summaryrefslogtreecommitdiff
path: root/kernel/sched/wait.c
blob: 01f5d3020589de0e85ed2fa706c30bc0be547687 (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
// SPDX-License-Identifier: GPL-2.0-only
/*
 * Generic waiting primitives.
 *
 * (C) 2004 Nadia Yvette Chambers, Oracle
 */
#include "sched.h"

void __init_waitqueue_head(struct wait_queue_head *wq_head, const char *name, struct lock_class_key *key)
{
	spin_lock_init(&wq_head->lock);
	lockdep_set_class_and_name(&wq_head->lock, key, name);
	INIT_LIST_HEAD(&wq_head->head);
}

EXPORT_SYMBOL(__init_waitqueue_head);

void add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
{
	unsigned long flags;

	wq_entry->flags &= ~WQ_FLAG_EXCLUSIVE;
	spin_lock_irqsave(&wq_head->lock, flags);
	__add_wait_queue(wq_head, wq_entry);
	spin_unlock_irqrestore(&wq_head->lock, flags);
}
EXPORT_SYMBOL(add_wait_queue);

void add_wait_queue_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
{
	unsigned long flags;

	wq_entry->flags |= WQ_FLAG_EXCLUSIVE;
	spin_lock_irqsave(&wq_head->lock, flags);
	__add_wait_queue_entry_tail(wq_head, wq_entry);
	spin_unlock_irqrestore(&wq_head->lock, flags);
}
EXPORT_SYMBOL(add_wait_queue_exclusive);

void remove_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
{
	unsigned long flags;

	spin_lock_irqsave(&wq_head->lock, flags);
	__remove_wait_queue(wq_head, wq_entry);
	spin_unlock_irqrestore(&wq_head->lock, flags);
}
EXPORT_SYMBOL(remove_wait_queue);

/*
 * Scan threshold to break wait queue walk.
 * This allows a waker to take a break from holding the
 * wait queue lock during the wait queue walk.
 */
#define WAITQUEUE_WALK_BREAK_CNT 64

/*
 * The core wakeup function. Non-exclusive wakeups (nr_exclusive == 0) just
 * wake everything up. If it's an exclusive wakeup (nr_exclusive == small +ve
 * number) then we wake all the non-exclusive tasks and one exclusive task.
 *
 * There are circumstances in which we can try to wake a task which has already
 * started to run but is not in state TASK_RUNNING. try_to_wake_up() returns
 * zero in this (rare) case, and we handle it by continuing to scan the queue.
 */
static int __wake_up_common(struct wait_queue_head *wq_head, unsigned int mode,
			int nr_exclusive, int wake_flags, void *key,
			wait_queue_entry_t *bookmark)
{
	wait_queue_entry_t *curr, *next;
	int cnt = 0;

	lockdep_assert_held(&wq_head->lock);

	if (bookmark && (bookmark->flags & WQ_FLAG_BOOKMARK)) {
		curr = list_next_entry(bookmark, entry);

		list_del(&bookmark->entry);
		bookmark->flags = 0;
	} else
		curr = list_first_entry(&wq_head->head, wait_queue_entry_t, entry);

	if (&curr->entry == &wq_head->head)
		return nr_exclusive;

	list_for_each_entry_safe_from(curr, next, &wq_head->head, entry) {
		unsigned flags = curr->flags;
		int ret;

		if (flags & WQ_FLAG_BOOKMARK)
			continue;

		ret = curr->func(curr, mode, wake_flags, key);
		if (ret < 0)
			break;
		if (ret && (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
			break;

		if (bookmark && (++cnt > WAITQUEUE_WALK_BREAK_CNT) &&
				(&next->entry != &wq_head->head)) {
			bookmark->flags = WQ_FLAG_BOOKMARK;
			list_add_tail(&bookmark->entry, &next->entry);
			break;
		}
	}

	return nr_exclusive;
}

static void __wake_up_common_lock(struct wait_queue_head *wq_head, unsigned int mode,
			int nr_exclusive, int wake_flags, void *key)
{
	unsigned long flags;
	wait_queue_entry_t bookmark;

	bookmark.flags = 0;
	bookmark.private = NULL;
	bookmark.func = NULL;
	INIT_LIST_HEAD(&bookmark.entry);

	do {
		spin_lock_irqsave(&wq_head->lock, flags);
		nr_exclusive = __wake_up_common(wq_head, mode, nr_exclusive,
						wake_flags, key, &bookmark);
		spin_unlock_irqrestore(&wq_head->lock, flags);
	} while (bookmark.flags & WQ_FLAG_BOOKMARK);
}

