kernel/sched/wait.c

   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 remove_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
  41 {
  42         unsigned long flags;
  43
  44         spin_lock_irqsave(&wq_head->lock, flags);
  45         __remove_wait_queue(wq_head, wq_entry);
  46         spin_unlock_irqrestore(&wq_head->lock, flags);
  47 }
  48 EXPORT_SYMBOL(remove_wait_queue);
  49
  50 /*
  51  * Scan threshold to break wait queue walk.
  52  * This allows a waker to take a break from holding the
  53  * wait queue lock during the wait queue walk.
  54  */
  55 #define WAITQUEUE_WALK_BREAK_CNT 64
  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 int __wake_up_common(struct wait_queue_head *wq_head, unsigned int mode,
  67                         int nr_exclusive, int wake_flags, void *key,
  68                         wait_queue_entry_t *bookmark)
  69 {
  70         wait_queue_entry_t *curr, *next;
  71         int cnt = 0;
  72
  73         lockdep_assert_held(&wq_head->lock);
  74
  75         if (bookmark && (bookmark->flags & WQ_FLAG_BOOKMARK)) {
  76                 curr = list_next_entry(bookmark, entry);
  77
  78                 list_del(&bookmark->entry);
  79                 bookmark->flags = 0;
  80         } else
  81                 curr = list_first_entry(&wq_head->head, wait_queue_entry_t, entry);
  82
  83         if (&curr->entry == &wq_head->head)
  84                 return nr_exclusive;
  85
  86         list_for_each_entry_safe_from(curr, next, &wq_head->head, entry) {
  87                 unsigned flags = curr->flags;
  88                 int ret;
  89
  90                 if (flags & WQ_FLAG_BOOKMARK)
  91                         continue;
  92
  93                 ret = curr->func(curr, mode, wake_flags, key);
  94                 if (ret < 0)
  95                         break;
  96                 if (ret && (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
  97                         break;
  98
  99                 if (bookmark && (++cnt > WAITQUEUE_WALK_BREAK_CNT) &&
 100                                 (&next->entry != &wq_head->head)) {
 101                         bookmark->flags = WQ_FLAG_BOOKMARK;
 102                         list_add_tail(&bookmark->entry, &next->entry);
 103                         break;
 104                 }
 105         }
 106
 107         return nr_exclusive;
 108 }
 109
 110 static void __wake_up_common_lock(struct wait_queue_head *wq_head, unsigned int mode,
 111                         int nr_exclusive, int wake_flags, void *key)
 112 {
 113         unsigned long flags;
 114         wait_queue_entry_t bookmark;
 115
 116         bookmark.flags = 0;
 117         bookmark.private = NULL;
 118         bookmark.func = NULL;
 119         INIT_LIST_HEAD(&bookmark.entry);
 120
 121         do {
 122                 spin_lock_irqsave(&wq_head->lock, flags);
 123                 nr_exclusive = __wake_up_common(wq_head, mode, nr_exclusive,
 124                                                 wake_flags, key, &bookmark);
 125                 spin_unlock_irqrestore(&wq_head->lock, flags);
 126         } while (bookmark.flags & WQ_FLAG_BOOKMARK);
 127 }
 128
 129 /**
 130  * __wake_up - wake up threads blocked on a waitqueue.
 131  * @wq_head: the waitqueue
 132  * @mode: which threads
 133  * @nr_exclusive: how many wake-one or wake-many threads to wake up
 134  * @key: is directly passed to the wakeup function
 135  *
 136  * If this function wakes up a task, it executes a full memory barrier before
 137  * accessing the task state.
 138  */
 139 void __wake_up(struct wait_queue_head *wq_head, unsigned int mode,
 140                         int nr_exclusive, void *key)
 141 {
 142         __wake_up_common_lock(wq_head, mode, nr_exclusive, 0, key);
 143 }
 144 EXPORT_SYMBOL(__wake_up);
 145
 146 /*
 147  * Same as __wake_up but called with the spinlock in wait_queue_head_t held.
 148  */
 149 void __wake_up_locked(struct wait_queue_head *wq_head, unsigned int mode, int nr)
 150 {
 151         __wake_up_common(wq_head, mode, nr, 0, NULL, NULL);
 152 }
 153 EXPORT_SYMBOL_GPL(__wake_up_locked);
 154
 155 void __wake_up_locked_key(struct wait_queue_head *wq_head, unsigned int mode, void *key)
 156 {
 157         __wake_up_common(wq_head, mode, 1, 0, key, NULL);
 158 }
 159 EXPORT_SYMBOL_GPL(__wake_up_locked_key);
 160
 161 void __wake_up_locked_key_bookmark(struct wait_queue_head *wq_head,
 162                 unsigned int mode, void *key, wait_queue_entry_t *bookmark)
 163 {
 164         __wake_up_common(wq_head, mode, 1, 0, key, bookmark);
 165 }
 166 EXPORT_SYMBOL_GPL(__wake_up_locked_key_bookmark);
 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  * If this function wakes up a task, it executes a full memory barrier before
 183  * accessing the task state.
