1/* Copyright (C) 2003-2020 Free Software Foundation, Inc.
2 This file is part of the GNU C Library.
3 Contributed by Martin Schwidefsky <schwidefsky@de.ibm.com>, 2003.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, see
17 <https://www.gnu.org/licenses/>. */
18
19#include <endian.h>
20#include <errno.h>
21#include <sysdep.h>
22#include <futex-internal.h>
23#include <pthread.h>
24#include <pthreadP.h>
25#include <sys/time.h>
26#include <atomic.h>
27#include <stdint.h>
28#include <stdbool.h>
29
30#include <shlib-compat.h>
31#include <stap-probe.h>
32#include <time.h>
33
34#include "pthread_cond_common.c"
35
36
37struct _condvar_cleanup_buffer
38{
39 uint64_t wseq;
40 pthread_cond_t *cond;
41 pthread_mutex_t *mutex;
42 int private;
43};
44
45
46/* Decrease the waiter reference count. */
47static void
48__condvar_confirm_wakeup (pthread_cond_t *cond, int private)
49{
50 /* If destruction is pending (i.e., the wake-request flag is nonzero) and we
51 are the last waiter (prior value of __wrefs was 1 << 3), then wake any
52 threads waiting in pthread_cond_destroy. Release MO to synchronize with
53 these threads. Don't bother clearing the wake-up request flag. */
54 if ((atomic_fetch_add_release (&cond->__data.__wrefs, -8) >> 2) == 3)
55 futex_wake (&cond->__data.__wrefs, INT_MAX, private);
56}
57
58
59/* Cancel waiting after having registered as a waiter previously. SEQ is our
60 position and G is our group index.
61 The goal of cancellation is to make our group smaller if that is still
62 possible. If we are in a closed group, this is not possible anymore; in
63 this case, we need to send a replacement signal for the one we effectively
64 consumed because the signal should have gotten consumed by another waiter
65 instead; we must not both cancel waiting and consume a signal.
66
67 Must not be called while still holding a reference on the group.
68
69 Returns true iff we consumed a signal.
70
71 On some kind of timeouts, we may be able to pretend that a signal we
72 effectively consumed happened before the timeout (i.e., similarly to first
73 spinning on signals before actually checking whether the timeout has
74 passed already). Doing this would allow us to skip sending a replacement
75 signal, but this case might happen rarely because the end of the timeout
76 must race with someone else sending a signal. Therefore, we don't bother
77 trying to optimize this. */
78static void
79__condvar_cancel_waiting (pthread_cond_t *cond, uint64_t seq, unsigned int g,
80 int private)
81{
82 bool consumed_signal = false;
83
84 /* No deadlock with group switching is possible here because we have do
85 not hold a reference on the group. */
86 __condvar_acquire_lock (cond, private);
87
88 uint64_t g1_start = __condvar_load_g1_start_relaxed (cond) >> 1;
89 if (g1_start > seq)
90 {
91 /* Our group is closed, so someone provided enough signals for it.
92 Thus, we effectively consumed a signal. */
93 consumed_signal = true;
94 }
95 else
96 {
97 if (g1_start + __condvar_get_orig_size (cond) <= seq)
98 {
99 /* We are in the current G2 and thus cannot have consumed a signal.
100 Reduce its effective size or handle overflow. Remember that in
101 G2, unsigned int size is zero or a negative value. */
102 if (cond->__data.__g_size[g] + __PTHREAD_COND_MAX_GROUP_SIZE > 0)
103 {
104 cond->__data.__g_size[g]--;
105 }
106 else
107 {
108 /* Cancellations would overflow the maximum group size. Just
109 wake up everyone spuriously to create a clean state. This
110 also means we do not consume a signal someone else sent. */
111 __condvar_release_lock (cond, private);
112 __pthread_cond_broadcast (cond);
113 return;
114 }
115 }
116 else
117 {
118 /* We are in current G1. If the group's size is zero, someone put
119 a signal in the group that nobody else but us can consume. */
120 if (cond->__data.__g_size[g] == 0)
121 consumed_signal = true;
122 else
123 {
124 /* Otherwise, we decrease the size of the group. This is
125 equivalent to atomically putting in a signal just for us and
126 consuming it right away. We do not consume a signal sent
127 by someone else. We also cannot have consumed a futex
128 wake-up because if we were cancelled or timed out in a futex
129 call, the futex will wake another waiter. */
130 cond->__data.__g_size[g]--;
131 }
132 }
133 }
134
135 __condvar_release_lock (cond, private);
136
137 if (consumed_signal)
138 {
139 /* We effectively consumed a signal even though we didn't want to.
