1/* Copyright (C) 2003-2022 Free Software Foundation, Inc.
2 This file is part of the GNU C Library.
4 The GNU C Library is free software; you can redistribute it and/or
5 modify it under the terms of the GNU Lesser General Public
6 License as published by the Free Software Foundation; either
7 version 2.1 of the License, or (at your option) any later version.
9 The GNU C Library is distributed in the hope that it will be useful,
10 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 Lesser General Public License for more details.
14 You should have received a copy of the GNU Lesser General Public
15 License along with the GNU C Library; if not, see
16 <https://www.gnu.org/licenses/>. */
18#include <endian.h>
19#include <errno.h>
20#include <sysdep.h>
21#include <futex-internal.h>
22#include <pthread.h>
23#include <pthreadP.h>
24#include <sys/time.h>
25#include <atomic.h>
26#include <stdint.h>
27#include <stdbool.h>
29#include <shlib-compat.h>
30#include <stap-probe.h>
31#include <time.h>
33#include "pthread_cond_common.c"
36struct _condvar_cleanup_buffer
38 uint64_t wseq;
39 pthread_cond_t *cond;
40 pthread_mutex_t *mutex;
41 int private;
45/* Decrease the waiter reference count. */
46static void
47__condvar_confirm_wakeup (pthread_cond_t *cond, int private)
49 /* If destruction is pending (i.e., the wake-request flag is nonzero) and we
50 are the last waiter (prior value of __wrefs was 1 << 3), then wake any
51 threads waiting in pthread_cond_destroy. Release MO to synchronize with
52 these threads. Don't bother clearing the wake-up request flag. */
53 if ((atomic_fetch_add_release (&cond->__data.__wrefs, -8) >> 2) == 3)
54 futex_wake (futex_word: &cond->__data.__wrefs, INT_MAX, private);
58/* Cancel waiting after having registered as a waiter previously. SEQ is our
59 position and G is our group index.
60 The goal of cancellation is to make our group smaller if that is still
61 possible. If we are in a closed group, this is not possible anymore; in
62 this case, we need to send a replacement signal for the one we effectively
63 consumed because the signal should have gotten consumed by another waiter
64 instead; we must not both cancel waiting and consume a signal.
66 Must not be called while still holding a reference on the group.
68 Returns true iff we consumed a signal.
70 On some kind of timeouts, we may be able to pretend that a signal we
71 effectively consumed happened before the timeout (i.e., similarly to first
72 spinning on signals before actually checking whether the timeout has
73 passed already). Doing this would allow us to skip sending a replacement
74 signal, but this case might happen rarely because the end of the timeout
75 must race with someone else sending a signal. Therefore, we don't bother
76 trying to optimize this. */
77static void
78__condvar_cancel_waiting (pthread_cond_t *cond, uint64_t seq, unsigned int g,
79 int private)
81 bool consumed_signal = false;
83 /* No deadlock with group switching is possible here because we do
84 not hold a reference on the group. */
85 __condvar_acquire_lock (cond, private);
87 uint64_t g1_start = __condvar_load_g1_start_relaxed (cond) >> 1;
88 if (g1_start > seq)
89 {
90 /* Our group is closed, so someone provided enough signals for it.
91 Thus, we effectively consumed a signal. */
92 consumed_signal = true;
93 }
94 else
95 {
96 if (g1_start + __condvar_get_orig_size (cond) <= seq)
97 {
98 /* We are in the current G2 and thus cannot have consumed a signal.
