1/*
  2 * kernel/mutex.c
  3 *
  4 * Mutexes: blocking mutual exclusion locks
  5 *
  6 * Started by Ingo Molnar:
  7 *
  8 *  Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
  9 *
 10 * Many thanks to Arjan van de Ven, Thomas Gleixner, Steven Rostedt and
 11 * David Howells for suggestions and improvements.
 12 *
 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
 16 *    and Sven Dietrich.
 17 *
 18 * Also see Documentation/mutex-design.txt.
 19 */
 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>
 26
 27/*
 28 * In the DEBUG case we are using the "NULL fastpath" for mutexes,
 29 * which forces all calls into the slowpath:
 30 */
 31#ifdef CONFIG_DEBUG_MUTEXES
 32# include "mutex-debug.h"
 33# include <asm-generic/mutex-null.h>
 34#else
 35# include "mutex.h"
 36# include <asm/mutex.h>
 37#endif
 38
 39void
 40__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
 41{
 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);
 46
 47	debug_mutex_init(lock, name, key);
 48}
 49
 50EXPORT_SYMBOL(__mutex_init);
 51
 52#ifndef CONFIG_DEBUG_LOCK_ALLOC
 53/*
 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.
 58 */
 59static __used noinline void __sched
 60__mutex_lock_slowpath(atomic_t *lock_count);
 61
 62/**
 63 * mutex_lock - acquire the mutex
 64 * @lock: the mutex to be acquired
 65 *
 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.
 68 *
 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.
 76 *
 77 * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging
 78 *   checks that will enforce the restrictions and will also do
 79 *   deadlock debugging. )
 80 *
 81 * This function is similar to (but not equivalent to) down().
 82 */
 83void __sched mutex_lock(struct mutex *lock)
 84{
 85	might_sleep();
 86	/*
 87	 * The locking fastpath is the 1->0 transition from
 88	 * 'unlocked' into 'locked' state.
 89	 */
 90	__mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
 91	mutex_set_owner(lock);
 92}
 93
 94EXPORT_SYMBOL(mutex_lock);
 95#endif
 96
 97static __used noinline void __sched __mutex_unlock_slowpath(atomic_t *lock_count);
 98
 99/**
100 * mutex_unlock - release the mutex
101 * @lock: the mutex to be released
102 *
103 * Unlock a mutex that has been locked by this task previously.
104 *
105 * This function must not be used in interrupt context. Unlocking
106 * of a not locked mutex is not allowed.
107 *
108 * This function is similar to (but not equivalent to) up().
109 */
110void __sched mutex_unlock(struct mutex *lock)
111{
112	/*
113	 * The unlocking fastpath is the 0->1 transition from 'locked'
114	 * into 'unlocked' state:
115	 */
116#ifndef CONFIG_DEBUG_MUTEXES
117	/*
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.
121	 */
122	mutex_clear_owner(lock);
123#endif
124	__mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
125}
126
127EXPORT_SYMBOL(mutex_unlock);
128
129/*
130 * Lock a mutex (possibly interruptible), slowpath:
131 */
132static inline int __sched
133__mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
134		    struct lockdep_map *nest_lock, unsigned long ip)
135{
136	struct task_struct *task = current;
137	struct mutex_waiter waiter;
138	unsigned long flags;
139
140	preempt_disable();
141	mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
142
143#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
144	/*
145	 * Optimistic spinning.
146	 *
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
149	 * (different) CPU.
150	 *
151	 * The rationale is that if the lock owner is running, it is likely to
152	 * release the lock soon.
153	 *
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.
157	 *
158	 * We can't do this for DEBUG_MUTEXES because that relies on wait_lock
159	 * to serialize everything.
160	 */
161
162	for (;;) {
163		struct task_struct *owner;
164
165		/*
166		 * If there's an owner, wait for it to either
167		 * release the lock or go to sleep.
168		 */
169		owner = ACCESS_ONCE(lock->owner);
170		if (owner && !mutex_spin_on_owner(lock, owner))
171			break;
172
173		if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
174			lock_acquired(&lock->dep_map, ip);
175			mutex_set_owner(lock);
176			preempt_enable();
177			return 0;
178		}
179
180		/*
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.
