1/* POSIX reader--writer lock: core parts.
2 Copyright (C) 2016-2019 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
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 <http://www.gnu.org/licenses/>. */
18
19#include <errno.h>
20#include <sysdep.h>
21#include <pthread.h>
22#include <pthreadP.h>
23#include <sys/time.h>
24#include <stap-probe.h>
25#include <atomic.h>
26#include <futex-internal.h>
27
28
29/* A reader--writer lock that fulfills the POSIX requirements (but operations
30 on this lock are not necessarily full barriers, as one may interpret the
31 POSIX requirement about "synchronizing memory"). All critical sections are
32 in a total order, writers synchronize with prior writers and readers, and
33 readers synchronize with prior writers.
34
35 A thread is allowed to acquire a read lock recursively (i.e., have rdlock
36 critical sections that overlap in sequenced-before) unless the kind of the
37 rwlock is set to PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP.
38
39 This lock is built so that workloads of mostly readers can be executed with
40 low runtime overheads. This matches that the default kind of the lock is
41 PTHREAD_RWLOCK_PREFER_READER_NP. Acquiring a read lock requires a single
42 atomic addition if the lock is or was previously acquired by other
43 readers; releasing the lock is a single CAS if there are no concurrent
44 writers.
45 Workloads consisting of mostly writers are of secondary importance.
46 An uncontended write lock acquisition is as fast as for a normal
47 exclusive mutex but writer contention is somewhat more costly due to
48 keeping track of the exact number of writers. If the rwlock kind requests
49 writers to be preferred (i.e., PTHREAD_RWLOCK_PREFER_WRITER_NP or the
50 no-recursive-readers variant of it), then writer--to--writer lock ownership
51 hand-over is fairly fast and bypasses lock acquisition attempts by readers.
52 The costs of lock ownership transfer between readers and writers vary. If
53 the program asserts that there are no recursive readers and writers are
54 preferred, then write lock acquisition attempts will block subsequent read
55 lock acquisition attempts, so that new incoming readers do not prolong a
56 phase in which readers have acquired the lock.
57
58 The main components of the rwlock are a writer-only lock that allows only
59 one of the concurrent writers to be the primary writer, and a
60 single-writer-multiple-readers lock that decides between read phases, in
61 which readers have acquired the rwlock, and write phases in which a primary
62 writer or a sequence of different primary writers have acquired the rwlock.
63
64 The single-writer-multiple-readers lock is the central piece of state
65 describing the rwlock and is encoded in the __readers field (see below for
66 a detailed explanation):
67
68 State WP WL R RW Notes
69 ---------------------------
70 #1 0 0 0 0 Lock is idle (and in a read phase).
71 #2 0 0 >0 0 Readers have acquired the lock.
72 #3 0 1 0 0 Lock is not acquired; a writer will try to start a
73 write phase.
74 #4 0 1 >0 0 Readers have acquired the lock; a writer is waiting
75 and explicit hand-over to the writer is required.
76 #4a 0 1 >0 1 Same as #4 except that there are further readers
77 waiting because the writer is to be preferred.
78 #5 1 0 0 0 Lock is idle (and in a write phase).
79 #6 1 0 >0 0 Write phase; readers will try to start a read phase
80 (requires explicit hand-over to all readers that
81 do not start the read phase).
82 #7 1 1 0 0 Lock is acquired by a writer.
83 #8 1 1 >0 0 Lock acquired by a writer and readers are waiting;
84 explicit hand-over to the readers is required.
85
86 WP (PTHREAD_RWLOCK_WRPHASE) is true if the lock is in a write phase, so
87 potentially acquired by a primary writer.
88 WL (PTHREAD_RWLOCK_WRLOCKED) is true if there is a primary writer (i.e.,
89 the thread that was able to set this bit from false to true).
90 R (all bits in __readers except the number of least-significant bits
91 denoted in PTHREAD_RWLOCK_READER_SHIFT) is the number of readers that have
92 or are trying to acquired the lock. There may be more readers waiting if
93 writers are preferred and there will be no recursive readers, in which
94 case RW (PTHREAD_RWLOCK_RWAITING) is true in state #4a.
95
96 We want to block using futexes but using __readers as a futex word directly
97 is not a good solution. First, we want to wait on different conditions
98 such as waiting for a phase change vs. waiting for the primary writer to
99 release the writer-only lock. Second, the number of readers could change
100 frequently, which would make it likely that a writer's futex_wait fails
101 frequently too because the expected value does not match the value of
102 __readers anymore.