/**
 * __wake_up - wake up threads blocked on a waitqueue.
 * @wq_head: the waitqueue
 * @mode: which threads
 * @nr_exclusive: how many wake-one or wake-many threads to wake up
 * @key: is directly passed to the wakeup function
 *
 * If this function wakes up a task, it executes a full memory barrier before
 * accessing the task state.
 */
void __wake_up(struct wait_queue_head *wq_head, unsigned int mode,
			int nr_exclusive, void *key)
{
	__wake_up_common_lock(wq_head, mode, nr_exclusive, 0, key);
}
EXPORT_SYMBOL(__wake_up);

/*
 * Same as __wake_up but called with the spinlock in wait_queue_head_t held.
 */
void __wake_up_locked(struct wait_queue_head *wq_head, unsigned int mode, int nr)
{
	__wake_up_common(wq_head, mode, nr, 0, NULL, NULL);
}
EXPORT_SYMBOL_GPL(__wake_up_locked);

void __wake_up_locked_key(struct wait_queue_head *wq_head, unsigned int mode, void *key)
{
	__wake_up_common(wq_head, mode, 1, 0, key, NULL);
}
EXPORT_SYMBOL_GPL(__wake_up_locked_key);

void __wake_up_locked_key_bookmark(struct wait_queue_head *wq_head,
		unsigned int mode, void *key, wait_queue_entry_t *bookmark)
{
	__wake_up_common(wq_head, mode, 1, 0, key, bookmark);
}
EXPORT_SYMBOL_GPL(__wake_up_locked_key_bookmark);

/**
 * __wake_up_sync_key - wake up threads blocked on a waitqueue.
 * @wq_head: the waitqueue
 * @mode: which threads
 * @key: opaque value to be passed to wakeup targets
 *
 * The sync wakeup differs that the waker knows that it will schedule
 * away soon, so while the target thread will be woken up, it will not
 * be migrated to another CPU - ie. the two threads are 'synchronized'
 * with each other. This can prevent needless bouncing between CPUs.
 *
 * On UP it can prevent extra preemption.
 *
 * If this function wakes up a task, it executes a full memory barrier before
 * accessing the task state.
 */
void __wake_up_sync_key(struct wait_queue_head *wq_head, unsigned int mode,
			void *key)
{
	if (unlikely(!wq_head))
		return;

	__wake_up_common_lock(wq_head, mode, 1, WF_SYNC, key);
}
EXPORT_SYMBOL_GPL(__wake_up_sync_key);

/**
 * __wake_up_locked_sync_key - wake up a thread blocked on a locked waitqueue.
 * @wq_head: the waitqueue
 * @mode: which threads
 * @key: opaque value to be passed to wakeup targets
 *
 * The sync wakeup differs in that the waker knows that it will schedule
 * away soon, so while the target thread will be woken up, it will not
 * be migrated to another CPU - ie. the two threads are 'synchronized'
 * with each other. This can prevent needless bouncing between CPUs.
 *
 * On UP it can prevent extra preemption.
 *
 * If this function wakes up a task, it executes a full memory barrier before
 * accessing the task state.
 */
void __wake_up_locked_sync_key(struct wait_queue_head *wq_head,
			       unsigned int mode, void *key)
{
        __wake_up_common(wq_head, mode, 1, WF_SYNC, key, NULL);
}
EXPORT_SYMBOL_GPL(__wake_up_locked_sync_key);

/*
 * __wake_up_sync - see __wake_up_sync_key()
 */
void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode)
{
	__wake_up_sync_key(wq_head, mode, NULL);
}
EXPORT_SYMBOL_GPL(__wake_up_sync);	/* For internal use only */