140 Therefore, we need to send a replacement signal.
141 If we would want to optimize this, we could do what
142 pthread_cond_signal does right in the critical section above. */
143 __pthread_cond_signal (cond);
144 }
145}
146
147/* Wake up any signalers that might be waiting. */
148static void
149__condvar_dec_grefs (pthread_cond_t *cond, unsigned int g, int private)
150{
151 /* Release MO to synchronize-with the acquire load in
152 __condvar_quiesce_and_switch_g1. */
153 if (atomic_fetch_add_release (cond->__data.__g_refs + g, -2) == 3)
154 {
155 /* Clear the wake-up request flag before waking up. We do not need more
156 than relaxed MO and it doesn't matter if we apply this for an aliased
157 group because we wake all futex waiters right after clearing the
158 flag. */
159 atomic_fetch_and_relaxed (cond->__data.__g_refs + g, ~(unsigned int) 1);
160 futex_wake (cond->__data.__g_refs + g, INT_MAX, private);
161 }
162}
163
164/* Clean-up for cancellation of waiters waiting for normal signals. We cancel
165 our registration as a waiter, confirm we have woken up, and re-acquire the
166 mutex. */
167static void
168__condvar_cleanup_waiting (void *arg)
169{
170 struct _condvar_cleanup_buffer *cbuffer =
171 (struct _condvar_cleanup_buffer *) arg;
172 pthread_cond_t *cond = cbuffer->cond;
173 unsigned g = cbuffer->wseq & 1;
174
175 __condvar_dec_grefs (cond, g, cbuffer->private);
176
177 __condvar_cancel_waiting (cond, cbuffer->wseq >> 1, g, cbuffer->private);
178 /* FIXME With the current cancellation implementation, it is possible that
179 a thread is cancelled after it has returned from a syscall. This could
180 result in a cancelled waiter consuming a futex wake-up that is then
181 causing another waiter in the same group to not wake up. To work around
182 this issue until we have fixed cancellation, just add a futex wake-up
183 conservatively. */
184 futex_wake (cond->__data.__g_signals + g, 1, cbuffer->private);
185
186 __condvar_confirm_wakeup (cond, cbuffer->private);
187
188 /* XXX If locking the mutex fails, should we just stop execution? This
189 might be better than silently ignoring the error. */
190 __pthread_mutex_cond_lock (cbuffer->mutex);
191}
192
193/* This condvar implementation guarantees that all calls to signal and
194 broadcast and all of the three virtually atomic parts of each call to wait
195 (i.e., (1) releasing the mutex and blocking, (2) unblocking, and (3) re-
196 acquiring the mutex) happen in some total order that is consistent with the
197 happens-before relations in the calling program. However, this order does
198 not necessarily result in additional happens-before relations being
199 established (which aligns well with spurious wake-ups being allowed).
200
201 All waiters acquire a certain position in a 64b waiter sequence (__wseq).
202 This sequence determines which waiters are allowed to consume signals.
203 A broadcast is equal to sending as many signals as are unblocked waiters.
204 When a signal arrives, it samples the current value of __wseq with a
205 relaxed-MO load (i.e., the position the next waiter would get). (This is
206 sufficient because it is consistent with happens-before; the caller can
207 enforce stronger ordering constraints by calling signal while holding the
208 mutex.) Only waiters with a position less than the __wseq value observed
209 by the signal are eligible to consume this signal.
210
211 This would be straight-forward to implement if waiters would just spin but
212 we need to let them block using futexes. Futexes give no guarantee of
213 waking in FIFO order, so we cannot reliably wake eligible waiters if we
214 just use a single futex. Also, futex words are 32b in size, but we need
215 to distinguish more than 1<<32 states because we need to represent the
216 order of wake-up (and thus which waiters are eligible to consume signals);
217 blocking in a futex is not atomic with a waiter determining its position in
218 the waiter sequence, so we need the futex word to reliably notify waiters
219 that they should not attempt to block anymore because they have been
220 already signaled in the meantime. While an ABA issue on a 32b value will
221 be rare, ignoring it when we are aware of it is not the right thing to do
222 either.