103 Therefore, we split out the futex words into the __wrphase_futex and
104 __writers_futex fields. The former tracks the value of the WP bit and is
105 changed after changing WP by the thread that changes WP. However, because
106 of the POSIX requirements regarding mutex/rwlock destruction (i.e., that
107 destroying a rwlock is allowed as soon as no thread has acquired or will
108 acquire the lock), we have to be careful and hand over lock ownership (via
109 a phase change) carefully to those threads waiting. Specifically, we must
110 prevent a situation in which we are not quite sure whether we still have
111 to unblock another thread through a change to memory (executing a
112 futex_wake on a former futex word that is now used for something else is
113 fine).
114 The scheme we use for __wrphase_futex is that waiting threads that may
115 use the futex word to block now all have to use the futex word to block; it
116 is not allowed to take the short-cut and spin-wait on __readers because
117 then the waking thread cannot just make one final change to memory to
118 unblock all potentially waiting threads. If, for example, a reader
119 increments R in states #7 or #8, it has to then block until __wrphase_futex
120 is 0 and it can confirm that the value of 0 was stored by the primary
121 writer; in turn, the primary writer has to change to a read phase too when
122 releasing WL (i.e., to state #2), and it must change __wrphase_futex to 0
123 as the next step. This ensures that the waiting reader will not be able to
124 acquire, release, and then destroy the lock concurrently with the pending
125 futex unblock operations by the former primary writer. This scheme is
126 called explicit hand-over in what follows.
127 Note that waiting threads can cancel waiting only if explicit hand-over has
128 not yet started (e.g., if __readers is still in states #7 or #8 in the
129 example above).
130
131 Writers determine the primary writer through WL. Blocking using futexes
132 is performed using __writers_futex as a futex word; primary writers will
133 enable waiting on this futex by setting it to 1 after they acquired the WL
134 bit and will disable waiting by setting it to 0 before they release WL.
135 This leaves small windows where blocking using futexes is not possible
136 although a primary writer exists, but in turn decreases complexity of the
137 writer--writer synchronization and does not affect correctness.
138 If writers are preferred, writers can hand over WL directly to other
139 waiting writers that registered by incrementing __writers: If the primary
140 writer can CAS __writers from a non-zero value to the same value with the
141 PTHREAD_RWLOCK_WRHANDOVER bit set, it effectively transfers WL ownership
142 to one of the registered waiting writers and does not reset WL; in turn,
143 a registered writer that can clear PTHREAD_RWLOCK_WRHANDOVER using a CAS
144 then takes over WL. Note that registered waiting writers can cancel
145 waiting by decrementing __writers, but the last writer to unregister must
146 become the primary writer if PTHREAD_RWLOCK_WRHANDOVER is set.
147 Also note that adding another state/bit to signal potential writer--writer
148 contention (e.g., as done in the normal mutex algorithm) would not be
149 helpful because we would have to conservatively assume that there is in
150 fact no other writer, and wake up readers too.
151
152 To avoid having to call futex_wake when no thread uses __wrphase_futex or
153 __writers_futex, threads will set the PTHREAD_RWLOCK_FUTEX_USED bit in the
154 respective futex words before waiting on it (using a CAS so it will only be
155 set if in a state in which waiting would be possible). In the case of
156 __writers_futex, we wake only one thread but several threads may share
157 PTHREAD_RWLOCK_FUTEX_USED, so we must assume that there are still others.
158 This is similar to what we do in pthread_mutex_lock. We do not need to
159 do this for __wrphase_futex because there, we always wake all waiting
160 threads.
161
162 Blocking in the state #4a simply uses __readers as futex word. This
163 simplifies the algorithm but suffers from some of the drawbacks discussed
164 before, though not to the same extent because R can only decrease in this
165 state, so the number of potentially failing futex_wait attempts will be
166 bounded. All threads moving from state #4a to another state must wake
167 up threads blocked on the __readers futex.
168
169 The ordering invariants that we have to take care of in the implementation
170 are primarily those necessary for a reader--writer lock; this is rather
171 straightforward and happens during write/read phase switching (potentially
172 through explicit hand-over), and between writers through synchronization
173 involving the PTHREAD_RWLOCK_WRLOCKED or PTHREAD_RWLOCK_WRHANDOVER bits.
174 Additionally, we need to take care that modifications of __writers_futex
175 and __wrphase_futex (e.g., by otherwise unordered readers) take place in
176 the writer critical sections or read/write phases, respectively, and that
177 explicit hand-over observes stores from the previous phase. How this is
178 done is explained in more detail in comments in the code.
