summaryrefslogtreecommitdiffstats
path: root/kernel/mutex.c
blob: f788a5ace24be889e337566845c6cf26f7b7c597 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
/*
 * kernel/mutex.c
 *
 * Mutexes: blocking mutual exclusion locks
 *
 * Started by Ingo Molnar:
 *
 *  Copyright (C) 2004, 2005, 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
 *
 * Many thanks to Arjan van de Ven, Thomas Gleixner, Steven Rostedt and
 * David Howells for suggestions and improvements.
 *
 *  - Adaptive spinning for mutexes by Peter Zijlstra. (Ported to mainline
 *    from the -rt tree, where it was originally implemented for rtmutexes
 *    by Steven Rostedt, based on work by Gregory Haskins, Peter Morreale
 *    and Sven Dietrich.
 *
 * Also see Documentation/mutex-design.txt.
 */
#include <linux/mutex.h>
#include <linux/sched.h>
#include <linux/module.h>
#include <linux/spinlock.h>
#include <linux/interrupt.h>
#include <linux/debug_locks.h>

/*
 * In the DEBUG case we are using the "NULL fastpath" for mutexes,
 * which forces all calls into the slowpath:
 */
#ifdef CONFIG_DEBUG_MUTEXES
# include "mutex-debug.h"
# include <asm-generic/mutex-null.h>
#else
# include "mutex.h"
# include <asm/mutex.h>
#endif

/***
 * mutex_init - initialize the mutex
 * @lock: the mutex to be initialized
 * @key: the lock_class_key for the class; used by mutex lock debugging
 *
 * Initialize the mutex to unlocked state.
 *
 * It is not allowed to initialize an already locked mutex.
 */
void
__mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
{
	atomic_set(&lock->count, 1);
	spin_lock_init(&lock->wait_lock);
	INIT_LIST_HEAD(&lock->wait_list);
	mutex_clear_owner(lock);

	debug_mutex_init(lock, name, key);
}

EXPORT_SYMBOL(__mutex_init);

#ifndef CONFIG_DEBUG_LOCK_ALLOC
/*
 * We split the mutex lock/unlock logic into separate fastpath and
 * slowpath functions, to reduce the register pressure on the fastpath.
 * We also put the fastpath first in the kernel image, to make sure the
 * branch is predicted by the CPU as default-untaken.
 */
static __used noinline void __sched
__mutex_lock_slowpath(atomic_t *lock_count);

/***
 * mutex_lock - acquire the mutex
 * @lock: the mutex to be acquired
 *
 * Lock the mutex exclusively for this task. If the mutex is not
 * available right now, it will sleep until it can get it.
 *
 * The mutex must later on be released by the same task that
 * acquired it. Recursive locking is not allowed. The task
 * may not exit without first unlocking the mutex. Also, kernel
 * memory where the mutex resides mutex must not be freed with
 * the mutex still locked. The mutex must first be initialized
 * (or statically defined) before it can be locked. memset()-ing
 * the mutex to 0 is not allowed.
 *
 * ( The CONFIG_DEBUG_MUTEXES .config option turns on debugging
 *   checks that will enforce the restrictions and will also do
 *   deadlock debugging. )
 *
 * This function is similar to (but not equivalent to) down().
 */
void __sched mutex_lock(struct mutex *lock)
{
	might_sleep();
	/*
	 * The locking fastpath is the 1->0 transition from
	 * 'unlocked' into 'locked' state.
	 */
	__mutex_fastpath_lock(&lock->count, __mutex_lock_slowpath);
	mutex_set_owner(lock);
}

EXPORT_SYMBOL(mutex_lock);
#endif

static __used noinline void __sched __mutex_unlock_slowpath(atomic_t *lock_count);