223
224 Therefore, we use a 64b counter to represent the waiter sequence (on
225 architectures which only support 32b atomics, we use a few bits less).
226 To deal with the blocking using futexes, we maintain two groups of waiters:
227 * Group G1 consists of waiters that are all eligible to consume signals;
228 incoming signals will always signal waiters in this group until all
229 waiters in G1 have been signaled.
230 * Group G2 consists of waiters that arrive when a G1 is present and still
231 contains waiters that have not been signaled. When all waiters in G1
232 are signaled and a new signal arrives, the new signal will convert G2
233 into the new G1 and create a new G2 for future waiters.
234
235 We cannot allocate new memory because of process-shared condvars, so we
236 have just two slots of groups that change their role between G1 and G2.
237 Each has a separate futex word, a number of signals available for
238 consumption, a size (number of waiters in the group that have not been
239 signaled), and a reference count.
240
241 The group reference count is used to maintain the number of waiters that
242 are using the group's futex. Before a group can change its role, the
243 reference count must show that no waiters are using the futex anymore; this
244 prevents ABA issues on the futex word.
245
246 To represent which intervals in the waiter sequence the groups cover (and
247 thus also which group slot contains G1 or G2), we use a 64b counter to
248 designate the start position of G1 (inclusive), and a single bit in the
249 waiter sequence counter to represent which group slot currently contains
250 G2. This allows us to switch group roles atomically wrt. waiters obtaining
251 a position in the waiter sequence. The G1 start position allows waiters to
252 figure out whether they are in a group that has already been completely
253 signaled (i.e., if the current G1 starts at a later position that the
254 waiter's position). Waiters cannot determine whether they are currently
255 in G2 or G1 -- but they do not have too because all they are interested in
256 is whether there are available signals, and they always start in G2 (whose
257 group slot they know because of the bit in the waiter sequence. Signalers
258 will simply fill the right group until it is completely signaled and can
259 be closed (they do not switch group roles until they really have to to
260 decrease the likelihood of having to wait for waiters still holding a
261 reference on the now-closed G1).
262
263 Signalers maintain the initial size of G1 to be able to determine where
264 G2 starts (G2 is always open-ended until it becomes G1). They track the
265 remaining size of a group; when waiters cancel waiting (due to PThreads
266 cancellation or timeouts), they will decrease this remaining size as well.
267
268 To implement condvar destruction requirements (i.e., that
269 pthread_cond_destroy can be called as soon as all waiters have been
270 signaled), waiters increment a reference count before starting to wait and
271 decrement it after they stopped waiting but right before they acquire the
272 mutex associated with the condvar.
273
274 pthread_cond_t thus consists of the following (bits that are used for
275 flags and are not part of the primary value of each field but necessary
276 to make some things atomic or because there was no space for them
277 elsewhere in the data structure):
278
279 __wseq: Waiter sequence counter
280 * LSB is index of current G2.
281 * Waiters fetch-add while having acquire the mutex associated with the
282 condvar. Signalers load it and fetch-xor it concurrently.
283 __g1_start: Starting position of G1 (inclusive)
284 * LSB is index of current G2.
285 * Modified by signalers while having acquired the condvar-internal lock
286 and observed concurrently by waiters.
287 __g1_orig_size: Initial size of G1
288 * The two least-significant bits represent the condvar-internal lock.
289 * Only accessed while having acquired the condvar-internal lock.
290 __wrefs: Waiter reference counter.
291 * Bit 2 is true if waiters should run futex_wake when they remove the
292 last reference. pthread_cond_destroy uses this as futex word.
293 * Bit 1 is the clock ID (0 == CLOCK_REALTIME, 1 == CLOCK_MONOTONIC).
294 * Bit 0 is true iff this is a process-shared condvar.
295 * Simple reference count used by both waiters and pthread_cond_destroy.
296 (If the format of __wrefs is changed, update nptl_lock_constants.pysym
297 and the pretty printers.)
298 For each of the two groups, we have:
299 __g_refs: Futex waiter reference count.
