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);
59 debug_mutex_init(lock, name, key);
62 EXPORT_SYMBOL(__mutex_init);
64 #ifndef CONFIG_DEBUG_LOCK_ALLOC
66 * We split the mutex lock/unlock logic into separate fastpath and
67 * slowpath functions, to reduce the register pressure on the fastpath.
68 * We also put the fastpath first in the kernel image, to make sure the
69 * branch is predicted by the CPU as default-untaken.
71 static __used noinline void __sched
72 __mutex_lock_slowpath(atomic_t *lock_count);
75 * mutex_lock - acquire the mutex
76 * @lock: the mutex to be acquired
78 * Lock the mutex exclusively for this task. If the mutex is not
79 * available right now, it will sleep until it can get it.
81 * The mutex must later on be released by the same task that
82 * acquired it. Recursive locking is not allowed. The task
83 * may not exit without first unlocking the mutex. Also, kernel
84 * memory where the mutex resides mutex must not be freed with
85 * the mutex still locked. The mutex must first be initialized
86 * (or statically defined) before it can be locked. memset()-ing
87 * the mutex to 0 is not allowed.
89 * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging
90 * checks that will enforce the restrictions and will also do
91 * deadlock debugging. )
93 * This function is similar to (but not equivalent to) down().
95 void __sched mutex_lock(struct mutex *lock)
99 * The locking fastpath is the 1->0 transition from
100 * 'unlocked' into 'locked' state.
102 __mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
103 mutex_set_owner(lock);
106 EXPORT_SYMBOL(mutex_lock);
109 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
111 * Mutex spinning code migrated from kernel/sched/core.c
114 static inline bool owner_running(struct mutex *lock, struct task_struct *owner)
116 if (lock->owner != owner)
120 * Ensure we emit the owner->on_cpu, dereference _after_ checking
121 * lock->owner still matches owner, if that fails, owner might
122 * point to free()d memory, if it still matches, the rcu_read_lock()
123 * ensures the memory stays valid.
127 return owner->on_cpu;
131 * Look out! "owner" is an entirely speculative pointer
132 * access and not reliable.
135 int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
138 while (owner_running(lock, owner)) {
142 arch_mutex_cpu_relax();
147 * We break out the loop above on need_resched() and when the
148 * owner changed, which is a sign for heavy contention. Return
149 * success only when lock->owner is NULL.
151 return lock->owner == NULL;
155 static __used noinline void __sched __mutex_unlock_slowpath(atomic_t *lock_count);
158 * mutex_unlock - release the mutex
159 * @lock: the mutex to be released
161 * Unlock a mutex that has been locked by this task previously.
163 * This function must not be used in interrupt context. Unlocking
164 * of a not locked mutex is not allowed.
166 * This function is similar to (but not equivalent to) up().
168 void __sched mutex_unlock(struct mutex *lock)
171 * The unlocking fastpath is the 0->1 transition from 'locked'
172 * into 'unlocked' state:
174 #ifndef CONFIG_DEBUG_MUTEXES
176 * When debugging is enabled we must not clear the owner before time,
177 * the slow path will always be taken, and that clears the owner field
178 * after verifying that it was indeed current.
180 mutex_clear_owner(lock);
182 __mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
185 EXPORT_SYMBOL(mutex_unlock);
188 * Lock a mutex (possibly interruptible), slowpath:
190 static inline int __sched
191 __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
192 struct lockdep_map *nest_lock, unsigned long ip)
194 struct task_struct *task = current;
195 struct mutex_waiter waiter;
199 mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
201 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
203 * Optimistic spinning.
205 * We try to spin for acquisition when we find that there are no
206 * pending waiters and the lock owner is currently running on a
209 * The rationale is that if the lock owner is running, it is likely to
210 * release the lock soon.
212 * Since this needs the lock owner, and this mutex implementation
213 * doesn't track the owner atomically in the lock field, we need to
214 * track it non-atomically.
216 * We can't do this for DEBUG_MUTEXES because that relies on wait_lock
217 * to serialize everything.
221 struct task_struct *owner;
224 * If there's an owner, wait for it to either
225 * release the lock or go to sleep.