185		 */
186		if (!owner && (need_resched() || rt_task(task)))
187			break;
188
189		/*
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.
194		 */
195		arch_mutex_cpu_relax();
196	}
197#endif
198	spin_lock_mutex(&lock->wait_lock, flags);
199
200	debug_mutex_lock_common(lock, &waiter);
201	debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));
202
203	/* add waiting tasks to the end of the waitqueue (FIFO): */
204	list_add_tail(&waiter.list, &lock->wait_list);
205	waiter.task = task;
206
207	if (atomic_xchg(&lock->count, -1) == 1)
208		goto done;
209
210	lock_contended(&lock->dep_map, ip);
211
212	for (;;) {
213		/*
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
220		 * other waiters:
221		 */
222		if (atomic_xchg(&lock->count, -1) == 1)
223			break;
224
225		/*
226		 * got a signal? (This code gets eliminated in the
227		 * TASK_UNINTERRUPTIBLE case.)
228		 */
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);
234
235			debug_mutex_free_waiter(&waiter);
236			preempt_enable();
237			return -EINTR;
238		}
239		__set_task_state(task, state);
240
241		/* didn't get the lock, go to sleep: */
242		spin_unlock_mutex(&lock->wait_lock, flags);
243		preempt_enable_no_resched();
244		schedule();
245		preempt_disable();
246		spin_lock_mutex(&lock->wait_lock, flags);
247	}
248
249done:
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);
254
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);
258
259	spin_unlock_mutex(&lock->wait_lock, flags);
260
261	debug_mutex_free_waiter(&waiter);
262	preempt_enable();
263
264	return 0;
265}
266
267#ifdef CONFIG_DEBUG_LOCK_ALLOC
268void __sched
269mutex_lock_nested(struct mutex *lock, unsigned int subclass)
270{
271	might_sleep();
272	__mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, subclass, NULL, _RET_IP_);
273}
274
275EXPORT_SYMBOL_GPL(mutex_lock_nested);
276
277void __sched
278_mutex_lock_nest_lock(struct mutex *lock, struct lockdep_map *nest)
279{
280	might_sleep();
281	__mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, nest, _RET_IP_);
282}
283
284EXPORT_SYMBOL_GPL(_mutex_lock_nest_lock);
285
286int __sched
287mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass)
288{
289	might_sleep();
290	return __mutex_lock_common(lock, TASK_KILLABLE, subclass, NULL, _RET_IP_);
291}
292EXPORT_SYMBOL_GPL(mutex_lock_killable_nested);
293
294int __sched
295mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
296{
297	might_sleep();
298	return __mutex_lock_common(lock, TASK_INTERRUPTIBLE,
299				   subclass, NULL, _RET_IP_);
300}
301
302EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);
303#endif
304
305/*
306 * Release the lock, slowpath:
307 */
308static inline void
309__mutex_unlock_common_slowpath(atomic_t *lock_count, int nested)
310{
311	struct mutex *lock = container_of(lock_count, struct mutex, count);
312	unsigned long flags;
313
314	spin_lock_mutex(&lock->wait_lock, flags);
315	mutex_release(&lock->dep_map, nested, _RET_IP_);
316	debug_mutex_unlock(lock);
317
318	/*
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
321	 * unlock it here
322	 */
323	if (__mutex_slowpath_needs_to_unlock())
324		atomic_set(&lock->count, 1);
325
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);
331
332		debug_mutex_wake_waiter(lock, waiter);
333
334		wake_up_process(waiter->task);
335	}
336
337	spin_unlock_mutex(&lock->wait_lock, flags);
338}
339
340/*
341 * Release the lock, slowpath:
342 */
343static __used noinline void
344__mutex_unlock_slowpath(atomic_t *lock_count)
345{
346	__mutex_unlock_common_slowpath(lock_count, 1);
347}
348
349#ifndef CONFIG_DEBUG_LOCK_ALLOC
350/*
351 * Here come the less common (and hence less performance-critical) APIs:
352 * mutex_lock_interruptible() and mutex_trylock().
