xref: /linux/kernel/sched/wait.c (revision 1c4b5ecb7ea190fa3e9f9d6891e6c90b60e04f24)
1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * Generic waiting primitives.
4  *
5  * (C) 2004 Nadia Yvette Chambers, Oracle
6  */
7 #include "sched.h"
8 
9 void __init_waitqueue_head(struct wait_queue_head *wq_head, const char *name, struct lock_class_key *key)
10 {
11 	spin_lock_init(&wq_head->lock);
12 	lockdep_set_class_and_name(&wq_head->lock, key, name);
13 	INIT_LIST_HEAD(&wq_head->head);
14 }
15 
16 EXPORT_SYMBOL(__init_waitqueue_head);
17 
18 void add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
19 {
20 	unsigned long flags;
21 
22 	wq_entry->flags &= ~WQ_FLAG_EXCLUSIVE;
23 	spin_lock_irqsave(&wq_head->lock, flags);
24 	__add_wait_queue(wq_head, wq_entry);
25 	spin_unlock_irqrestore(&wq_head->lock, flags);
26 }
27 EXPORT_SYMBOL(add_wait_queue);
28 
29 void add_wait_queue_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
30 {
31 	unsigned long flags;
32 
33 	wq_entry->flags |= WQ_FLAG_EXCLUSIVE;
34 	spin_lock_irqsave(&wq_head->lock, flags);
35 	__add_wait_queue_entry_tail(wq_head, wq_entry);
36 	spin_unlock_irqrestore(&wq_head->lock, flags);
37 }
38 EXPORT_SYMBOL(add_wait_queue_exclusive);
39 
40 void add_wait_queue_priority(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
41 {
42 	unsigned long flags;
43 
44 	wq_entry->flags |= WQ_FLAG_EXCLUSIVE | WQ_FLAG_PRIORITY;
45 	spin_lock_irqsave(&wq_head->lock, flags);
46 	__add_wait_queue(wq_head, wq_entry);
47 	spin_unlock_irqrestore(&wq_head->lock, flags);
48 }
49 EXPORT_SYMBOL_GPL(add_wait_queue_priority);
50 
51 void remove_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
52 {
53 	unsigned long flags;
54 
55 	spin_lock_irqsave(&wq_head->lock, flags);
56 	__remove_wait_queue(wq_head, wq_entry);
57 	spin_unlock_irqrestore(&wq_head->lock, flags);
58 }
59 EXPORT_SYMBOL(remove_wait_queue);
60 
61 /*
62  * Scan threshold to break wait queue walk.
63  * This allows a waker to take a break from holding the
64  * wait queue lock during the wait queue walk.
65  */
66 #define WAITQUEUE_WALK_BREAK_CNT 64
67 
68 /*
69  * The core wakeup function. Non-exclusive wakeups (nr_exclusive == 0) just
70  * wake everything up. If it's an exclusive wakeup (nr_exclusive == small +ve
71  * number) then we wake that number of exclusive tasks, and potentially all
72  * the non-exclusive tasks. Normally, exclusive tasks will be at the end of
73  * the list and any non-exclusive tasks will be woken first. A priority task
74  * may be at the head of the list, and can consume the event without any other
75  * tasks being woken.
76  *
77  * There are circumstances in which we can try to wake a task which has already
78  * started to run but is not in state TASK_RUNNING. try_to_wake_up() returns
79  * zero in this (rare) case, and we handle it by continuing to scan the queue.