99 Reduce its effective size or handle overflow. Remember that in
100 G2, unsigned int size is zero or a negative value. */
101 if (cond->__data.__g_size[g] + __PTHREAD_COND_MAX_GROUP_SIZE > 0)
102 {
103 cond->__data.__g_size[g]--;
104 }
105 else
106 {
107 /* Cancellations would overflow the maximum group size. Just
108 wake up everyone spuriously to create a clean state. This
109 also means we do not consume a signal someone else sent. */
110 __condvar_release_lock (cond, private);
111 __pthread_cond_broadcast (cond);
112 return;
113 }
114 }
115 else
116 {
117 /* We are in current G1. If the group's size is zero, someone put
118 a signal in the group that nobody else but us can consume. */
119 if (cond->__data.__g_size[g] == 0)
120 consumed_signal = true;
121 else
122 {
123 /* Otherwise, we decrease the size of the group. This is
124 equivalent to atomically putting in a signal just for us and
125 consuming it right away. We do not consume a signal sent
126 by someone else. We also cannot have consumed a futex
127 wake-up because if we were cancelled or timed out in a futex
128 call, the futex will wake another waiter. */
129 cond->__data.__g_size[g]--;
130 }
131 }
132 }
134 __condvar_release_lock (cond, private);
136 if (consumed_signal)
137 {
138 /* We effectively consumed a signal even though we didn't want to.
139 Therefore, we need to send a replacement signal.
140 If we would want to optimize this, we could do what
141 pthread_cond_signal does right in the critical section above. */
142 __pthread_cond_signal (cond);
143 }
146/* Wake up any signalers that might be waiting. */
147static void
148__condvar_dec_grefs (pthread_cond_t *cond, unsigned int g, int private)
150 /* Release MO to synchronize-with the acquire load in
151 __condvar_quiesce_and_switch_g1. */
152 if (atomic_fetch_add_release (cond->__data.__g_refs + g, -2) == 3)
153 {
154 /* Clear the wake-up request flag before waking up. We do not need more
155 than relaxed MO and it doesn't matter if we apply this for an aliased
156 group because we wake all futex waiters right after clearing the
157 flag. */
158 atomic_fetch_and_relaxed (cond->__data.__g_refs + g, ~(unsigned int) 1);
159 futex_wake (futex_word: cond->__data.__g_refs + g, INT_MAX, private);
160 }
163/* Clean-up for cancellation of waiters waiting for normal signals. We cancel
164 our registration as a waiter, confirm we have woken up, and re-acquire the
165 mutex. */
166static void
167__condvar_cleanup_waiting (void *arg)
169 struct _condvar_cleanup_buffer *cbuffer =
170 (struct _condvar_cleanup_buffer *) arg;
171 pthread_cond_t *cond = cbuffer->cond;
172 unsigned g = cbuffer->wseq & 1;
174 __condvar_dec_grefs (cond, g, private: cbuffer->private);
176 __condvar_cancel_waiting (cond, seq: cbuffer->wseq >> 1, g, private: cbuffer->private);
177 /* FIXME With the current cancellation implementation, it is possible that
178 a thread is cancelled after it has returned from a syscall. This could
179 result in a cancelled waiter consuming a futex wake-up that is then
180 causing another waiter in the same group to not wake up. To work around
181 this issue until we have fixed cancellation, just add a futex wake-up
182 conservatively. */
183 futex_wake (futex_word: cond->__data.__g_signals + g, processes_to_wake: 1, private: cbuffer->private);
185 __condvar_confirm_wakeup (cond, private: cbuffer->private);
187 /* XXX If locking the mutex fails, should we just stop execution? This
188 might be better than silently ignoring the error. */
189 __pthread_mutex_cond_lock (mutex: cbuffer->mutex);
192/* This condvar implementation guarantees that all calls to signal and
193 broadcast and all of the three virtually atomic parts of each call to wait
194 (i.e., (1) releasing the mutex and blocking, (2) unblocking, and (3) re-
195 acquiring the mutex) happen in some total order that is consistent with the
196 happens-before relations in the calling program. However, this order does
197 not necessarily result in additional happens-before relations being
198 established (which aligns well with spurious wake-ups being allowed).