227 owner = ACCESS_ONCE(lock->owner);
228 if (owner && !mutex_spin_on_owner(lock, owner))
231 if ((atomic_read(&lock->count) == 1) &&
232 (atomic_cmpxchg(&lock->count, 1, 0) == 1)) {
233 lock_acquired(&lock->dep_map, ip);
234 mutex_set_owner(lock);
240 * When there's no owner, we might have preempted between the
241 * owner acquiring the lock and setting the owner field. If
242 * we're an RT task that will live-lock because we won't let
243 * the owner complete.
245 if (!owner && (need_resched() || rt_task(task)))
249 * The cpu_relax() call is a compiler barrier which forces
250 * everything in this loop to be re-loaded. We don't need
251 * memory barriers as we'll eventually observe the right
252 * values at the cost of a few extra spins.
254 arch_mutex_cpu_relax();
257 spin_lock_mutex(&lock->wait_lock, flags);
259 debug_mutex_lock_common(lock, &waiter);
260 debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));
262 /* add waiting tasks to the end of the waitqueue (FIFO): */
263 list_add_tail(&waiter.list, &lock->wait_list);
266 if (MUTEX_SHOW_NO_WAITER(lock) && (atomic_xchg(&lock->count, -1) == 1))
269 lock_contended(&lock->dep_map, ip);
273 * Lets try to take the lock again - this is needed even if
274 * we get here for the first time (shortly after failing to
275 * acquire the lock), to make sure that we get a wakeup once
276 * it's unlocked. Later on, if we sleep, this is the
277 * operation that gives us the lock. We xchg it to -1, so
278 * that when we release the lock, we properly wake up the
281 if (MUTEX_SHOW_NO_WAITER(lock) &&
282 (atomic_xchg(&lock->count, -1) == 1))
286 * got a signal? (This code gets eliminated in the
287 * TASK_UNINTERRUPTIBLE case.)
289 if (unlikely(signal_pending_state(state, task))) {
290 mutex_remove_waiter(lock, &waiter,
291 task_thread_info(task));
292 mutex_release(&lock->dep_map, 1, ip);
293 spin_unlock_mutex(&lock->wait_lock, flags);
295 debug_mutex_free_waiter(&waiter);
299 __set_task_state(task, state);
301 /* didn't get the lock, go to sleep: */
302 spin_unlock_mutex(&lock->wait_lock, flags);
303 schedule_preempt_disabled();
304 spin_lock_mutex(&lock->wait_lock, flags);
308 lock_acquired(&lock->dep_map, ip);
309 /* got the lock - rejoice! */
310 mutex_remove_waiter(lock, &waiter, current_thread_info());
311 mutex_set_owner(lock);
313 /* set it to 0 if there are no waiters left: */
314 if (likely(list_empty(&lock->wait_list)))
315 atomic_set(&lock->count, 0);
317 spin_unlock_mutex(&lock->wait_lock, flags);
319 debug_mutex_free_waiter(&waiter);
325 #ifdef CONFIG_DEBUG_LOCK_ALLOC
327 mutex_lock_nested(struct mutex *lock, unsigned int subclass)
330 __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, subclass, NULL, _RET_IP_);
333 EXPORT_SYMBOL_GPL(mutex_lock_nested);
336 _mutex_lock_nest_lock(struct mutex *lock, struct lockdep_map *nest)
339 __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, nest, _RET_IP_);
342 EXPORT_SYMBOL_GPL(_mutex_lock_nest_lock);
345 mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass)
348 return __mutex_lock_common(lock, TASK_KILLABLE, subclass, NULL, _RET_IP_);
350 EXPORT_SYMBOL_GPL(mutex_lock_killable_nested);
353 mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
356 return __mutex_lock_common(lock, TASK_INTERRUPTIBLE,
357 subclass, NULL, _RET_IP_);
360 EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);
364 * Release the lock, slowpath:
367 __mutex_unlock_common_slowpath(atomic_t *lock_count, int nested)
369 struct mutex *lock = container_of(lock_count, struct mutex, count);
372 spin_lock_mutex(&lock->wait_lock, flags);
373 mutex_release(&lock->dep_map, nested, _RET_IP_);
374 debug_mutex_unlock(lock);
377 * some architectures leave the lock unlocked in the fastpath failure
378 * case, others need to leave it locked. In the later case we have to
381 if (__mutex_slowpath_needs_to_unlock())
382 atomic_set(&lock->count, 1);
384 if (!list_empty(&lock->wait_list)) {
385 /* get the first entry from the wait-list: */
386 struct mutex_waiter *waiter =
387 list_entry(lock->wait_list.next,
388 struct mutex_waiter, list);
390 debug_mutex_wake_waiter(lock, waiter);
392 wake_up_process(waiter->task);
395 spin_unlock_mutex(&lock->wait_lock, flags);
399 * Release the lock, slowpath:
401 static __used noinline void
402 __mutex_unlock_slowpath(atomic_t *lock_count)
404 __mutex_unlock_common_slowpath(lock_count, 1);
407 #ifndef CONFIG_DEBUG_LOCK_ALLOC
409 * Here come the less common (and hence less performance-critical) APIs:
410 * mutex_lock_interruptible() and mutex_trylock().