/***
 * mutex_unlock - release the mutex
 * @lock: the mutex to be released
 *
 * Unlock a mutex that has been locked by this task previously.
 *
 * This function must not be used in interrupt context. Unlocking
 * of a not locked mutex is not allowed.
 *
 * This function is similar to (but not equivalent to) up().
 */
void __sched mutex_unlock(struct mutex *lock)
{
	/*
	 * The unlocking fastpath is the 0->1 transition from 'locked'
	 * into 'unlocked' state:
	 */
#ifndef CONFIG_DEBUG_MUTEXES
	/*
	 * When debugging is enabled we must not clear the owner before time,
	 * the slow path will always be taken, and that clears the owner field
	 * after verifying that it was indeed current.
	 */
	mutex_clear_owner(lock);
#endif
	__mutex_fastpath_unlock(&lock->count, __mutex_unlock_slowpath);
}

EXPORT_SYMBOL(mutex_unlock);

/*
 * Lock a mutex (possibly interruptible), slowpath:
 */
static inline int __sched
__mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
	       	unsigned long ip)
{
	struct task_struct *task = current;
	struct mutex_waiter waiter;
	unsigned long flags;

	preempt_disable();
	mutex_acquire(&lock->dep_map, subclass, 0, ip);
#if defined(CONFIG_SMP) && !defined(CONFIG_DEBUG_MUTEXES) && \
    !defined(CONFIG_HAVE_DEFAULT_NO_SPIN_MUTEXES)
	/*
	 * Optimistic spinning.
	 *
	 * We try to spin for acquisition when we find that there are no
	 * pending waiters and the lock owner is currently running on a
	 * (different) CPU.
	 *
	 * The rationale is that if the lock owner is running, it is likely to
	 * release the lock soon.
	 *
	 * Since this needs the lock owner, and this mutex implementation
	 * doesn't track the owner atomically in the lock field, we need to
	 * track it non-atomically.
	 *
	 * We can't do this for DEBUG_MUTEXES because that relies on wait_lock
	 * to serialize everything.
	 */

	for (;;) {
		struct thread_info *owner;

		/*
		 * If there's an owner, wait for it to either
		 * release the lock or go to sleep.
		 */
		owner = ACCESS_ONCE(lock->owner);
		if (owner && !mutex_spin_on_owner(lock, owner))
			break;

		if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
			lock_acquired(&lock->dep_map, ip);
			mutex_set_owner(lock);
			preempt_enable();
			return 0;
		}

		/*
		 * When there's no owner, we might have preempted between the
		 * owner acquiring the lock and setting the owner field. If
		 * we're an RT task that will live-lock because we won't let
		 * the owner complete.
		 */
		if (!owner && (need_resched() || rt_task(task)))
			break;

		/*
		 * The cpu_relax() call is a compiler barrier which forces
		 * everything in this loop to be re-loaded. We don't need
		 * memory barriers as we'll eventually observe the right
		 * values at the cost of a few extra spins.
		 */
		cpu_relax();
	}
#endif
	spin_lock_mutex(&lock->wait_lock, flags);

	debug_mutex_lock_common(lock, &waiter);
	debug_mutex_add_waiter(lock, &waiter, task_thread_info(task));

	/* add waiting tasks to the end of the waitqueue (FIFO): */
	list_add_tail(&waiter.list, &lock->wait_list);
	waiter.task = task;

	if (atomic_xchg(&lock->count, -1) == 1)
		goto done;

	lock_contended(&lock->dep_map, ip);

	for (;;) {
		/*
		 * Lets try to take the lock again - this is needed even if
		 * we get here for the first time (shortly after failing to
		 * acquire the lock), to make sure that we get a wakeup once
		 * it's unlocked. Later on, if we sleep, this is the
		 * operation that gives us the lock. We xchg it to -1, so
		 * that when we release the lock, we properly wake up the
		 * other waiters:
		 */
		if (atomic_xchg(&lock->count, -1) == 1)
			break;