300 * LSB is true if waiters should run futex_wake when they remove the
301 last reference.
302 * Reference count used by waiters concurrently with signalers that have
303 acquired the condvar-internal lock.
304 __g_signals: The number of signals that can still be consumed.
305 * Used as a futex word by waiters. Used concurrently by waiters and
306 signalers.
307 * LSB is true iff this group has been completely signaled (i.e., it is
308 closed).
309 __g_size: Waiters remaining in this group (i.e., which have not been
310 signaled yet.
311 * Accessed by signalers and waiters that cancel waiting (both do so only
312 when having acquired the condvar-internal lock.
313 * The size of G2 is always zero because it cannot be determined until
314 the group becomes G1.
315 * Although this is of unsigned type, we rely on using unsigned overflow
316 rules to make this hold effectively negative values too (in
317 particular, when waiters in G2 cancel waiting).
318
319 A PTHREAD_COND_INITIALIZER condvar has all fields set to zero, which yields
320 a condvar that has G2 starting at position 0 and a G1 that is closed.
321
322 Because waiters do not claim ownership of a group right when obtaining a
323 position in __wseq but only reference count the group when using futexes
324 to block, it can happen that a group gets closed before a waiter can
325 increment the reference count. Therefore, waiters have to check whether
326 their group is already closed using __g1_start. They also have to perform
327 this check when spinning when trying to grab a signal from __g_signals.
328 Note that for these checks, using relaxed MO to load __g1_start is
329 sufficient because if a waiter can see a sufficiently large value, it could
330 have also consume a signal in the waiters group.
331
332 Waiters try to grab a signal from __g_signals without holding a reference
333 count, which can lead to stealing a signal from a more recent group after
334 their own group was already closed. They cannot always detect whether they
335 in fact did because they do not know when they stole, but they can
336 conservatively add a signal back to the group they stole from; if they
337 did so unnecessarily, all that happens is a spurious wake-up. To make this
338 even less likely, __g1_start contains the index of the current g2 too,
339 which allows waiters to check if there aliasing on the group slots; if
340 there wasn't, they didn't steal from the current G1, which means that the
341 G1 they stole from must have been already closed and they do not need to
342 fix anything.
343
344 It is essential that the last field in pthread_cond_t is __g_signals[1]:
345 The previous condvar used a pointer-sized field in pthread_cond_t, so a
346 PTHREAD_COND_INITIALIZER from that condvar implementation might only
347 initialize 4 bytes to zero instead of the 8 bytes we need (i.e., 44 bytes
348 in total instead of the 48 we need). __g_signals[1] is not accessed before
349 the first group switch (G2 starts at index 0), which will set its value to
350 zero after a harmless fetch-or whose return value is ignored. This
351 effectively completes initialization.
352
353
354 Limitations:
355 * This condvar isn't designed to allow for more than
356 __PTHREAD_COND_MAX_GROUP_SIZE * (1 << 31) calls to __pthread_cond_wait.
357 * More than __PTHREAD_COND_MAX_GROUP_SIZE concurrent waiters are not
358 supported.
359 * Beyond what is allowed as errors by POSIX or documented, we can also
360 return the following errors:
361 * EPERM if MUTEX is a recursive mutex and the caller doesn't own it.
362 * EOWNERDEAD or ENOTRECOVERABLE when using robust mutexes. Unlike
363 for other errors, this can happen when we re-acquire the mutex; this
364 isn't allowed by POSIX (which requires all errors to virtually happen
365 before we release the mutex or change the condvar state), but there's
366 nothing we can do really.
367 * When using PTHREAD_MUTEX_PP_* mutexes, we can also return all errors
368 returned by __pthread_tpp_change_priority. We will already have
369 released the mutex in such cases, so the caller cannot expect to own
370 MUTEX.
371
372 Other notes:
373 * Instead of the normal mutex unlock / lock functions, we use
374 __pthread_mutex_unlock_usercnt(m, 0) / __pthread_mutex_cond_lock(m)
375 because those will not change the mutex-internal users count, so that it
376 can be detected when a condvar is still associated with a particular
377 mutex because there is a waiter blocked on this condvar using this mutex.