200 All waiters acquire a certain position in a 64b waiter sequence (__wseq).
201 This sequence determines which waiters are allowed to consume signals.
202 A broadcast is equal to sending as many signals as are unblocked waiters.
203 When a signal arrives, it samples the current value of __wseq with a
204 relaxed-MO load (i.e., the position the next waiter would get). (This is
205 sufficient because it is consistent with happens-before; the caller can
206 enforce stronger ordering constraints by calling signal while holding the
207 mutex.) Only waiters with a position less than the __wseq value observed
208 by the signal are eligible to consume this signal.
210 This would be straight-forward to implement if waiters would just spin but
211 we need to let them block using futexes. Futexes give no guarantee of
212 waking in FIFO order, so we cannot reliably wake eligible waiters if we
213 just use a single futex. Also, futex words are 32b in size, but we need
214 to distinguish more than 1<<32 states because we need to represent the
215 order of wake-up (and thus which waiters are eligible to consume signals);
216 blocking in a futex is not atomic with a waiter determining its position in
217 the waiter sequence, so we need the futex word to reliably notify waiters
218 that they should not attempt to block anymore because they have been
219 already signaled in the meantime. While an ABA issue on a 32b value will
220 be rare, ignoring it when we are aware of it is not the right thing to do
221 either.
223 Therefore, we use a 64b counter to represent the waiter sequence (on
224 architectures which only support 32b atomics, we use a few bits less).
225 To deal with the blocking using futexes, we maintain two groups of waiters:
226 * Group G1 consists of waiters that are all eligible to consume signals;
227 incoming signals will always signal waiters in this group until all
228 waiters in G1 have been signaled.
229 * Group G2 consists of waiters that arrive when a G1 is present and still
230 contains waiters that have not been signaled. When all waiters in G1
231 are signaled and a new signal arrives, the new signal will convert G2
232 into the new G1 and create a new G2 for future waiters.
234 We cannot allocate new memory because of process-shared condvars, so we
235 have just two slots of groups that change their role between G1 and G2.
236 Each has a separate futex word, a number of signals available for
237 consumption, a size (number of waiters in the group that have not been
238 signaled), and a reference count.
240 The group reference count is used to maintain the number of waiters that
241 are using the group's futex. Before a group can change its role, the
242 reference count must show that no waiters are using the futex anymore; this
243 prevents ABA issues on the futex word.
245 To represent which intervals in the waiter sequence the groups cover (and
246 thus also which group slot contains G1 or G2), we use a 64b counter to
247 designate the start position of G1 (inclusive), and a single bit in the
248 waiter sequence counter to represent which group slot currently contains
249 G2. This allows us to switch group roles atomically wrt. waiters obtaining
250 a position in the waiter sequence. The G1 start position allows waiters to
251 figure out whether they are in a group that has already been completely
252 signaled (i.e., if the current G1 starts at a later position that the
253 waiter's position). Waiters cannot determine whether they are currently
254 in G2 or G1 -- but they do not have too because all they are interested in
255 is whether there are available signals, and they always start in G2 (whose
256 group slot they know because of the bit in the waiter sequence. Signalers
257 will simply fill the right group until it is completely signaled and can
258 be closed (they do not switch group roles until they really have to to
259 decrease the likelihood of having to wait for waiters still holding a
260 reference on the now-closed G1).
262 Signalers maintain the initial size of G1 to be able to determine where
263 G2 starts (G2 is always open-ended until it becomes G1). They track the
264 remaining size of a group; when waiters cancel waiting (due to PThreads
265 cancellation or timeouts), they will decrease this remaining size as well.
267 To implement condvar destruction requirements (i.e., that
268 pthread_cond_destroy can be called as soon as all waiters have been
269 signaled), waiters increment a reference count before starting to wait and
270 decrement it after they stopped waiting but right before they acquire the
271 mutex associated with the condvar.
273 pthread_cond_t thus consists of the following (bits that are used for
274 flags and are not part of the primary value of each field but necessary
275 to make some things atomic or because there was no space for them
276 elsewhere in the data structure):
278 __wseq: Waiter sequence counter
279 * LSB is index of current G2.