 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 void __wake_up_pollfree(struct wait_queue_head *wq_head)
 210 {
 211         __wake_up(wq_head, TASK_NORMAL, 0, poll_to_key(EPOLLHUP | POLLFREE));
 212         /* POLLFREE must have cleared the queue. */
 213         WARN_ON_ONCE(waitqueue_active(wq_head));
 214 }
 215
 216 /*
 217  * Note: we use "set_current_state()" _after_ the wait-queue add,
 218  * because we need a memory barrier there on SMP, so that any
 219  * wake-function that tests for the wait-queue being active
 220  * will be guaranteed to see waitqueue addition _or_ subsequent
 221  * tests in this thread will see the wakeup having taken place.
 222  *
 223  * The spin_unlock() itself is semi-permeable and only protects
 224  * one way (it only protects stuff inside the critical region and
 225  * stops them from bleeding out - it would still allow subsequent
 226  * loads to move into the critical region).
 227  */
 228 void
 229 prepare_to_wait(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
 230 {
 231         unsigned long flags;
 232
 233         wq_entry->flags &= ~WQ_FLAG_EXCLUSIVE;
 234         spin_lock_irqsave(&wq_head->lock, flags);
 235         if (list_empty(&wq_entry->entry))
 236                 __add_wait_queue(wq_head, wq_entry);
 237         set_current_state(state);
 238         spin_unlock_irqrestore(&wq_head->lock, flags);
 239 }
 240 EXPORT_SYMBOL(prepare_to_wait);
 241
 242 /* Returns true if we are the first waiter in the queue, false otherwise. */
 243 bool
 244 prepare_to_wait_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
 245 {
 246         unsigned long flags;
 247         bool was_empty = false;
 248
 249         wq_entry->flags |= WQ_FLAG_EXCLUSIVE;
 250         spin_lock_irqsave(&wq_head->lock, flags);
 251         if (list_empty(&wq_entry->entry)) {
 252                 was_empty = list_empty(&wq_head->head);
 253                 __add_wait_queue_entry_tail(wq_head, wq_entry);
 254         }
 255         set_current_state(state);
 256         spin_unlock_irqrestore(&wq_head->lock, flags);
 257         return was_empty;
 258 }
 259 EXPORT_SYMBOL(prepare_to_wait_exclusive);
 260
 261 void init_wait_entry(struct wait_queue_entry *wq_entry, int flags)
 262 {
 263         wq_entry->flags = flags;
 264         wq_entry->private = current;
 265         wq_entry->func = autoremove_wake_function;
 266         INIT_LIST_HEAD(&wq_entry->entry);
 267 }
 268 EXPORT_SYMBOL(init_wait_entry);
 269
 270 long prepare_to_wait_event(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry, int state)
 271 {
 272         unsigned long flags;
 273         long ret = 0;
 274
 275         spin_lock_irqsave(&wq_head->lock, flags);
 276         if (signal_pending_state(state, current)) {
 277                 /*
 278                  * Exclusive waiter must not fail if it was selected by wakeup,
 279                  * it should "consume" the condition we were waiting for.
 280                  *
 281                  * The caller will recheck the condition and return success if
 282                  * we were already woken up, we can not miss the event because
 283                  * wakeup locks/unlocks the same wq_head->lock.
 284                  *
 285                  * But we need to ensure that set-condition + wakeup after that
 286                  * can't see us, it should wake up another exclusive waiter if
 287                  * we fail.
 288                  */
 289                 list_del_init(&wq_entry->entry);
 290                 ret = -ERESTARTSYS;
 291         } else {
 292                 if (list_empty(&wq_entry->entry)) {
 293                         if (wq_entry->flags & WQ_FLAG_EXCLUSIVE)
 294                                 __add_wait_queue_entry_tail(wq_head, wq_entry);
 295                         else
 296                                 __add_wait_queue(wq_head, wq_entry);
 297                 }
 298                 set_current_state(state);
 299         }
 300         spin_unlock_irqrestore(&wq_head->lock, flags);
 301
 302         return ret;
 303 }
 304 EXPORT_SYMBOL(prepare_to_wait_event);
 305
 306 /*
 307  * Note! These two wait functions are entered with the
 308  * wait-queue lock held (and interrupts off in the _irq
 309  * case), so there is no race with testing the wakeup
 310  * condition in the caller before they add the wait
 311  * entry to the wake queue.