378*/
379static __always_inline int
380__pthread_cond_wait_common (pthread_cond_t *cond, pthread_mutex_t *mutex,
381 clockid_t clockid,
382 const struct timespec *abstime)
383{
384 const int maxspin = 0;
385 int err;
386 int result = 0;
387
388 LIBC_PROBE (cond_wait, 2, cond, mutex);
389
390 /* clockid will already have been checked by
391 __pthread_cond_clockwait or pthread_condattr_setclock, or we
392 don't use it if abstime is NULL, so we don't need to check it
393 here. */
394
395 /* Acquire a position (SEQ) in the waiter sequence (WSEQ). We use an
396 atomic operation because signals and broadcasts may update the group
397 switch without acquiring the mutex. We do not need release MO here
398 because we do not need to establish any happens-before relation with
399 signalers (see __pthread_cond_signal); modification order alone
400 establishes a total order of waiters/signals. We do need acquire MO
401 to synchronize with group reinitialization in
402 __condvar_quiesce_and_switch_g1. */
403 uint64_t wseq = __condvar_fetch_add_wseq_acquire (cond, 2);
404 /* Find our group's index. We always go into what was G2 when we acquired
405 our position. */
406 unsigned int g = wseq & 1;
407 uint64_t seq = wseq >> 1;
408
409 /* Increase the waiter reference count. Relaxed MO is sufficient because
410 we only need to synchronize when decrementing the reference count. */
411 unsigned int flags = atomic_fetch_add_relaxed (&cond->__data.__wrefs, 8);
412 int private = __condvar_get_private (flags);
413
414 /* Now that we are registered as a waiter, we can release the mutex.
415 Waiting on the condvar must be atomic with releasing the mutex, so if
416 the mutex is used to establish a happens-before relation with any
417 signaler, the waiter must be visible to the latter; thus, we release the
418 mutex after registering as waiter.
419 If releasing the mutex fails, we just cancel our registration as a
420 waiter and confirm that we have woken up. */
421 err = __pthread_mutex_unlock_usercnt (mutex, 0);
422 if (__glibc_unlikely (err != 0))
423 {
424 __condvar_cancel_waiting (cond, seq, g, private);
425 __condvar_confirm_wakeup (cond, private);
426 return err;
427 }
428
429 /* Now wait until a signal is available in our group or it is closed.
430 Acquire MO so that if we observe a value of zero written after group
431 switching in __condvar_quiesce_and_switch_g1, we synchronize with that
432 store and will see the prior update of __g1_start done while switching
433 groups too. */
434 unsigned int signals = atomic_load_acquire (cond->__data.__g_signals + g);
435
436 do
437 {
438 while (1)
439 {
440 /* Spin-wait first.
441 Note that spinning first without checking whether a timeout
442 passed might lead to what looks like a spurious wake-up even
443 though we should return ETIMEDOUT (e.g., if the caller provides
444 an absolute timeout that is clearly in the past). However,
445 (1) spurious wake-ups are allowed, (2) it seems unlikely that a
446 user will (ab)use pthread_cond_wait as a check for whether a
447 point in time is in the past, and (3) spinning first without
448 having to compare against the current time seems to be the right
449 choice from a performance perspective for most use cases. */
450 unsigned int spin = maxspin;
451 while (signals == 0 && spin > 0)
452 {
453 /* Check that we are not spinning on a group that's already
454 closed. */
455 if (seq < (__condvar_load_g1_start_relaxed (cond) >> 1))
456 goto done;
457
458 /* TODO Back off. */
459
460 /* Reload signals. See above for MO. */
461 signals = atomic_load_acquire (cond->__data.__g_signals + g);
462 spin--;
463 }
464
465 /* If our group will be closed as indicated by the flag on signals,
466 don't bother grabbing a signal. */
467 if (signals & 1)
468 goto done;
469
470 /* If there is an available signal, don't block. */
471 if (signals != 0)
472 break;
473
474 /* No signals available after spinning, so prepare to block.
475 We first acquire a group reference and use acquire MO for that so
476 that we synchronize with the dummy read-modify-write in
477 __condvar_quiesce_and_switch_g1 if we read from that. In turn,
478 in this case this will make us see the closed flag on __g_signals
479 that designates a concurrent attempt to reuse the group's slot.