		/*
		 * got a signal? (This code gets eliminated in the
		 * TASK_UNINTERRUPTIBLE case.)
		 */
		if (unlikely(signal_pending_state(state, task))) {
			mutex_remove_waiter(lock, &waiter,
					    task_thread_info(task));
			mutex_release(&lock->dep_map, 1, ip);
			spin_unlock_mutex(&lock->wait_lock, flags);

			debug_mutex_free_waiter(&waiter);
			preempt_enable();
			return -EINTR;
		}
		__set_task_state(task, state);

		/* didnt get the lock, go to sleep: */
		spin_unlock_mutex(&lock->wait_lock, flags);
		__schedule();
		spin_lock_mutex(&lock->wait_lock, flags);
	}

done:
	lock_acquired(&lock->dep_map, ip);
	/* got the lock - rejoice! */
	mutex_remove_waiter(lock, &waiter, current_thread_info());
	mutex_set_owner(lock);

	/* set it to 0 if there are no waiters left: */
	if (likely(list_empty(&lock->wait_list)))
		atomic_set(&lock->count, 0);

	spin_unlock_mutex(&lock->wait_lock, flags);

	debug_mutex_free_waiter(&waiter);
	preempt_enable();

	return 0;
}

#ifdef CONFIG_DEBUG_LOCK_ALLOC
void __sched
mutex_lock_nested(struct mutex *lock, unsigned int subclass)
{
	might_sleep();
	__mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, subclass, _RET_IP_);
}

EXPORT_SYMBOL_GPL(mutex_lock_nested);

int __sched
mutex_lock_killable_nested(struct mutex *lock, unsigned int subclass)
{
	might_sleep();
	return __mutex_lock_common(lock, TASK_KILLABLE, subclass, _RET_IP_);
}
EXPORT_SYMBOL_GPL(mutex_lock_killable_nested);

int __sched
mutex_lock_interruptible_nested(struct mutex *lock, unsigned int subclass)
{
	might_sleep();
	return __mutex_lock_common(lock, TASK_INTERRUPTIBLE,
				   subclass, _RET_IP_);
}

EXPORT_SYMBOL_GPL(mutex_lock_interruptible_nested);
#endif

/*
 * Release the lock, slowpath:
 */
static inline void
__mutex_unlock_common_slowpath(atomic_t *lock_count, int nested)
{
	struct mutex *lock = container_of(lock_count, struct mutex, count);
	unsigned long flags;

	spin_lock_mutex(&lock->wait_lock, flags);
	mutex_release(&lock->dep_map, nested, _RET_IP_);
	debug_mutex_unlock(lock);

	/*
	 * some architectures leave the lock unlocked in the fastpath failure
	 * case, others need to leave it locked. In the later case we have to
	 * unlock it here
	 */
	if (__mutex_slowpath_needs_to_unlock())
		atomic_set(&lock->count, 1);

	if (!list_empty(&lock->wait_list)) {
		/* get the first entry from the wait-list: */
		struct mutex_waiter *waiter =
				list_entry(lock->wait_list.next,
					   struct mutex_waiter, list);

		debug_mutex_wake_waiter(lock, waiter);

		wake_up_process(waiter->task);
	}

	spin_unlock_mutex(&lock->wait_lock, flags);
}

/*
 * Release the lock, slowpath:
 */
static __used noinline void
__mutex_unlock_slowpath(atomic_t *lock_count)
{
	__mutex_unlock_common_slowpath(lock_count, 1);
}

#ifndef CONFIG_DEBUG_LOCK_ALLOC
/*
 * Here come the less common (and hence less performance-critical) APIs:
 * mutex_lock_interruptible() and mutex_trylock().
 */
static noinline int __sched
__mutex_lock_killable_slowpath(atomic_t *lock_count);

static noinline int __sched
__mutex_lock_interruptible_slowpath(atomic_t *lock_count);