80  */
81 static int __wake_up_common(struct wait_queue_head *wq_head, unsigned int mode,
82 			int nr_exclusive, int wake_flags, void *key,
83 			wait_queue_entry_t *bookmark)
84 {
85 	wait_queue_entry_t *curr, *next;
86 	int cnt = 0;
87 
88 	lockdep_assert_held(&wq_head->lock);
89 
90 	if (bookmark && (bookmark->flags & WQ_FLAG_BOOKMARK)) {
91 		curr = list_next_entry(bookmark, entry);
92 
93 		list_del(&bookmark->entry);
94 		bookmark->flags = 0;
95 	} else
96 		curr = list_first_entry(&wq_head->head, wait_queue_entry_t, entry);
97 
98 	if (&curr->entry == &wq_head->head)
99 		return nr_exclusive;
100 
101 	list_for_each_entry_safe_from(curr, next, &wq_head->head, entry) {
102 		unsigned flags = curr->flags;
103 		int ret;
104 
105 		if (flags & WQ_FLAG_BOOKMARK)
106 			continue;
107 
108 		ret = curr->func(curr, mode, wake_flags, key);
109 		if (ret < 0)
110 			break;
111 		if (ret && (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
112 			break;
113 
114 		if (bookmark && (++cnt > WAITQUEUE_WALK_BREAK_CNT) &&
115 				(&next->entry != &wq_head->head)) {
116 			bookmark->flags = WQ_FLAG_BOOKMARK;
117 			list_add_tail(&bookmark->entry, &next->entry);
118 			break;
119 		}
120 	}
121 
122 	return nr_exclusive;
123 }
124 
125 static void __wake_up_common_lock(struct wait_queue_head *wq_head, unsigned int mode,
126 			int nr_exclusive, int wake_flags, void *key)
127 {
128 	unsigned long flags;
129 	wait_queue_entry_t bookmark;
130 
131 	bookmark.flags = 0;
132 	bookmark.private = NULL;
133 	bookmark.func = NULL;
134 	INIT_LIST_HEAD(&bookmark.entry);
135 
136 	do {
137 		spin_lock_irqsave(&wq_head->lock, flags);
138 		nr_exclusive = __wake_up_common(wq_head, mode, nr_exclusive,
139 						wake_flags, key, &bookmark);
140 		spin_unlock_irqrestore(&wq_head->lock, flags);
141 	} while (bookmark.flags & WQ_FLAG_BOOKMARK);
142 }
143 
144 /**
145  * __wake_up - wake up threads blocked on a waitqueue.
146  * @wq_head: the waitqueue
147  * @mode: which threads
148  * @nr_exclusive: how many wake-one or wake-many threads to wake up
149  * @key: is directly passed to the wakeup function
150  *
151  * If this function wakes up a task, it executes a full memory barrier before
152  * accessing the task state.
153  */
154 void __wake_up(struct wait_queue_head *wq_head, unsigned int mode,
155 			int nr_exclusive, void *key)
156 {
157 	__wake_up_common_lock(wq_head, mode, nr_exclusive, 0, key);
158 }
159 EXPORT_SYMBOL(__wake_up);
160 
161 /*
162  * Same as __wake_up but called with the spinlock in wait_queue_head_t held.
163  */
164 void __wake_up_locked(struct wait_queue_head *wq_head, unsigned int mode, int nr)
165 {
166 	__wake_up_common(wq_head, mode, nr, 0, NULL, NULL);
167 }
168 EXPORT_SYMBOL_GPL(__wake_up_locked);
169 
170 void __wake_up_locked_key(struct wait_queue_head *wq_head, unsigned int mode, void *key)
171 {
172 	__wake_up_common(wq_head, mode, 1, 0, key, NULL);
173 }
174 EXPORT_SYMBOL_GPL(__wake_up_locked_key);
175 
176 void __wake_up_locked_key_bookmark(struct wait_queue_head *wq_head,
177 		unsigned int mode, void *key, wait_queue_entry_t *bookmark)
178 {
179 	__wake_up_common(wq_head, mode, 1, 0, key, bookmark);
180 }
181 EXPORT_SYMBOL_GPL(__wake_up_locked_key_bookmark);
182 
183 /**
184  * __wake_up_sync_key - wake up threads blocked on a waitqueue.
185  * @wq_head: the waitqueue
186  * @mode: which threads
187  * @key: opaque value to be passed to wakeup targets
188  *
189  * The sync wakeup differs that the waker knows that it will schedule
190  * away soon, so while the target thread will be woken up, it will not
191  * be migrated to another CPU - ie. the two threads are 'synchronized'
192  * with each other. This can prevent needless bouncing between CPUs.