179
180 Many of the accesses to the futex words just need relaxed MO. This is
181 possible because we essentially drive both the core rwlock synchronization
182 and the futex synchronization in parallel. For example, an unlock will
183 unlock the rwlock and take part in the futex synchronization (using
184 PTHREAD_RWLOCK_FUTEX_USED, see above); even if they are not tightly
185 ordered in some way, the futex synchronization ensures that there are no
186 lost wake-ups, and woken threads will then eventually see the most recent
187 state of the rwlock. IOW, waiting threads will always be woken up, while
188 not being able to wait using futexes (which can happen) is harmless; in
189 turn, this means that waiting threads don't need special ordering wrt.
190 waking threads.
191
192 The futex synchronization consists of the three-state futex word:
193 (1) cannot block on it, (2) can block on it, and (3) there might be a
194 thread blocked on it (i.e., with PTHREAD_RWLOCK_FUTEX_USED set).
195 Relaxed-MO atomic read-modify-write operations are sufficient to maintain
196 this (e.g., using a CAS to go from (2) to (3) but not from (1) to (3)),
197 but we need ordering of the futex word modifications by the waking threads
198 so that they collectively make correct state changes between (1)-(3).
199 The futex-internal synchronization (i.e., the conceptual critical sections
200 around futex operations in the kernel) then ensures that even an
201 unconstrained load (i.e., relaxed MO) inside of futex_wait will not lead to
202 lost wake-ups because either the waiting thread will see the change from
203 (3) to (1) when a futex_wake came first, or this futex_wake will wake this
204 waiting thread because the waiting thread came first.
205
206
207 POSIX allows but does not require rwlock acquisitions to be a cancellation
208 point. We do not support cancellation.
209
210 TODO We do not try to elide any read or write lock acquisitions currently.
211 While this would be possible, it is unclear whether HTM performance is
212 currently predictable enough and our runtime tuning is good enough at
213 deciding when to use elision so that enabling it would lead to consistently
214 better performance. */
215
216
217static int
218__pthread_rwlock_get_private (pthread_rwlock_t *rwlock)
219{
220 return rwlock->__data.__shared != 0 ? FUTEX_SHARED : FUTEX_PRIVATE;
221}
222
223static __always_inline void
224__pthread_rwlock_rdunlock (pthread_rwlock_t *rwlock)
225{
226 int private = __pthread_rwlock_get_private (rwlock);
227 /* We decrease the number of readers, and if we are the last reader and
228 there is a primary writer, we start a write phase. We use a CAS to
229 make this atomic so that it is clear whether we must hand over ownership
230 explicitly. */
231 unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
232 unsigned int rnew;
233 for (;;)
234 {
235 rnew = r - (1 << PTHREAD_RWLOCK_READER_SHIFT);
236 /* If we are the last reader, we also need to unblock any readers
237 that are waiting for a writer to go first (PTHREAD_RWLOCK_RWAITING)
238 so that they can register while the writer is active. */
239 if ((rnew >> PTHREAD_RWLOCK_READER_SHIFT) == 0)
240 {
241 if ((rnew & PTHREAD_RWLOCK_WRLOCKED) != 0)
242 rnew |= PTHREAD_RWLOCK_WRPHASE;
243 rnew &= ~(unsigned int) PTHREAD_RWLOCK_RWAITING;
244 }
245 /* We need release MO here for three reasons. First, so that we
246 synchronize with subsequent writers. Second, we might have been the
247 first reader and set __wrphase_futex to 0, so we need to synchronize
248 with the last reader that will set it to 1 (note that we will always
249 change __readers before the last reader, or we are the last reader).