/*
 * Note: we use "set_current_state()" _after_ the wait-queue add,
 * because we need a memory barrier there on SMP, so that any
 * wake-function that tests for the wait-queue being active
 * will be guaranteed to see waitqueue addition _or_ subsequent
 * tests in this thread will see the wakeup having taken place.
 *
 * The spin_unlock() itself is semi-permeable and only protects
 * one way (it only protects stuff inside the critical region and
 * stops them from bleeding out - it would still allow subsequent
 * loads to move into the critical region).
 */
void
prepare_to_wait(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
{
	unsigned long flags;

	wq_entry->flags &= ~WQ_FLAG_EXCLUSIVE;
	spin_lock_irqsave(&wq_head->lock, flags);
	if (list_empty(&wq_entry->entry))
		__add_wait_queue(wq_head, wq_entry);
	set_current_state(state);
	spin_unlock_irqrestore(&wq_head->lock, flags);
}
EXPORT_SYMBOL(prepare_to_wait);

void
prepare_to_wait_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
{
	unsigned long flags;

	wq_entry->flags |= WQ_FLAG_EXCLUSIVE;
	spin_lock_irqsave(&wq_head->lock, flags);
	if (list_empty(&wq_entry->entry))
		__add_wait_queue_entry_tail(wq_head, wq_entry);
	set_current_state(state);
	spin_unlock_irqrestore(&wq_head->lock, flags);
}
EXPORT_SYMBOL(prepare_to_wait_exclusive);

void init_wait_entry(struct wait_queue_entry *wq_entry, int flags)
{
	wq_entry->flags = flags;
	wq_entry->private = current;
	wq_entry->func = autoremove_wake_function;
	INIT_LIST_HEAD(&wq_entry->entry);
}
EXPORT_SYMBOL(init_wait_entry);

long prepare_to_wait_event(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
{
	unsigned long flags;
	long ret = 0;

	spin_lock_irqsave(&wq_head->lock, flags);
	if (signal_pending_state(state, current)) {
		/*
		 * Exclusive waiter must not fail if it was selected by wakeup,
		 * it should "consume" the condition we were waiting for.
		 *
		 * The caller will recheck the condition and return success if
		 * we were already woken up, we can not miss the event because
		 * wakeup locks/unlocks the same wq_head->lock.
		 *
		 * But we need to ensure that set-condition + wakeup after that
		 * can't see us, it should wake up another exclusive waiter if
		 * we fail.
		 */
		list_del_init(&wq_entry->entry);
		ret = -ERESTARTSYS;
	} else {
		if (list_empty(&wq_entry->entry)) {
			if (wq_entry->flags & WQ_FLAG_EXCLUSIVE)
				__add_wait_queue_entry_tail(wq_head, wq_entry);
			else
				__add_wait_queue(wq_head, wq_entry);
		}
		set_current_state(state);
	}
	spin_unlock_irqrestore(&wq_head->lock, flags);

	return ret;
}
EXPORT_SYMBOL(prepare_to_wait_event);

/*
 * Note! These two wait functions are entered with the
 * wait-queue lock held (and interrupts off in the _irq
 * case), so there is no race with testing the wakeup
 * condition in the caller before they add the wait
 * entry to the wake queue.
 */
int do_wait_intr(wait_queue_head_t *wq, wait_queue_entry_t *wait)
{
	if (likely(list_empty(&wait->entry)))
		__add_wait_queue_entry_tail(wq, wait);

	set_current_state(TASK_INTERRUPTIBLE);
	if (signal_pending(current))
		return -ERESTARTSYS;

	spin_unlock(&wq->lock);
	schedule();
	spin_lock(&wq->lock);

	return 0;
}
EXPORT_SYMBOL(do_wait_intr);

int do_wait_intr_irq(wait_queue_head_t *wq, wait_queue_entry_t *wait)
{
	if (likely(list_empty(&wait->entry)))
		__add_wait_queue_entry_tail(wq, wait);

	set_current_state(TASK_INTERRUPTIBLE);
	if (signal_pending(current))
		return -ERESTARTSYS;

	spin_unlock_irq(&wq->lock);
	schedule();
	spin_lock_irq(&wq->lock);

	return 0;
}
EXPORT_SYMBOL(do_wait_intr_irq);

