(lock_count, struct mutex, count);
413 return __mutex_lock_common(lock, TASK_KILLABLE, 0, NULL, _RET_IP_);
416 static noinline int __sched
417 __mutex_lock_interruptible_slowpath(atomic_t *lock_count)
419 struct mutex *lock = container_of(lock_count, struct mutex, count);
421 return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0, NULL, _RET_IP_);
426 * Spinlock based trylock, we take the spinlock and check whether we
429 static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
431 struct mutex *lock = container_of(lock_count, struct mutex, count);
435 spin_lock_mutex(&lock->wait_lock, flags);
437 prev = atomic_xchg(&lock->count, -1);
438 if (likely(prev == 1)) {
439 mutex_set_owner(lock);
440 mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
443 /* Set it back to 0 if there are no waiters: */
444 if (likely(list_empty(&lock->wait_list)))
445 atomic_set(&lock->count, 0);
447 spin_unlock_mutex(&lock->wait_lock, flags);
453 * mutex_trylock - try to acquire the mutex, without waiting
454 * @lock: the mutex to be acquired
456 * Try to acquire the mutex atomically. Returns 1 if the mutex
457 * has been acquired successfully, and 0 on contention.
459 * NOTE: this function follows the spin_trylock() convention, so
460 * it is negated from the down_trylock() return values! Be careful
461 * about this when converting semaphore users to mutexes.
463 * This function must not be used in interrupt context. The
464 * mutex must be released by the same task that acquired it.
466 int __sched mutex_trylock(struct mutex *lock)
470 ret = __mutex_fastpath_trylock(&lock->count, __mutex_trylock_slowpath);
472 mutex_set_owner(lock);
476 EXPORT_SYMBOL(mutex_trylock);
479 * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
480 * @cnt: the atomic which we are to dec
481 * @lock: the mutex to return holding if we dec to 0
483 * return true and hold lock if we dec to 0, return false otherwise
485 int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
487 /* dec if we can't possibly hit 0 */
488 if (atomic_add_unless(cnt, -1, 1))
490 /* we might hit 0, so take the lock */
492 if (!atomic_dec_and_test(cnt)) {
493 /* when we actually did the dec, we didn't hit 0 */
497 /* we hit 0, and we hold the lock */
500 EXPORT_SYMBOL(atomic_dec_and_mutex_lock);