280 * Waiters fetch-add while having acquire the mutex associated with the
281 condvar. Signalers load it and fetch-xor it concurrently.
282 __g1_start: Starting position of G1 (inclusive)
283 * LSB is index of current G2.
284 * Modified by signalers while having acquired the condvar-internal lock
285 and observed concurrently by waiters.
286 __g1_orig_size: Initial size of G1
287 * The two least-significant bits represent the condvar-internal lock.
288 * Only accessed while having acquired the condvar-internal lock.
289 __wrefs: Waiter reference counter.
290 * Bit 2 is true if waiters should run futex_wake when they remove the
291 last reference. pthread_cond_destroy uses this as futex word.
292 * Bit 1 is the clock ID (0 == CLOCK_REALTIME, 1 == CLOCK_MONOTONIC).
293 * Bit 0 is true iff this is a process-shared condvar.
294 * Simple reference count used by both waiters and pthread_cond_destroy.
295 (If the format of __wrefs is changed, update nptl_lock_constants.pysym
296 and the pretty printers.)
297 For each of the two groups, we have:
298 __g_refs: Futex waiter reference count.
299 * LSB is true if waiters should run futex_wake when they remove the
300 last reference.
301 * Reference count used by waiters concurrently with signalers that have
302 acquired the condvar-internal lock.
303 __g_signals: The number of signals that can still be consumed.
304 * Used as a futex word by waiters. Used concurrently by waiters and
305 signalers.
306 * LSB is true iff this group has been completely signaled (i.e., it is
307 closed).
308 __g_size: Waiters remaining in this group (i.e., which have not been
309 signaled yet.
310 * Accessed by signalers and waiters that cancel waiting (both do so only
311 when having acquired the condvar-internal lock.
312 * The size of G2 is always zero because it cannot be determined until
313 the group becomes G1.
314 * Although this is of unsigned type, we rely on using unsigned overflow
315 rules to make this hold effectively negative values too (in
316 particular, when waiters in G2 cancel waiting).
318 A PTHREAD_COND_INITIALIZER condvar has all fields set to zero, which yields
319 a condvar that has G2 starting at position 0 and a G1 that is closed.
321 Because waiters do not claim ownership of a group right when obtaining a
322 position in __wseq but only reference count the group when using futexes
323 to block, it can happen that a group gets closed before a waiter can
324 increment the reference count. Therefore, waiters have to check whether
325 their group is already closed using __g1_start. They also have to perform
326 this check when spinning when trying to grab a signal from __g_signals.
327 Note that for these checks, using relaxed MO to load __g1_start is
328 sufficient because if a waiter can see a sufficiently large value, it could
329 have also consume a signal in the waiters group.
331 Waiters try to grab a signal from __g_signals without holding a reference
332 count, which can lead to stealing a signal from a more recent group after
333 their own group was already closed. They cannot always detect whether they
334 in fact did because they do not know when they stole, but they can
335 conservatively add a signal back to the group they stole from; if they
336 did so unnecessarily, all that happens is a spurious wake-up. To make this
337 even less likely, __g1_start contains the index of the current g2 too,
338 which allows waiters to check if there aliasing on the group slots; if
339 there wasn't, they didn't steal from the current G1, which means that the
340 G1 they stole from must have been already closed and they do not need to
341 fix anything.
343 It is essential that the last field in pthread_cond_t is __g_signals[1]:
344 The previous condvar used a pointer-sized field in pthread_cond_t, so a
345 PTHREAD_COND_INITIALIZER from that condvar implementation might only
346 initialize 4 bytes to zero instead of the 8 bytes we need (i.e., 44 bytes
347 in total instead of the 48 we need). __g_signals[1] is not accessed before
348 the first group switch (G2 starts at index 0), which will set its value to
349 zero after a harmless fetch-or whose return value is ignored. This
350 effectively completes initialization.