/**
 * finish_wait - clean up after waiting in a queue
 * @wq_head: waitqueue waited on
 * @wq_entry: wait descriptor
 *
 * Sets current thread back to running state and removes
 * the wait descriptor from the given waitqueue if still
 * queued.
 */
void finish_wait(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
{
	unsigned long flags;

	__set_current_state(TASK_RUNNING);
	/*
	 * We can check for list emptiness outside the lock
	 * IFF:
	 *  - we use the "careful" check that verifies both
	 *    the next and prev pointers, so that there cannot
	 *    be any half-pending updates in progress on other
	 *    CPU's that we haven't seen yet (and that might
	 *    still change the stack area.
	 * and
	 *  - all other users take the lock (ie we can only
	 *    have _one_ other CPU that looks at or modifies
	 *    the list).
	 */
	if (!list_empty_careful(&wq_entry->entry)) {
		spin_lock_irqsave(&wq_head->lock, flags);
		list_del_init(&wq_entry->entry);
		spin_unlock_irqrestore(&wq_head->lock, flags);
	}
}
EXPORT_SYMBOL(finish_wait);

int autoremove_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int sync, void *key)
{
	int ret = default_wake_function(wq_entry, mode, sync, key);

	if (ret)
		list_del_init_careful(&wq_entry->entry);

	return ret;
}
EXPORT_SYMBOL(autoremove_wake_function);

static inline bool is_kthread_should_stop(void)
{
	return (current->flags & PF_KTHREAD) && kthread_should_stop();
}

/*
 * DEFINE_WAIT_FUNC(wait, woken_wake_func);
 *
 * add_wait_queue(&wq_head, &wait);
 * for (;;) {
 *     if (condition)
 *         break;
 *
 *     // in wait_woken()			// in woken_wake_function()
 *
 *     p->state = mode;				wq_entry->flags |= WQ_FLAG_WOKEN;
 *     smp_mb(); // A				try_to_wake_up():
 *     if (!(wq_entry->flags & WQ_FLAG_WOKEN))	   <full barrier>
 *         schedule()				   if (p->state & mode)
 *     p->state = TASK_RUNNING;			      p->state = TASK_RUNNING;
 *     wq_entry->flags &= ~WQ_FLAG_WOKEN;	~~~~~~~~~~~~~~~~~~
 *     smp_mb(); // B				condition = true;
 * }						smp_mb(); // C
 * remove_wait_queue(&wq_head, &wait);		wq_entry->flags |= WQ_FLAG_WOKEN;
 */
long wait_woken(struct wait_queue_entry *wq_entry, unsigned mode, long timeout)
{
	/*
	 * The below executes an smp_mb(), which matches with the full barrier
	 * executed by the try_to_wake_up() in woken_wake_function() such that
	 * either we see the store to wq_entry->flags in woken_wake_function()
	 * or woken_wake_function() sees our store to current->state.
	 */
	set_current_state(mode); /* A */
	if (!(wq_entry->flags & WQ_FLAG_WOKEN) && !is_kthread_should_stop())
		timeout = schedule_timeout(timeout);
	__set_current_state(TASK_RUNNING);

	/*
	 * The below executes an smp_mb(), which matches with the smp_mb() (C)
	 * in woken_wake_function() such that either we see the wait condition
	 * being true or the store to wq_entry->flags in woken_wake_function()
	 * follows ours in the coherence order.
	 */
	smp_store_mb(wq_entry->flags, wq_entry->flags & ~WQ_FLAG_WOKEN); /* B */

	return timeout;
}
EXPORT_SYMBOL(wait_woken);

int woken_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int sync, void *key)
{
	/* Pairs with the smp_store_mb() in wait_woken(). */
	smp_mb(); /* C */
	wq_entry->flags |= WQ_FLAG_WOKEN;

	return default_wake_function(wq_entry, mode, sync, key);
}
EXPORT_SYMBOL(woken_wake_function);