480 We use acquire MO for the __g_signals check to make the
481 __g1_start check work (see spinning above).
482 Note that the group reference acquisition will not mask the
483 release MO when decrementing the reference count because we use
484 an atomic read-modify-write operation and thus extend the release
485 sequence. */
486 atomic_fetch_add_acquire (cond->__data.__g_refs + g, 2);
487 if (((atomic_load_acquire (cond->__data.__g_signals + g) & 1) != 0)
488 || (seq < (__condvar_load_g1_start_relaxed (cond) >> 1)))
489 {
490 /* Our group is closed. Wake up any signalers that might be
491 waiting. */
492 __condvar_dec_grefs (cond, g, private);
493 goto done;
494 }
495
496 // Now block.
497 struct _pthread_cleanup_buffer buffer;
498 struct _condvar_cleanup_buffer cbuffer;
499 cbuffer.wseq = wseq;
500 cbuffer.cond = cond;
501 cbuffer.mutex = mutex;
502 cbuffer.private = private;
503 __pthread_cleanup_push (&buffer, __condvar_cleanup_waiting, &cbuffer);
504
505 if (abstime == NULL)
506 {
507 /* Block without a timeout. */
508 err = futex_wait_cancelable (
509 cond->__data.__g_signals + g, 0, private);
510 }
511 else
512 {
513 /* Block, but with a timeout.
514 Work around the fact that the kernel rejects negative timeout
515 values despite them being valid. */
516 if (__glibc_unlikely (abstime->tv_sec < 0))
517 err = ETIMEDOUT;
518 else
519 {
520 err = futex_abstimed_wait_cancelable
521 (cond->__data.__g_signals + g, 0, clockid, abstime,
522 private);
523 }
524 }
525
526 __pthread_cleanup_pop (&buffer, 0);
527
528 if (__glibc_unlikely (err == ETIMEDOUT))
529 {
530 __condvar_dec_grefs (cond, g, private);
531 /* If we timed out, we effectively cancel waiting. Note that
532 we have decremented __g_refs before cancellation, so that a
533 deadlock between waiting for quiescence of our group in
534 __condvar_quiesce_and_switch_g1 and us trying to acquire
535 the lock during cancellation is not possible. */
536 __condvar_cancel_waiting (cond, seq, g, private);
537 result = ETIMEDOUT;
538 goto done;
539 }
540 else
541 __condvar_dec_grefs (cond, g, private);
542
543 /* Reload signals. See above for MO. */
544 signals = atomic_load_acquire (cond->__data.__g_signals + g);
545 }
546
547 }
548 /* Try to grab a signal. Use acquire MO so that we see an up-to-date value
549 of __g1_start below (see spinning above for a similar case). In
550 particular, if we steal from a more recent group, we will also see a
551 more recent __g1_start below. */
552 while (!atomic_compare_exchange_weak_acquire (cond->__data.__g_signals + g,
553 &signals, signals - 2));
554
555 /* We consumed a signal but we could have consumed from a more recent group
556 that aliased with ours due to being in the same group slot. If this
557 might be the case our group must be closed as visible through
558 __g1_start. */
559 uint64_t g1_start = __condvar_load_g1_start_relaxed (cond);
560 if (seq < (g1_start >> 1))
561 {
562 /* We potentially stole a signal from a more recent group but we do not
563 know which group we really consumed from.
564 We do not care about groups older than current G1 because they are
565 closed; we could have stolen from these, but then we just add a
566 spurious wake-up for the current groups.
567 We will never steal a signal from current G2 that was really intended
568 for G2 because G2 never receives signals (until it becomes G1). We
569 could have stolen a signal from G2 that was conservatively added by a
570 previous waiter that also thought it stole a signal -- but given that
571 that signal was added unnecessarily, it's not a problem if we steal
572 it.
573 Thus, the remaining case is that we could have stolen from the current
574 G1, where "current" means the __g1_start value we observed. However,
575 if the current G1 does not have the same slot index as we do, we did
576 not steal from it and do not need to undo that. This is the reason
577 for putting a bit with G2's index into__g1_start as well. */
578 if (((g1_start & 1) ^ 1) == g)
579 {
580 /* We have to conservatively undo our potential mistake of stealing
581 a signal. We can stop trying to do that when the current G1
582 changes because other spinning waiters will notice this too and
583 __condvar_quiesce_and_switch_g1 has checked that there are no
584 futex waiters anymore before switching G1.