353 */
354static noinline int __sched
355__mutex_lock_killable_slowpath(atomic_t *lock_count);
356
357static noinline int __sched
358__mutex_lock_interruptible_slowpath(atomic_t *lock_count);
359
360/**
361 * mutex_lock_interruptible - acquire the mutex, interruptible
362 * @lock: the mutex to be acquired
363 *
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
367 * returns -EINTR.
368 *
369 * This function is similar to (but not equivalent to) down_interruptible().
370 */
371int __sched mutex_lock_interruptible(struct mutex *lock)
372{
373	int ret;
374
375	might_sleep();
376	ret =  __mutex_fastpath_lock_retval
377			(&lock->count, __mutex_lock_interruptible_slowpath);
378	if (!ret)
379		mutex_set_owner(lock);
380
381	return ret;
382}
383
384EXPORT_SYMBOL(mutex_lock_interruptible);
385
386int __sched mutex_lock_killable(struct mutex *lock)
387{
388	int ret;
389
390	might_sleep();
391	ret = __mutex_fastpath_lock_retval
392			(&lock->count, __mutex_lock_killable_slowpath);
393	if (!ret)
394		mutex_set_owner(lock);
395
396	return ret;
397}
398EXPORT_SYMBOL(mutex_lock_killable);
399
400static __used noinline void __sched
401__mutex_lock_slowpath(atomic_t *lock_count)
402{
403	struct mutex *lock = container_of(lock_count, struct mutex, count);
404
405	__mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, NULL, _RET_IP_);
406}
407
408static noinline int __sched
409__mutex_lock_killable_slowpath(atomic_t *lock_count)
410{
411	struct mutex *lock = container_of(lock_count, struct mutex, count);
412
413	return __mutex_lock_common(lock, TASK_KILLABLE, 0, NULL, _RET_IP_);
414}
415
416static noinline int __sched
417__mutex_lock_interruptible_slowpath(atomic_t *lock_count)
418{
419	struct mutex *lock = container_of(lock_count, struct mutex, count);
420
421	return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0, NULL, _RET_IP_);
422}
423#endif
424
425/*
426 * Spinlock based trylock, we take the spinlock and check whether we
427 * can get the lock:
428 */
429static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
430{
431	struct mutex *lock = container_of(lock_count, struct mutex, count);
432	unsigned long flags;
433	int prev;
434
435	spin_lock_mutex(&lock->wait_lock, flags);
436
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_);
441	}
442
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);
446
447	spin_unlock_mutex(&lock->wait_lock, flags);
448
449	return prev == 1;
450}
451
452/**
453 * mutex_trylock - try to acquire the mutex, without waiting
454 * @lock: the mutex to be acquired
455 *
456 * Try to acquire the mutex atomically. Returns 1 if the mutex
457 * has been acquired successfully, and 0 on contention.
458 *
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.
462 *
463 * This function must not be used in interrupt context. The
464 * mutex must be released by the same task that acquired it.
465 */
466int __sched mutex_trylock(struct mutex *lock)
467{
468	int ret;
469
470	ret = __mutex_fastpath_trylock(&lock->count, __mutex_trylock_slowpath);
471	if (ret)
472		mutex_set_owner(lock);
473
474	return ret;
475}
476EXPORT_SYMBOL(mutex_trylock);
477
478/**
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
482 *
483 * return true and hold lock if we dec to 0, return false otherwise
484 */
485int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
486{
487	/* dec if we can't possibly hit 0 */
488	if (atomic_add_unless(cnt, -1, 1))
489		return 0;
490	/* we might hit 0, so take the lock */
491	mutex_lock(lock);
492	if (!atomic_dec_and_test(cnt)) {
493		/* when we actually did the dec, we didn't hit 0 */
494		mutex_unlock(lock);
495		return 0;
496	}
497	/* we hit 0, and we hold the lock */
498	return 1;
499}
500EXPORT_SYMBOL(atomic_dec_and_mutex_lock);