193  *
194  * On UP it can prevent extra preemption.
195  *
196  * If this function wakes up a task, it executes a full memory barrier before
197  * accessing the task state.
198  */
199 void __wake_up_sync_key(struct wait_queue_head *wq_head, unsigned int mode,
200 			void *key)
201 {
202 	if (unlikely(!wq_head))
203 		return;
204 
205 	__wake_up_common_lock(wq_head, mode, 1, WF_SYNC, key);
206 }
207 EXPORT_SYMBOL_GPL(__wake_up_sync_key);
208 
209 /**
210  * __wake_up_locked_sync_key - wake up a thread blocked on a locked waitqueue.
211  * @wq_head: the waitqueue
212  * @mode: which threads
213  * @key: opaque value to be passed to wakeup targets
214  *
215  * The sync wakeup differs in that the waker knows that it will schedule
216  * away soon, so while the target thread will be woken up, it will not
217  * be migrated to another CPU - ie. the two threads are 'synchronized'
218  * with each other. This can prevent needless bouncing between CPUs.
219  *
220  * On UP it can prevent extra preemption.
221  *
222  * If this function wakes up a task, it executes a full memory barrier before
223  * accessing the task state.
224  */
225 void __wake_up_locked_sync_key(struct wait_queue_head *wq_head,
226 			       unsigned int mode, void *key)
227 {
228         __wake_up_common(wq_head, mode, 1, WF_SYNC, key, NULL);
229 }
230 EXPORT_SYMBOL_GPL(__wake_up_locked_sync_key);
231 
232 /*
233  * __wake_up_sync - see __wake_up_sync_key()
234  */
235 void __wake_up_sync(struct wait_queue_head *wq_head, unsigned int mode)
236 {
237 	__wake_up_sync_key(wq_head, mode, NULL);
238 }
239 EXPORT_SYMBOL_GPL(__wake_up_sync);	/* For internal use only */
240 
241 void __wake_up_pollfree(struct wait_queue_head *wq_head)
242 {
243 	__wake_up(wq_head, TASK_NORMAL, 0, poll_to_key(EPOLLHUP | POLLFREE));
244 	/* POLLFREE must have cleared the queue. */
245 	WARN_ON_ONCE(waitqueue_active(wq_head));
246 }
247 
248 /*
249  * Note: we use "set_current_state()" _after_ the wait-queue add,
250  * because we need a memory barrier there on SMP, so that any
251  * wake-function that tests for the wait-queue being active
252  * will be guaranteed to see waitqueue addition _or_ subsequent
253  * tests in this thread will see the wakeup having taken place.
254  *
255  * The spin_unlock() itself is semi-permeable and only protects
256  * one way (it only protects stuff inside the critical region and
257  * stops them from bleeding out - it would still allow subsequent
258  * loads to move into the critical region).