250 Third, a writer that takes part in explicit hand-over needs to see
251 the first reader's store to __wrphase_futex (or a later value) if
252 the writer observes that a write phase has been started. */
253 if (atomic_compare_exchange_weak_release (&rwlock->__data.__readers,
254 &r, rnew))
255 break;
256 /* TODO Back-off. */
257 }
258 if ((rnew & PTHREAD_RWLOCK_WRPHASE) != 0)
259 {
260 /* We need to do explicit hand-over. We need the acquire MO fence so
261 that our modification of _wrphase_futex happens after a store by
262 another reader that started a read phase. Relaxed MO is sufficient
263 for the modification of __wrphase_futex because it is just used
264 to delay acquisition by a writer until all threads are unblocked
265 irrespective of whether they are looking at __readers or
266 __wrphase_futex; any other synchronizes-with relations that are
267 necessary are established through __readers. */
268 atomic_thread_fence_acquire ();
269 if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 1)
270 & PTHREAD_RWLOCK_FUTEX_USED) != 0)
271 futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private);
272 }
273 /* Also wake up waiting readers if we did reset the RWAITING flag. */
274 if ((r & PTHREAD_RWLOCK_RWAITING) != (rnew & PTHREAD_RWLOCK_RWAITING))
275 futex_wake (&rwlock->__data.__readers, INT_MAX, private);
276}
277
278
279static __always_inline int
280__pthread_rwlock_rdlock_full (pthread_rwlock_t *rwlock,
281 clockid_t clockid,
282 const struct timespec *abstime)
283{
284 unsigned int r;
285
286 /* Make sure any passed in clockid and timeout value are valid. Note that
287 the previous implementation assumed that this check *must* not be
288 performed if there would in fact be no blocking; however, POSIX only
289 requires that "the validity of the abstime parameter need not be checked
290 if the lock can be immediately acquired" (i.e., we need not but may check
291 it). */
292 if (abstime && __glibc_unlikely (!futex_abstimed_supported_clockid (clockid)
293 || abstime->tv_nsec >= 1000000000
294 || abstime->tv_nsec < 0))
295 return EINVAL;
296
297 /* Make sure we are not holding the rwlock as a writer. This is a deadlock
298 situation we recognize and report. */
299 if (__glibc_unlikely (atomic_load_relaxed (&rwlock->__data.__cur_writer)
300 == THREAD_GETMEM (THREAD_SELF, tid)))
301 return EDEADLK;
302
303 /* If we prefer writers, recursive rdlock is disallowed, we are in a read
304 phase, and there are other readers present, we try to wait without
305 extending the read phase. We will be unblocked by either one of the
306 other active readers, or if the writer gives up WRLOCKED (e.g., on
307 timeout).
308 If there are no other readers, we simply race with any existing primary
309 writer; it would have been a race anyway, and changing the odds slightly
310 will likely not make a big difference. */
311 if (rwlock->__data.__flags == PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP)
312 {
313 r = atomic_load_relaxed (&rwlock->__data.__readers);
314 while ((r & PTHREAD_RWLOCK_WRPHASE) == 0
315 && (r & PTHREAD_RWLOCK_WRLOCKED) != 0
316 && (r >> PTHREAD_RWLOCK_READER_SHIFT) > 0)
317 {
318 /* TODO Spin first. */
319 /* Try setting the flag signaling that we are waiting without having
320 incremented the number of readers. Relaxed MO is fine because
321 this is just about waiting for a state change in __readers. */
322 if (atomic_compare_exchange_weak_relaxed
323 (&rwlock->__data.__readers, &r, r | PTHREAD_RWLOCK_RWAITING))
324 {
325 /* Wait for as long as the flag is set. An ABA situation is
326 harmless because the flag is just about the state of
327 __readers, and all threads set the flag under the same
328 conditions. */
329 while (((r = atomic_load_relaxed (&rwlock->__data.__readers))
330 & PTHREAD_RWLOCK_RWAITING) != 0)
331 {
332 int private = __pthread_rwlock_get_private (rwlock);
333 int err = futex_abstimed_wait (&rwlock->__data.__readers,
334 r, clockid, abstime, private);
335 /* We ignore EAGAIN and EINTR. On time-outs, we can just
336 return because we don't need to clean up anything. */
337 if (err == ETIMEDOUT)
338 return err;
339 }
340 /* It makes sense to not break out of the outer loop here
341 because we might be in the same situation again. */
342 }
343 else
344 {
345 /* TODO Back-off. */
346 }
347 }
348 }
349 /* Register as a reader, using an add-and-fetch so that R can be used as
350 expected value for future operations. Acquire MO so we synchronize with
351 prior writers as well as the last reader of the previous read phase (see
352 below). */
353 r = (atomic_fetch_add_acquire (&rwlock->__data.__readers,
354 (1 << PTHREAD_RWLOCK_READER_SHIFT))
355 + (1 << PTHREAD_RWLOCK_READER_SHIFT));
356
357 /* Check whether there is an overflow in the number of readers. We assume
358 that the total number of threads is less than half the maximum number
359 of readers that we have bits for in __readers (i.e., with 32-bit int and
360 PTHREAD_RWLOCK_READER_SHIFT of 3, we assume there are less than
361 1 << (32-3-1) concurrent threads).