412 static noinline int __sched
413 __mutex_lock_killable_slowpath(atomic_t *lock_count);
415 static noinline int __sched
416 __mutex_lock_interruptible_slowpath(atomic_t *lock_count);
419 * mutex_lock_interruptible - acquire the mutex, interruptible
420 * @lock: the mutex to be acquired
422 * Lock the mutex like mutex_lock(), and return 0 if the mutex has
423 * been acquired or sleep until the mutex becomes available. If a
424 * signal arrives while waiting for the lock then this function
427 * This function is similar to (but not equivalent to) down_interruptible().
429 int __sched mutex_lock_interruptible(struct mutex *lock)
434 ret = __mutex_fastpath_lock_retval
435 (&lock->count, __mutex_lock_interruptible_slowpath);
437 mutex_set_owner(lock);
442 EXPORT_SYMBOL(mutex_lock_interruptible);
444 int __sched mutex_lock_killable(struct mutex *lock)
449 ret = __mutex_fastpath_lock_retval
450 (&lock->count, __mutex_lock_killable_slowpath);
452 mutex_set_owner(lock);
456 EXPORT_SYMBOL(mutex_lock_killable);
458 static __used noinline void __sched
459 __mutex_lock_slowpath(atomic_t *lock_count)
461 struct mutex *lock = container_of(lock_count, struct mutex, count);
463 __mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, NULL, _RET_IP_);
466 static noinline int __sched
467 __mutex_lock_killable_slowpath(atomic_t *lock_count)
469 struct mutex *lock = container_of(lock_count, struct mutex, count);
471 return __mutex_lock_common(lock, TASK_KILLABLE, 0, NULL, _RET_IP_);
474 static noinline int __sched
475 __mutex_lock_interruptible_slowpath(atomic_t *lock_count)
477 struct mutex *lock = container_of(lock_count, struct mutex, count);
479 return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0, NULL, _RET_IP_);
484 * Spinlock based trylock, we take the spinlock and check whether we
487 static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
489 struct mutex *lock = container_of(lock_count, struct mutex, count);
493 spin_lock_mutex(&lock->wait_lock, flags);
495 prev = atomic_xchg(&lock->count, -1);
496 if (likely(prev == 1)) {
497 mutex_set_owner(lock);
498 mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
501 /* Set it back to 0 if there are no waiters: */
502 if (likely(list_empty(&lock->wait_list)))
503 atomic_set(&lock->count, 0);
505 spin_unlock_mutex(&lock->wait_lock, flags);
511 * mutex_trylock - try to acquire the mutex, without waiting
512 * @lock: the mutex to be acquired
514 * Try to acquire the mutex atomically. Returns 1 if the mutex
515 * has been acquired successfully, and 0 on contention.
517 * NOTE: this function follows the spin_trylock() convention, so
518 * it is negated from the down_trylock() return values! Be careful
519 * about this when converting semaphore users to mutexes.
521 * This function must not be used in interrupt context. The
522 * mutex must be released by the same task that acquired it.
524 int __sched mutex_trylock(struct mutex *lock)
528 ret = __mutex_fastpath_trylock(&lock->count, __mutex_trylock_slowpath);
530 mutex_set_owner(lock);
534 EXPORT_SYMBOL(mutex_trylock);
537 * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
538 * @cnt: the atomic which we are to dec
539 * @lock: the mutex to return holding if we dec to 0
541 * return true and hold lock if we dec to 0, return false otherwise
543 int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
545 /* dec if we can't possibly hit 0 */
546 if (atomic_add_unless(cnt, -1, 1))
548 /* we might hit 0, so take the lock */
550 if (!atomic_dec_and_test(cnt)) {
551 /* when we actually did the dec, we didn't hit 0 */
555 /* we hit 0, and we hold the lock */
558 EXPORT_SYMBOL(atomic_dec_and_mutex_lock);