/***
 * mutex_lock_interruptible - acquire the mutex, interruptable
 * @lock: the mutex to be acquired
 *
 * Lock the mutex like mutex_lock(), and return 0 if the mutex has
 * been acquired or sleep until the mutex becomes available. If a
 * signal arrives while waiting for the lock then this function
 * returns -EINTR.
 *
 * This function is similar to (but not equivalent to) down_interruptible().
 */
int __sched mutex_lock_interruptible(struct mutex *lock)
{
	int ret;

	might_sleep();
	ret =  __mutex_fastpath_lock_retval
			(&lock->count, __mutex_lock_interruptible_slowpath);
	if (!ret)
		mutex_set_owner(lock);

	return ret;
}

EXPORT_SYMBOL(mutex_lock_interruptible);

int __sched mutex_lock_killable(struct mutex *lock)
{
	int ret;

	might_sleep();
	ret = __mutex_fastpath_lock_retval
			(&lock->count, __mutex_lock_killable_slowpath);
	if (!ret)
		mutex_set_owner(lock);

	return ret;
}
EXPORT_SYMBOL(mutex_lock_killable);

static __used noinline void __sched
__mutex_lock_slowpath(atomic_t *lock_count)
{
	struct mutex *lock = container_of(lock_count, struct mutex, count);

	__mutex_lock_common(lock, TASK_UNINTERRUPTIBLE, 0, _RET_IP_);
}

static noinline int __sched
__mutex_lock_killable_slowpath(atomic_t *lock_count)
{
	struct mutex *lock = container_of(lock_count, struct mutex, count);

	return __mutex_lock_common(lock, TASK_KILLABLE, 0, _RET_IP_);
}

static noinline int __sched
__mutex_lock_interruptible_slowpath(atomic_t *lock_count)
{
	struct mutex *lock = container_of(lock_count, struct mutex, count);

	return __mutex_lock_common(lock, TASK_INTERRUPTIBLE, 0, _RET_IP_);
}
#endif

/*
 * Spinlock based trylock, we take the spinlock and check whether we
 * can get the lock:
 */
static inline int __mutex_trylock_slowpath(atomic_t *lock_count)
{
	struct mutex *lock = container_of(lock_count, struct mutex, count);
	unsigned long flags;
	int prev;

	spin_lock_mutex(&lock->wait_lock, flags);

	prev = atomic_xchg(&lock->count, -1);
	if (likely(prev == 1)) {
		mutex_set_owner(lock);
		mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
	}

	/* Set it back to 0 if there are no waiters: */
	if (likely(list_empty(&lock->wait_list)))
		atomic_set(&lock->count, 0);

	spin_unlock_mutex(&lock->wait_lock, flags);

	return prev == 1;
}

/***
 * mutex_trylock - try acquire the mutex, without waiting
 * @lock: the mutex to be acquired
 *
 * Try to acquire the mutex atomically. Returns 1 if the mutex
 * has been acquired successfully, and 0 on contention.
 *
 * NOTE: this function follows the spin_trylock() convention, so
 * it is negated to the down_trylock() return values! Be careful
 * about this when converting semaphore users to mutexes.
 *
 * This function must not be used in interrupt context. The
 * mutex must be released by the same task that acquired it.
 */
int __sched mutex_trylock(struct mutex *lock)
{
	int ret;

	ret = __mutex_fastpath_trylock(&lock->count, __mutex_trylock_slowpath);
	if (ret)
		mutex_set_owner(lock);

	return ret;
}
EXPORT_SYMBOL(mutex_trylock);

/**
 * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
 * @cnt: the atomic which we are to dec
 * @lock: the mutex to return holding if we dec to 0
 *
 * return true and hold lock if we dec to 0, return false otherwise
 */
int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock)
{
	/* dec if we can't possibly hit 0 */
	if (atomic_add_unless(cnt, -1, 1))
		return 0;
	/* we might hit 0, so take the lock */
	mutex_lock(lock);
	if (!atomic_dec_and_test(cnt)) {
		/* when we actually did the dec, we didn't hit 0 */
		mutex_unlock(lock);
		return 0;
	}
	/* we hit 0, and we hold the lock */
	return 1;
}
EXPORT_SYMBOL(atomic_dec_and_mutex_lock);