353 Limitations:
354 * This condvar isn't designed to allow for more than
355 __PTHREAD_COND_MAX_GROUP_SIZE * (1 << 31) calls to __pthread_cond_wait.
356 * More than __PTHREAD_COND_MAX_GROUP_SIZE concurrent waiters are not
357 supported.
358 * Beyond what is allowed as errors by POSIX or documented, we can also
359 return the following errors:
360 * EPERM if MUTEX is a recursive mutex and the caller doesn't own it.
361 * EOWNERDEAD or ENOTRECOVERABLE when using robust mutexes. Unlike
362 for other errors, this can happen when we re-acquire the mutex; this
363 isn't allowed by POSIX (which requires all errors to virtually happen
364 before we release the mutex or change the condvar state), but there's
365 nothing we can do really.
366 * When using PTHREAD_MUTEX_PP_* mutexes, we can also return all errors
367 returned by __pthread_tpp_change_priority. We will already have
368 released the mutex in such cases, so the caller cannot expect to own
369 MUTEX.
371 Other notes:
372 * Instead of the normal mutex unlock / lock functions, we use
373 __pthread_mutex_unlock_usercnt(m, 0) / __pthread_mutex_cond_lock(m)
374 because those will not change the mutex-internal users count, so that it
375 can be detected when a condvar is still associated with a particular
376 mutex because there is a waiter blocked on this condvar using this mutex.
378static __always_inline int
379__pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
380 clockid_t clockid, const struct __timespec64 *abstime)
382 const int maxspin = 0;
383 int err;
384 int result = 0;
386 LIBC_PROBE (cond_wait, 2, cond, mutex);
388 /* clockid will already have been checked by
389 __pthread_cond_clockwait or pthread_condattr_setclock, or we
390 don't use it if abstime is NULL, so we don't need to check it
391 here. */
393 /* Acquire a position (SEQ) in the waiter sequence (WSEQ). We use an
394 atomic operation because signals and broadcasts may update the group
395 switch without acquiring the mutex. We do not need release MO here
396 because we do not need to establish any happens-before relation with
397 signalers (see __pthread_cond_signal); modification order alone
398 establishes a total order of waiters/signals. We do need acquire MO
399 to synchronize with group reinitialization in
400 __condvar_quiesce_and_switch_g1. */
401 uint64_t wseq = __condvar_fetch_add_wseq_acquire (cond, val: 2);
402 /* Find our group's index. We always go into what was G2 when we acquired
403 our position. */
404 unsigned int g = wseq & 1;
405 uint64_t seq = wseq >> 1;
407 /* Increase the waiter reference count. Relaxed MO is sufficient because
408 we only need to synchronize when decrementing the reference count. */
409 unsigned int flags = atomic_fetch_add_relaxed (&cond->__data.__wrefs, 8);
410 int private = __condvar_get_private (flags);
412 /* Now that we are registered as a waiter, we can release the mutex.
413 Waiting on the condvar must be atomic with releasing the mutex, so if
414 the mutex is used to establish a happens-before relation with any
415 signaler, the waiter must be visible to the latter; thus, we release the
416 mutex after registering as waiter.
417 If releasing the mutex fails, we just cancel our registration as a
418 waiter and confirm that we have woken up. */
419 err = __pthread_mutex_unlock_usercnt (mutex, 0);
420 if (__glibc_unlikely (err != 0))
421 {
422 __condvar_cancel_waiting (cond, seq, g, private);
423 __condvar_confirm_wakeup (cond, private);
424 return err;
425 }
427 /* Now wait until a signal is available in our group or it is closed.
428 Acquire MO so that if we observe a value of zero written after group
429 switching in __condvar_quiesce_and_switch_g1, we synchronize with that
430 store and will see the prior update of __g1_start done while switching
431 groups too. */
432 unsigned int signals = atomic_load_acquire (cond->__data.__g_signals + g);
434 do
435 {
436 while (1)
437 {
438 /* Spin-wait first.
