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TOMOYO Linux Cross Reference
Linux/kernel/sched/wait.c

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

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