259  */
260 void
261 prepare_to_wait(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
262 {
263 	unsigned long flags;
264 
265 	wq_entry->flags &= ~WQ_FLAG_EXCLUSIVE;
266 	spin_lock_irqsave(&wq_head->lock, flags);
267 	if (list_empty(&wq_entry->entry))
268 		__add_wait_queue(wq_head, wq_entry);
269 	set_current_state(state);
270 	spin_unlock_irqrestore(&wq_head->lock, flags);
271 }
272 EXPORT_SYMBOL(prepare_to_wait);
273 
274 /* Returns true if we are the first waiter in the queue, false otherwise. */
275 bool
276 prepare_to_wait_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
277 {
278 	unsigned long flags;
279 	bool was_empty = false;
280 
281 	wq_entry->flags |= WQ_FLAG_EXCLUSIVE;
282 	spin_lock_irqsave(&wq_head->lock, flags);
283 	if (list_empty(&wq_entry->entry)) {
284 		was_empty = list_empty(&wq_head->head);
285 		__add_wait_queue_entry_tail(wq_head, wq_entry);
286 	}
287 	set_current_state(state);
288 	spin_unlock_irqrestore(&wq_head->lock, flags);
289 	return was_empty;
290 }
291 EXPORT_SYMBOL(prepare_to_wait_exclusive);
292 
293 void init_wait_entry(struct wait_queue_entry *wq_entry, int flags)
294 {
295 	wq_entry->flags = flags;
296 	wq_entry->private = current;
297 	wq_entry->func = autoremove_wake_function;
298 	INIT_LIST_HEAD(&wq_entry->entry);
299 }
300 EXPORT_SYMBOL(init_wait_entry);
301 
302 long prepare_to_wait_event(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
303 {
304 	unsigned long flags;
305 	long ret = 0;
306 
307 	spin_lock_irqsave(&wq_head->lock, flags);
308 	if (signal_pending_state(state, current)) {
309 		/*
310 		 * Exclusive waiter must not fail if it was selected by wakeup,
311 		 * it should "consume" the condition we were waiting for.
312 		 *
313 		 * The caller will recheck the condition and return success if
314 		 * we were already woken up, we can not miss the event because
315 		 * wakeup locks/unlocks the same wq_head->lock.
316 		 *
317 		 * But we need to ensure that set-condition + wakeup after that
318 		 * can't see us, it should wake up another exclusive waiter if
319 		 * we fail.
320 		 */
321 		list_del_init(&wq_entry->entry);
322 		ret = -ERESTARTSYS;
323 	} else {
324 		if (list_empty(&wq_entry->entry)) {
325 			if (wq_entry->flags & WQ_FLAG_EXCLUSIVE)
326 				__add_wait_queue_entry_tail(wq_head, wq_entry);
327 			else
328 				__add_wait_queue(wq_head, wq_entry);
329 		}
330 		set_current_state(state);
331 	}
332 	spin_unlock_irqrestore(&wq_head->lock, flags);
333 
334 	return ret;
335 }
336 EXPORT_SYMBOL(prepare_to_wait_event);
337 
338 /*
339  * Note! These two wait functions are entered with the
340  * wait-queue lock held (and interrupts off in the _irq
341  * case), so there is no race with testing the wakeup
342  * condition in the caller before they add the wait
343  * entry to the wake queue.
344  */
345 int do_wait_intr(wait_queue_head_t *wq, wait_queue_entry_t *wait)
346 {
347 	if (likely(list_empty(&wait->entry)))
348 		__add_wait_queue_entry_tail(wq, wait);
349 
350 	set_current_state(TASK_INTERRUPTIBLE);
351 	if (signal_pending(current))
352 		return -ERESTARTSYS;
353 
354 	spin_unlock(&wq->lock);
355 	schedule();
356 	spin_lock(&wq->lock);
357 
358 	return 0;
359 }
360 EXPORT_SYMBOL(do_wait_intr);
361 
362 int do_wait_intr_irq(wait_queue_head_t *wq, wait_queue_entry_t *wait)
363 {
364 	if (likely(list_empty(&wait->entry)))
365 		__add_wait_queue_entry_tail(wq, wait);
366 
367 	set_current_state(TASK_INTERRUPTIBLE);
368 	if (signal_pending(current))
369 		return -ERESTARTSYS;
370 
371 	spin_unlock_irq(&wq->lock);
372 	schedule();
373 	spin_lock_irq(&wq->lock);
374 
375 	return 0;
376 }
377 EXPORT_SYMBOL(do_wait_intr_irq);
378 
379 /**
380  * finish_wait - clean up after waiting in a queue
381  * @wq_head: waitqueue waited on
382  * @wq_entry: wait descriptor
383  *
384  * Sets current thread back to running state and removes
385  * the wait descriptor from the given waitqueue if still
386  * queued.