439 Note that spinning first without checking whether a timeout
440 passed might lead to what looks like a spurious wake-up even
441 though we should return ETIMEDOUT (e.g., if the caller provides
442 an absolute timeout that is clearly in the past). However,
443 (1) spurious wake-ups are allowed, (2) it seems unlikely that a
444 user will (ab)use pthread_cond_wait as a check for whether a
445 point in time is in the past, and (3) spinning first without
446 having to compare against the current time seems to be the right
447 choice from a performance perspective for most use cases. */
448 unsigned int spin = maxspin;
449 while (signals == 0 && spin > 0)
450 {
451 /* Check that we are not spinning on a group that's already
452 closed. */
453 if (seq < (__condvar_load_g1_start_relaxed (cond) >> 1))
454 goto done;
456 /* TODO Back off. */
458 /* Reload signals. See above for MO. */
459 signals = atomic_load_acquire (cond->__data.__g_signals + g);
460 spin--;
461 }
463 /* If our group will be closed as indicated by the flag on signals,
464 don't bother grabbing a signal. */
465 if (signals & 1)
466 goto done;
468 /* If there is an available signal, don't block. */
469 if (signals != 0)
470 break;
472 /* No signals available after spinning, so prepare to block.
473 We first acquire a group reference and use acquire MO for that so
474 that we synchronize with the dummy read-modify-write in
475 __condvar_quiesce_and_switch_g1 if we read from that. In turn,
476 in this case this will make us see the closed flag on __g_signals
477 that designates a concurrent attempt to reuse the group's slot.
478 We use acquire MO for the __g_signals check to make the
479 __g1_start check work (see spinning above).
480 Note that the group reference acquisition will not mask the
481 release MO when decrementing the reference count because we use
482 an atomic read-modify-write operation and thus extend the release
483 sequence. */
484 atomic_fetch_add_acquire (cond->__data.__g_refs + g, 2);
485 if (((atomic_load_acquire (cond->__data.__g_signals + g) & 1) != 0)
486 || (seq < (__condvar_load_g1_start_relaxed (cond) >> 1)))
487 {
488 /* Our group is closed. Wake up any signalers that might be
489 waiting. */
490 __condvar_dec_grefs (cond, g, private);
491 goto done;
492 }
494 // Now block.
495 struct _pthread_cleanup_buffer buffer;
496 struct _condvar_cleanup_buffer cbuffer;
497 cbuffer.wseq = wseq;
498 cbuffer.cond = cond;
499 cbuffer.mutex = mutex;
500 cbuffer.private = private;
501 __pthread_cleanup_push (&buffer, __condvar_cleanup_waiting, &cbuffer);
503 err = __futex_abstimed_wait_cancelable64 (
504 cond->__data.__g_signals + g, 0, clockid, abstime, private);
506 __pthread_cleanup_pop (&buffer, 0);
508 if (__glibc_unlikely (err == ETIMEDOUT || err == EOVERFLOW))
509 {
510 __condvar_dec_grefs (cond, g, private);
511 /* If we timed out, we effectively cancel waiting. Note that
512 we have decremented __g_refs before cancellation, so that a
513 deadlock between waiting for quiescence of our group in
514 __condvar_quiesce_and_switch_g1 and us trying to acquire
515 the lock during cancellation is not possible. */
516 __condvar_cancel_waiting (cond, seq, g, private);
517 result = err;
518 goto done;
519 }
520 else
521 __condvar_dec_grefs (cond, g, private);
523 /* Reload signals. See above for MO. */
524 signals = atomic_load_acquire (cond->__data.__g_signals + g);
525 }
527 }
528 /* Try to grab a signal. Use acquire MO so that we see an up-to-date value
529 of __g1_start below (see spinning above for a similar case). In
530 particular, if we steal from a more recent group, we will also see a
531 more recent __g1_start below. */
532 while (!atomic_compare_exchange_weak_acquire (cond->__data.__g_signals + g,
533 &signals, signals - 2));
535 /* We consumed a signal but we could have consumed from a more recent group
536 that aliased with ours due to being in the same group slot. If this
537 might be the case our group must be closed as visible through
538 __g1_start. */
539 uint64_t g1_start = __condvar_load_g1_start_relaxed (cond);
540 if (seq < (g1_start >> 1))
541 {
542 /* We potentially stole a signal from a more recent group but we do not
543 know which group we really consumed from.