362 If there is an overflow, we use a CAS to try to decrement the number of
363 readers if there still is an overflow situation. If so, we return
364 EAGAIN; if not, we are not a thread causing an overflow situation, and so
365 we just continue. Using a fetch-add instead of the CAS isn't possible
366 because other readers might release the lock concurrently, which could
367 make us the last reader and thus responsible for handing ownership over
368 to writers (which requires a CAS too to make the decrement and ownership
369 transfer indivisible). */
370 while (__glibc_unlikely (r >= PTHREAD_RWLOCK_READER_OVERFLOW))
371 {
372 /* Relaxed MO is okay because we just want to undo our registration and
373 cannot have changed the rwlock state substantially if the CAS
374 succeeds. */
375 if (atomic_compare_exchange_weak_relaxed
376 (&rwlock->__data.__readers,
377 &r, r - (1 << PTHREAD_RWLOCK_READER_SHIFT)))
378 return EAGAIN;
379 }
380
381 /* We have registered as a reader, so if we are in a read phase, we have
382 acquired a read lock. This is also the reader--reader fast-path.
383 Even if there is a primary writer, we just return. If writers are to
384 be preferred and we are the only active reader, we could try to enter a
385 write phase to let the writer proceed. This would be okay because we
386 cannot have acquired the lock previously as a reader (which could result
387 in deadlock if we would wait for the primary writer to run). However,
388 this seems to be a corner case and handling it specially not be worth the
389 complexity. */
390 if (__glibc_likely ((r & PTHREAD_RWLOCK_WRPHASE) == 0))
391 return 0;
392 /* Otherwise, if we were in a write phase (states #6 or #8), we must wait
393 for explicit hand-over of the read phase; the only exception is if we
394 can start a read phase if there is no primary writer currently. */
395 while ((r & PTHREAD_RWLOCK_WRPHASE) != 0
396 && (r & PTHREAD_RWLOCK_WRLOCKED) == 0)
397 {
398 /* Try to enter a read phase: If the CAS below succeeds, we have
399 ownership; if it fails, we will simply retry and reassess the
400 situation.
401 Acquire MO so we synchronize with prior writers. */
402 if (atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers, &r,
403 r ^ PTHREAD_RWLOCK_WRPHASE))
404 {
405 /* We started the read phase, so we are also responsible for
406 updating the write-phase futex. Relaxed MO is sufficient.
407 We have to do the same steps as a writer would when handing
408 over the read phase to us because other readers cannot
409 distinguish between us and the writer; this includes
410 explicit hand-over and potentially having to wake other readers
411 (but we can pretend to do the setting and unsetting of WRLOCKED
412 atomically, and thus can skip this step). */
413 if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 0)
414 & PTHREAD_RWLOCK_FUTEX_USED) != 0)
415 {
416 int private = __pthread_rwlock_get_private (rwlock);
417 futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private);
418 }
419 return 0;
420 }
421 else
422 {
423 /* TODO Back off before retrying. Also see above. */
424 }
425 }
426
427 /* We were in a write phase but did not install the read phase. We cannot
428 distinguish between a writer and another reader starting the read phase,
429 so we must wait for explicit hand-over via __wrphase_futex.
430 However, __wrphase_futex might not have been set to 1 yet (either
431 because explicit hand-over to the writer is still ongoing, or because
432 the writer has started the write phase but has not yet updated
433 __wrphase_futex). The least recent value of __wrphase_futex we can
434 read from here is the modification of the last read phase (because
435 we synchronize with the last reader in this read phase through
436 __readers; see the use of acquire MO on the fetch_add above).
437 Therefore, if we observe a value of 0 for __wrphase_futex, we need
438 to subsequently check that __readers now indicates a read phase; we
439 need to use acquire MO for this so that if we observe a read phase,
440 we will also see the modification of __wrphase_futex by the previous
441 writer. We then need to load __wrphase_futex again and continue to
442 wait if it is not 0, so that we do not skip explicit hand-over.
