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