544 We do not care about groups older than current G1 because they are
545 closed; we could have stolen from these, but then we just add a
546 spurious wake-up for the current groups.
547 We will never steal a signal from current G2 that was really intended
548 for G2 because G2 never receives signals (until it becomes G1). We
549 could have stolen a signal from G2 that was conservatively added by a
550 previous waiter that also thought it stole a signal -- but given that
551 that signal was added unnecessarily, it's not a problem if we steal
552 it.
553 Thus, the remaining case is that we could have stolen from the current
554 G1, where "current" means the __g1_start value we observed. However,
555 if the current G1 does not have the same slot index as we do, we did
556 not steal from it and do not need to undo that. This is the reason
557 for putting a bit with G2's index into__g1_start as well. */
558 if (((g1_start & 1) ^ 1) == g)
559 {
560 /* We have to conservatively undo our potential mistake of stealing
561 a signal. We can stop trying to do that when the current G1
562 changes because other spinning waiters will notice this too and
563 __condvar_quiesce_and_switch_g1 has checked that there are no
564 futex waiters anymore before switching G1.
565 Relaxed MO is fine for the __g1_start load because we need to
566 merely be able to observe this fact and not have to observe
567 something else as well.
568 ??? Would it help to spin for a little while to see whether the
569 current G1 gets closed? This might be worthwhile if the group is
570 small or close to being closed. */
571 unsigned int s = atomic_load_relaxed (cond->__data.__g_signals + g);
572 while (__condvar_load_g1_start_relaxed (cond) == g1_start)
573 {
574 /* Try to add a signal. We don't need to acquire the lock
575 because at worst we can cause a spurious wake-up. If the
576 group is in the process of being closed (LSB is true), this
577 has an effect similar to us adding a signal. */
578 if (((s & 1) != 0)
579 || atomic_compare_exchange_weak_relaxed
580 (cond->__data.__g_signals + g, &s, s + 2))
581 {
582 /* If we added a signal, we also need to add a wake-up on
583 the futex. We also need to do that if we skipped adding
584 a signal because the group is being closed because
585 while __condvar_quiesce_and_switch_g1 could have closed
586 the group, it might stil be waiting for futex waiters to
587 leave (and one of those waiters might be the one we stole
588 the signal from, which cause it to block using the
589 futex). */
590 futex_wake (futex_word: cond->__data.__g_signals + g, processes_to_wake: 1, private);
591 break;
592 }
593 /* TODO Back off. */
594 }
595 }
596 }
598 done:
600 /* Confirm that we have been woken. We do that before acquiring the mutex
601 to allow for execution of pthread_cond_destroy while having acquired the
602 mutex. */
603 __condvar_confirm_wakeup (cond, private);
605 /* Woken up; now re-acquire the mutex. If this doesn't fail, return RESULT,
606 which is set to ETIMEDOUT if a timeout occured, or zero otherwise. */
607 err = __pthread_mutex_cond_lock (mutex: mutex);
608 /* XXX Abort on errors that are disallowed by POSIX? */
609 return (err != 0) ? err : result;
613/* See __pthread_cond_wait_common. */
615___pthread_cond_wait (pthread_cond_t *cond, pthread_mutex_t *mutex)