 312  */
 313 int do_wait_intr(wait_queue_head_t *wq, wait_queue_entry_t *wait)
 314 {
 315         if (likely(list_empty(&wait->entry)))
 316                 __add_wait_queue_entry_tail(wq, wait);
 317
 318         set_current_state(TASK_INTERRUPTIBLE);
 319         if (signal_pending(current))
 320                 return -ERESTARTSYS;
 321
 322         spin_unlock(&wq->lock);
 323         schedule();
 324         spin_lock(&wq->lock);
 325
 326         return 0;
 327 }
 328 EXPORT_SYMBOL(do_wait_intr);
 329
 330 int do_wait_intr_irq(wait_queue_head_t *wq, wait_queue_entry_t *wait)
 331 {
 332         if (likely(list_empty(&wait->entry)))
 333                 __add_wait_queue_entry_tail(wq, wait);
 334
 335         set_current_state(TASK_INTERRUPTIBLE);
 336         if (signal_pending(current))
 337                 return -ERESTARTSYS;
 338
 339         spin_unlock_irq(&wq->lock);
 340         schedule();
 341         spin_lock_irq(&wq->lock);
 342
 343         return 0;
 344 }
 345 EXPORT_SYMBOL(do_wait_intr_irq);
 346
 347 /**
 348  * finish_wait - clean up after waiting in a queue
 349  * @wq_head: waitqueue waited on
 350  * @wq_entry: wait descriptor
 351  *
 352  * Sets current thread back to running state and removes
 353  * the wait descriptor from the given waitqueue if still
 354  * queued.
 355  */
 356 void finish_wait(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry)
 357 {
 358         unsigned long flags;
 359
 360         __set_current_state(TASK_RUNNING);
 361         /*
 362          * We can check for list emptiness outside the lock
 363          * IFF:
 364          *  - we use the "careful" check that verifies both
 365          *    the next and prev pointers, so that there cannot
 366          *    be any half-pending updates in progress on other
 367          *    CPU's that we haven't seen yet (and that might
 368          *    still change the stack area.
 369          * and
 370          *  - all other users take the lock (ie we can only
 371          *    have _one_ other CPU that looks at or modifies
 372          *    the list).
 373          */
 374         if (!list_empty_careful(&wq_entry->entry)) {
 375                 spin_lock_irqsave(&wq_head->lock, flags);
 376                 list_del_init(&wq_entry->entry);
 377                 spin_unlock_irqrestore(&wq_head->lock, flags);
 378         }
 379 }
 380 EXPORT_SYMBOL(finish_wait);
 381
 382 int autoremove_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int sync, void *key)
 383 {
 384         int ret = default_wake_function(wq_entry, mode, sync, key);
 385
 386         if (ret)
 387                 list_del_init(&wq_entry->entry);
 388
 389         return ret;
 390 }
 391 EXPORT_SYMBOL(autoremove_wake_function);
 392
 393 static inline bool is_kthread_should_stop(void)
 394 {
 395         return (current->flags & PF_KTHREAD) && kthread_should_stop();
 396 }
 397
 398 /*
 399  * DEFINE_WAIT_FUNC(wait, woken_wake_func);
 400  *
 401  * add_wait_queue(&wq_head, &wait);
 402  * for (;;) {
 403  *     if (condition)
 404  *         break;
 405  *
 406  *     // in wait_woken()                       // in woken_wake_function()
 407  *
 408  *     p->state = mode;                         wq_entry->flags |= WQ_FLAG_WOKEN;
 409  *     smp_mb(); // A                           try_to_wake_up():
 410  *     if (!(wq_entry->flags & WQ_FLAG_WOKEN))     <full barrier>
 411  *         schedule()                              if (p->state & mode)
 412  *     p->state = TASK_RUNNING;                       p->state = TASK_RUNNING;
 413  *     wq_entry->flags &= ~WQ_FLAG_WOKEN;       ~~~~~~~~~~~~~~~~~~
 414  *     smp_mb(); // B                           condition = true;
 415  * }                                            smp_mb(); // C
 416  * remove_wait_queue(&wq_head, &wait);          wq_entry->flags |= WQ_FLAG_WOKEN;
 417  */
 418 long wait_woken(struct wait_queue_entry *wq_entry, unsigned mode, long timeout)
 419 {
 420         /*
 421          * The below executes an smp_mb(), which matches with the full barrier
 422          * executed by the try_to_wake_up() in woken_wake_function() such that
 423          * either we see the store to wq_entry->flags in woken_wake_function()
 424          * or woken_wake_function() sees our store to current->state.
 425          */
 426         set_current_state(mode); /* A */
 427         if (!(wq_entry->flags & WQ_FLAG_WOKEN) && !is_kthread_should_stop())
 428                 timeout = schedule_timeout(timeout);
 429         __set_current_state(TASK_RUNNING);
 430
 431         /*
 432          * The below executes an smp_mb(), which matches with the smp_mb() (C)
 433          * in woken_wake_function() such that either we see the wait condition
 434          * being true or the store to wq_entry->flags in woken_wake_function()
 435          * follows ours in the coherence order.
 436          */
 437         smp_store_mb(wq_entry->flags, wq_entry->flags & ~WQ_FLAG_WOKEN); /* B */
 438
 439         return timeout;
 440 }
 441 EXPORT_SYMBOL(wait_woken);
 442
 443 int woken_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int sync, void *key)
 444 {
 445         /* Pairs with the smp_store_mb() in wait_woken(). */
 446         smp_mb(); /* C */
 447         wq_entry->flags |= WQ_FLAG_WOKEN;
 448
 449         return default_wake_function(wq_entry, mode, sync, key);
 450 }
 451 EXPORT_SYMBOL(woken_wake_function);