443 Relaxed MO is sufficient for the load from __wrphase_futex because
444 we just use it as an indicator for when we can proceed; we use
445 __readers and the acquire MO accesses to it to eventually read from
446 the proper stores to __wrphase_futex. */
447 unsigned int wpf;
448 bool ready = false;
449 for (;;)
450 {
451 while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex))
452 | PTHREAD_RWLOCK_FUTEX_USED) == (1 | PTHREAD_RWLOCK_FUTEX_USED))
453 {
454 int private = __pthread_rwlock_get_private (rwlock);
455 if (((wpf & PTHREAD_RWLOCK_FUTEX_USED) == 0)
456 && (!atomic_compare_exchange_weak_relaxed
457 (&rwlock->__data.__wrphase_futex,
458 &wpf, wpf | PTHREAD_RWLOCK_FUTEX_USED)))
459 continue;
460 int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex,
461 1 | PTHREAD_RWLOCK_FUTEX_USED,
462 clockid, abstime, private);
463 if (err == ETIMEDOUT)
464 {
465 /* If we timed out, we need to unregister. If no read phase
466 has been installed while we waited, we can just decrement
467 the number of readers. Otherwise, we just acquire the
468 lock, which is allowed because we give no precise timing
469 guarantees, and because the timeout is only required to
470 be in effect if we would have had to wait for other
471 threads (e.g., if futex_wait would time-out immediately
472 because the given absolute time is in the past). */
473 r = atomic_load_relaxed (&rwlock->__data.__readers);
474 while ((r & PTHREAD_RWLOCK_WRPHASE) != 0)
475 {
476 /* We don't need to make anything else visible to
477 others besides unregistering, so relaxed MO is
478 sufficient. */
479 if (atomic_compare_exchange_weak_relaxed
480 (&rwlock->__data.__readers, &r,
481 r - (1 << PTHREAD_RWLOCK_READER_SHIFT)))
482 return ETIMEDOUT;
483 /* TODO Back-off. */
484 }
485 /* Use the acquire MO fence to mirror the steps taken in the
486 non-timeout case. Note that the read can happen both
487 in the atomic_load above as well as in the failure case
488 of the CAS operation. */
489 atomic_thread_fence_acquire ();
490 /* We still need to wait for explicit hand-over, but we must
491 not use futex_wait anymore because we would just time out
492 in this case and thus make the spin-waiting we need
493 unnecessarily expensive. */
494 while ((atomic_load_relaxed (&rwlock->__data.__wrphase_futex)
495 | PTHREAD_RWLOCK_FUTEX_USED)
496 == (1 | PTHREAD_RWLOCK_FUTEX_USED))
497 {
498 /* TODO Back-off? */
499 }
500 ready = true;
501 break;
502 }
503 /* If we got interrupted (EINTR) or the futex word does not have the
504 expected value (EAGAIN), retry. */
505 }
506 if (ready)
507 /* See below. */
508 break;
509 /* We need acquire MO here so that we synchronize with the lock
510 release of the writer, and so that we observe a recent value of
511 __wrphase_futex (see below). */
512 if ((atomic_load_acquire (&rwlock->__data.__readers)
513 & PTHREAD_RWLOCK_WRPHASE) == 0)
514 /* We are in a read phase now, so the least recent modification of
515 __wrphase_futex we can read from is the store by the writer
516 with value 1. Thus, only now we can assume that if we observe
517 a value of 0, explicit hand-over is finished. Retry the loop
518 above one more time. */
519 ready = true;
520 }
521
522 return 0;
523}
524
525
526static __always_inline void
527__pthread_rwlock_wrunlock (pthread_rwlock_t *rwlock)
528{
529 int private = __pthread_rwlock_get_private (rwlock);
530
531 atomic_store_relaxed (&rwlock->__data.__cur_writer, 0);
532 /* Disable waiting by writers. We will wake up after we decided how to
533 proceed. */
534 bool wake_writers
535 = ((atomic_exchange_relaxed (&rwlock->__data.__writers_futex, 0)
536 & PTHREAD_RWLOCK_FUTEX_USED) != 0);
537
538 if (rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP)
539 {
540 /* First, try to hand over to another writer. */
541 unsigned int w = atomic_load_relaxed (&rwlock->__data.__writers);
542 while (w != 0)
543 {
544 /* Release MO so that another writer that gets WRLOCKED from us will
545 synchronize with us and thus can take over our view of
546 __readers (including, for example, whether we are in a write
547 phase or not). */
548 if (atomic_compare_exchange_weak_release
549 (&rwlock->__data.__writers, &w, w | PTHREAD_RWLOCK_WRHANDOVER))
550 /* Another writer will take over. */
551 goto done;
552 /* TODO Back-off. */
553 }
554 }
555
556 /* We have done everything we needed to do to prefer writers, so now we
557 either hand over explicitly to readers if there are any, or we simply
558 stay in a write phase. See pthread_rwlock_rdunlock for more details. */