617 /* clockid is unused when abstime is NULL. */
618 return __pthread_cond_wait_common (cond, mutex, clockid: 0, NULL);
621versioned_symbol (libc, ___pthread_cond_wait, pthread_cond_wait,
622 GLIBC_2_3_2);
623libc_hidden_ver (___pthread_cond_wait, __pthread_cond_wait)
624#ifndef SHARED
625strong_alias (___pthread_cond_wait, __pthread_cond_wait)
628/* See __pthread_cond_wait_common. */
630___pthread_cond_timedwait64 (pthread_cond_t *cond, pthread_mutex_t *mutex,
631 const struct __timespec64 *abstime)
633 /* Check parameter validity. This should also tell the compiler that
634 it can assume that abstime is not NULL. */
635 if (! valid_nanoseconds (ns: abstime->tv_nsec))
636 return EINVAL;
638 /* Relaxed MO is suffice because clock ID bit is only modified
639 in condition creation. */
640 unsigned int flags = atomic_load_relaxed (&cond->__data.__wrefs);
641 clockid_t clockid = (flags & __PTHREAD_COND_CLOCK_MONOTONIC_MASK)
643 return __pthread_cond_wait_common (cond, mutex, clockid, abstime);
646#if __TIMESIZE == 64
647strong_alias (___pthread_cond_timedwait64, ___pthread_cond_timedwait)
649strong_alias (___pthread_cond_timedwait64, __pthread_cond_timedwait64)
650libc_hidden_def (__pthread_cond_timedwait64)
653___pthread_cond_timedwait (pthread_cond_t *cond, pthread_mutex_t *mutex,
654 const struct timespec *abstime)
656 struct __timespec64 ts64 = valid_timespec_to_timespec64 (*abstime);
658 return __pthread_cond_timedwait64 (cond, mutex, &ts64);
660#endif /* __TIMESIZE == 64 */
661versioned_symbol (libc, ___pthread_cond_timedwait,
662 pthread_cond_timedwait, GLIBC_2_3_2);
663libc_hidden_ver (___pthread_cond_timedwait, __pthread_cond_timedwait)
664#ifndef SHARED
665strong_alias (___pthread_cond_timedwait, __pthread_cond_timedwait)
668/* See __pthread_cond_wait_common. */
670___pthread_cond_clockwait64 (pthread_cond_t *cond, pthread_mutex_t *mutex,
671 clockid_t clockid,
672 const struct __timespec64 *abstime)
674 /* Check parameter validity. This should also tell the compiler that
675 it can assume that abstime is not NULL. */
676 if (! valid_nanoseconds (ns: abstime->tv_nsec))
677 return EINVAL;
679 if (!futex_abstimed_supported_clockid (clockid))
680 return EINVAL;
682 return __pthread_cond_wait_common (cond, mutex, clockid, abstime);
685#if __TIMESIZE == 64
686strong_alias (___pthread_cond_clockwait64, ___pthread_cond_clockwait)
688strong_alias (___pthread_cond_clockwait64, __pthread_cond_clockwait64);
689libc_hidden_def (__pthread_cond_clockwait64)
692___pthread_cond_clockwait (pthread_cond_t *cond, pthread_mutex_t *mutex,
693 clockid_t clockid,
694 const struct timespec *abstime)
696 struct __timespec64 ts64 = valid_timespec_to_timespec64 (*abstime);
698 return __pthread_cond_clockwait64 (cond, mutex, clockid, &ts64);
700#endif /* __TIMESIZE == 64 */
701libc_hidden_ver (___pthread_cond_clockwait, __pthread_cond_clockwait)
702#ifndef SHARED
703strong_alias (___pthread_cond_clockwait, __pthread_cond_clockwait)
705versioned_symbol (libc, ___pthread_cond_clockwait,
706 pthread_cond_clockwait, GLIBC_2_34);
707#if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_30, GLIBC_2_34)
708compat_symbol (libpthread, ___pthread_cond_clockwait,
709 pthread_cond_clockwait, GLIBC_2_30);

source code of glibc/nptl/pthread_cond_wait.c