387  */
388 void finish_wait(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
389 {
390 	unsigned long flags;
391 
392 	__set_current_state(TASK_RUNNING);
393 	/*
394 	 * We can check for list emptiness outside the lock
395 	 * IFF:
396 	 *  - we use the "careful" check that verifies both
397 	 *    the next and prev pointers, so that there cannot
398 	 *    be any half-pending updates in progress on other
399 	 *    CPU's that we haven't seen yet (and that might
400 	 *    still change the stack area.
401 	 * and
402 	 *  - all other users take the lock (ie we can only
403 	 *    have _one_ other CPU that looks at or modifies
404 	 *    the list).
405 	 */
406 	if (!list_empty_careful(&wq_entry->entry)) {
407 		spin_lock_irqsave(&wq_head->lock, flags);
408 		list_del_init(&wq_entry->entry);
409 		spin_unlock_irqrestore(&wq_head->lock, flags);
410 	}
411 }
412 EXPORT_SYMBOL(finish_wait);
413 
414 int autoremove_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int sync, void *key)
415 {
416 	int ret = default_wake_function(wq_entry, mode, sync, key);
417 
418 	if (ret)
419 		list_del_init_careful(&wq_entry->entry);
420 
421 	return ret;
422 }
423 EXPORT_SYMBOL(autoremove_wake_function);
424 
425 static inline bool is_kthread_should_stop(void)
426 {
427 	return (current->flags & PF_KTHREAD) && kthread_should_stop();
428 }
429 
430 /*
431  * DEFINE_WAIT_FUNC(wait, woken_wake_func);
432  *
433  * add_wait_queue(&wq_head, &wait);
434  * for (;;) {
435  *     if (condition)
436  *         break;
437  *
438  *     // in wait_woken()			// in woken_wake_function()
439  *
440  *     p->state = mode;				wq_entry->flags |= WQ_FLAG_WOKEN;
441  *     smp_mb(); // A				try_to_wake_up():
442  *     if (!(wq_entry->flags & WQ_FLAG_WOKEN))	   <full barrier>
443  *         schedule()				   if (p->state & mode)
444  *     p->state = TASK_RUNNING;			      p->state = TASK_RUNNING;
445  *     wq_entry->flags &= ~WQ_FLAG_WOKEN;	~~~~~~~~~~~~~~~~~~
446  *     smp_mb(); // B				condition = true;
447  * }						smp_mb(); // C
448  * remove_wait_queue(&wq_head, &wait);		wq_entry->flags |= WQ_FLAG_WOKEN;
449  */
450 long wait_woken(struct wait_queue_entry *wq_entry, unsigned mode, long timeout)
451 {
452 	/*
453 	 * The below executes an smp_mb(), which matches with the full barrier
454 	 * executed by the try_to_wake_up() in woken_wake_function() such that
455 	 * either we see the store to wq_entry->flags in woken_wake_function()
456 	 * or woken_wake_function() sees our store to current->state.
457 	 */
458 	set_current_state(mode); /* A */
459 	if (!(wq_entry->flags & WQ_FLAG_WOKEN) && !is_kthread_should_stop())
460 		timeout = schedule_timeout(timeout);
461 	__set_current_state(TASK_RUNNING);
462 
463 	/*
464 	 * The below executes an smp_mb(), which matches with the smp_mb() (C)
465 	 * in woken_wake_function() such that either we see the wait condition
466 	 * being true or the store to wq_entry->flags in woken_wake_function()
467 	 * follows ours in the coherence order.
468 	 */
469 	smp_store_mb(wq_entry->flags, wq_entry->flags & ~WQ_FLAG_WOKEN); /* B */
470 
471 	return timeout;
472 }
473 EXPORT_SYMBOL(wait_woken);
474 
475 int woken_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int sync, void *key)
476 {
477 	/* Pairs with the smp_store_mb() in wait_woken(). */
478 	smp_mb(); /* C */
479 	wq_entry->flags |= WQ_FLAG_WOKEN;
480 
481 	return default_wake_function(wq_entry, mode, sync, key);
482 }
483 EXPORT_SYMBOL(woken_wake_function);
484