559 unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
560 /* Release MO so that subsequent readers or writers synchronize with us. */
561 while (!atomic_compare_exchange_weak_release
562 (&rwlock->__data.__readers, &r,
563 ((r ^ PTHREAD_RWLOCK_WRLOCKED)
564 ^ ((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0 ? 0
565 : PTHREAD_RWLOCK_WRPHASE))))
566 {
567 /* TODO Back-off. */
568 }
569 if ((r >> PTHREAD_RWLOCK_READER_SHIFT) != 0)
570 {
571 /* We must hand over explicitly through __wrphase_futex. Relaxed MO is
572 sufficient because it is just used to delay acquisition by a writer;
573 any other synchronizes-with relations that are necessary are
574 established through __readers. */
575 if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 0)
576 & PTHREAD_RWLOCK_FUTEX_USED) != 0)
577 futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private);
578 }
579
580 done:
581 /* We released WRLOCKED in some way, so wake a writer. */
582 if (wake_writers)
583 futex_wake (&rwlock->__data.__writers_futex, 1, private);
584}
585
586
587static __always_inline int
588__pthread_rwlock_wrlock_full (pthread_rwlock_t *rwlock,
589 clockid_t clockid,
590 const struct timespec *abstime)
591{
592 /* Make sure any passed in clockid and timeout value are valid. Note that
593 the previous implementation assumed that this check *must* not be
594 performed if there would in fact be no blocking; however, POSIX only
595 requires that "the validity of the abstime parameter need not be checked
596 if the lock can be immediately acquired" (i.e., we need not but may check
597 it). */
598 if (abstime && __glibc_unlikely (!futex_abstimed_supported_clockid (clockid)
599 || abstime->tv_nsec >= 1000000000
600 || abstime->tv_nsec < 0))
601 return EINVAL;
602
603 /* Make sure we are not holding the rwlock as a writer. This is a deadlock
604 situation we recognize and report. */
605 if (__glibc_unlikely (atomic_load_relaxed (&rwlock->__data.__cur_writer)
606 == THREAD_GETMEM (THREAD_SELF, tid)))
607 return EDEADLK;
608
609 /* First we try to acquire the role of primary writer by setting WRLOCKED;
610 if it was set before, there already is a primary writer. Acquire MO so
611 that we synchronize with previous primary writers.
612
613 We do not try to change to a write phase right away using a fetch_or
614 because we would have to reset it again and wake readers if there are
615 readers present (some readers could try to acquire the lock more than
616 once, so setting a write phase in the middle of this could cause
617 deadlock). Changing to a write phase eagerly would only speed up the
618 transition from a read phase to a write phase in the uncontended case,
619 but it would slow down the contended case if readers are preferred (which
620 is the default).
621 We could try to CAS from a state with no readers to a write phase, but
622 this could be less scalable if readers arrive and leave frequently. */
623 bool may_share_futex_used_flag = false;
624 unsigned int r = atomic_fetch_or_acquire (&rwlock->__data.__readers,
625 PTHREAD_RWLOCK_WRLOCKED);
626 if (__glibc_unlikely ((r & PTHREAD_RWLOCK_WRLOCKED) != 0))
627 {
628 /* There is another primary writer. */
629 bool prefer_writer
630 = (rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP);
631 if (prefer_writer)
632 {
633 /* We register as a waiting writer, so that we can make use of
634 writer--writer hand-over. Relaxed MO is fine because we just
635 want to register. We assume that the maximum number of threads
636 is less than the capacity in __writers. */
637 atomic_fetch_add_relaxed (&rwlock->__data.__writers, 1);
638 }
639 for (;;)
640 {
641 /* TODO Spin until WRLOCKED is 0 before trying the CAS below.
642 But pay attention to not delay trying writer--writer hand-over
643 for too long (which we must try eventually anyway). */
644 if ((r & PTHREAD_RWLOCK_WRLOCKED) == 0)
645 {
646 /* Try to become the primary writer or retry. Acquire MO as in
647 the fetch_or above. */
648 if (atomic_compare_exchange_weak_acquire
649 (&rwlock->__data.__readers, &r, r | PTHREAD_RWLOCK_WRLOCKED))
650 {
651 if (prefer_writer)
652 {
653 /* Unregister as a waiting writer. Note that because we
654 acquired WRLOCKED, WRHANDOVER will not be set.
655 Acquire MO on the CAS above ensures that
656 unregistering happens after the previous writer;
657 this sorts the accesses to __writers by all
658 primary writers in a useful way (e.g., any other
659 primary writer acquiring after us or getting it from
660 us through WRHANDOVER will see both our changes to
661 __writers).