585 Relaxed MO is fine for the __g1_start load because we need to
586 merely be able to observe this fact and not have to observe
587 something else as well.
588 ??? Would it help to spin for a little while to see whether the
589 current G1 gets closed? This might be worthwhile if the group is
590 small or close to being closed. */
591 unsigned int s = atomic_load_relaxed (cond->__data.__g_signals + g);
592 while (__condvar_load_g1_start_relaxed (cond) == g1_start)
593 {
594 /* Try to add a signal. We don't need to acquire the lock
595 because at worst we can cause a spurious wake-up. If the
596 group is in the process of being closed (LSB is true), this
597 has an effect similar to us adding a signal. */
598 if (((s & 1) != 0)
599 || atomic_compare_exchange_weak_relaxed
600 (cond->__data.__g_signals + g, &s, s + 2))
601 {
602 /* If we added a signal, we also need to add a wake-up on
603 the futex. We also need to do that if we skipped adding
604 a signal because the group is being closed because
605 while __condvar_quiesce_and_switch_g1 could have closed
606 the group, it might stil be waiting for futex waiters to
607 leave (and one of those waiters might be the one we stole
608 the signal from, which cause it to block using the
609 futex). */
610 futex_wake (cond->__data.__g_signals + g, 1, private);
611 break;
612 }
613 /* TODO Back off. */
614 }
615 }
616 }
617
618 done:
619
620 /* Confirm that we have been woken. We do that before acquiring the mutex
621 to allow for execution of pthread_cond_destroy while having acquired the
622 mutex. */
623 __condvar_confirm_wakeup (cond, private);
624
625 /* Woken up; now re-acquire the mutex. If this doesn't fail, return RESULT,
626 which is set to ETIMEDOUT if a timeout occured, or zero otherwise. */
627 err = __pthread_mutex_cond_lock (mutex);
628 /* XXX Abort on errors that are disallowed by POSIX? */
629 return (err != 0) ? err : result;
630}
631
632
633/* See __pthread_cond_wait_common. */
634int
635__pthread_cond_wait (pthread_cond_t *cond, pthread_mutex_t *mutex)
636{
637 /* clockid is unused when abstime is NULL. */
638 return __pthread_cond_wait_common (cond, mutex, 0, NULL);
639}
640
641/* See __pthread_cond_wait_common. */
642int
643__pthread_cond_timedwait (pthread_cond_t *cond, pthread_mutex_t *mutex,
644 const struct timespec *abstime)
645{
646 /* Check parameter validity. This should also tell the compiler that
647 it can assume that abstime is not NULL. */
648 if (! valid_nanoseconds (abstime->tv_nsec))
649 return EINVAL;
650
651 /* Relaxed MO is suffice because clock ID bit is only modified
652 in condition creation. */
653 unsigned int flags = atomic_load_relaxed (&cond->__data.__wrefs);
654 clockid_t clockid = (flags & __PTHREAD_COND_CLOCK_MONOTONIC_MASK)
655 ? CLOCK_MONOTONIC : CLOCK_REALTIME;
656 return __pthread_cond_wait_common (cond, mutex, clockid, abstime);
657}
658versioned_symbol (libpthread, __pthread_cond_wait, pthread_cond_wait,
659 GLIBC_2_3_2);
660versioned_symbol (libpthread, __pthread_cond_timedwait, pthread_cond_timedwait,
661 GLIBC_2_3_2);
662
663/* See __pthread_cond_wait_common. */
664int
665__pthread_cond_clockwait (pthread_cond_t *cond, pthread_mutex_t *mutex,
666 clockid_t clockid,
667 const struct timespec *abstime)
668{
669 /* Check parameter validity. This should also tell the compiler that
670 it can assume that abstime is not NULL. */
671 if (! valid_nanoseconds (abstime->tv_nsec))
672 return EINVAL;
673
674 if (!futex_abstimed_supported_clockid (clockid))
675 return EINVAL;
676
677 return __pthread_cond_wait_common (cond, mutex, clockid, abstime);
678}
679weak_alias (__pthread_cond_clockwait, pthread_cond_clockwait);
680