kernel/sched/wait.c

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