662 ??? Perhaps this is not strictly necessary for
663 reasons we do not yet know of. */
664 atomic_fetch_add_relaxed (&rwlock->__data.__writers, -1);
665 }
666 break;
667 }
668 /* Retry if the CAS fails (r will have been updated). */
669 continue;
670 }
671 /* If writer--writer hand-over is available, try to become the
672 primary writer this way by grabbing the WRHANDOVER token. If we
673 succeed, we own WRLOCKED. */
674 if (prefer_writer)
675 {
676 unsigned int w = atomic_load_relaxed (&rwlock->__data.__writers);
677 if ((w & PTHREAD_RWLOCK_WRHANDOVER) != 0)
678 {
679 /* Acquire MO is required here so that we synchronize with
680 the writer that handed over WRLOCKED. We also need this
681 for the reload of __readers below because our view of
682 __readers must be at least as recent as the view of the
683 writer that handed over WRLOCKED; we must avoid an ABA
684 through WRHANDOVER, which could, for example, lead to us
685 assuming we are still in a write phase when in fact we
686 are not. */
687 if (atomic_compare_exchange_weak_acquire
688 (&rwlock->__data.__writers,
689 &w, (w - PTHREAD_RWLOCK_WRHANDOVER - 1)))
690 {
691 /* Reload so our view is consistent with the view of
692 the previous owner of WRLOCKED. See above. */
693 r = atomic_load_relaxed (&rwlock->__data.__readers);
694 break;
695 }
696 /* We do not need to reload __readers here. We should try
697 to perform writer--writer hand-over if possible; if it
698 is not possible anymore, we will reload __readers
699 elsewhere in this loop. */
700 continue;
701 }
702 }
703 /* We did not acquire WRLOCKED nor were able to use writer--writer
704 hand-over, so we block on __writers_futex. */
705 int private = __pthread_rwlock_get_private (rwlock);
706 unsigned int wf
707 = atomic_load_relaxed (&rwlock->__data.__writers_futex);
708 if (((wf & ~(unsigned int) PTHREAD_RWLOCK_FUTEX_USED) != 1)
709 || ((wf != (1 | PTHREAD_RWLOCK_FUTEX_USED))
710 && (!atomic_compare_exchange_weak_relaxed
711 (&rwlock->__data.__writers_futex, &wf,
712 1 | PTHREAD_RWLOCK_FUTEX_USED))))
713 {
714 /* If we cannot block on __writers_futex because there is no
715 primary writer, or we cannot set PTHREAD_RWLOCK_FUTEX_USED,
716 we retry. We must reload __readers here in case we cannot
717 block on __writers_futex so that we can become the primary
718 writer and are not stuck in a loop that just continuously
719 fails to block on __writers_futex. */
720 r = atomic_load_relaxed (&rwlock->__data.__readers);
721 continue;
722 }
723 /* We set the flag that signals that the futex is used, or we could
724 have set it if we had been faster than other waiters. As a
725 result, we may share the flag with an unknown number of other
726 writers. Therefore, we must keep this flag set when we acquire
727 the lock. We do not need to do this when we do not reach this
728 point here because then we are not part of the group that may
729 share the flag, and another writer will wake one of the writers
730 in this group. */
731 may_share_futex_used_flag = true;
732 int err = futex_abstimed_wait (&rwlock->__data.__writers_futex,
733 1 | PTHREAD_RWLOCK_FUTEX_USED,
734 clockid, abstime, private);
735 if (err == ETIMEDOUT)
736 {
737 if (prefer_writer)
738 {
739 /* We need to unregister as a waiting writer. If we are the
740 last writer and writer--writer hand-over is available,
741 we must make use of it because nobody else will reset
742 WRLOCKED otherwise. (If we use it, we simply pretend
743 that this happened before the timeout; see
744 pthread_rwlock_rdlock_full for the full reasoning.)
745 Also see the similar code above. */
746 unsigned int w
747 = atomic_load_relaxed (&rwlock->__data.__writers);
748 while (!atomic_compare_exchange_weak_acquire
749 (&rwlock->__data.__writers, &w,
750 (w == PTHREAD_RWLOCK_WRHANDOVER + 1 ? 0 : w - 1)))
751 {
752 /* TODO Back-off. */
753 }
754 if (w == PTHREAD_RWLOCK_WRHANDOVER + 1)
755 {
756 /* We must continue as primary writer. See above. */
757 r = atomic_load_relaxed (&rwlock->__data.__readers);
758 break;
759 }
760 }
761 /* We cleaned up and cannot have stolen another waiting writer's
762 futex wake-up, so just return. */
763 return ETIMEDOUT;
764 }
765 /* If we got interrupted (EINTR) or the futex word does not have the
766 expected value (EAGAIN), retry after reloading __readers. */
767 r = atomic_load_relaxed (&rwlock->__data.__readers);
768 }
769 /* Our snapshot of __readers is up-to-date at this point because we
770 either set WRLOCKED using a CAS (and update r accordingly below,
771 which was used as expected value for the CAS) or got WRLOCKED from
772 another writer whose snapshot of __readers we inherit. */
773 r |= PTHREAD_RWLOCK_WRLOCKED;
774 }
775
776 /* We are the primary writer; enable blocking on __writers_futex. Relaxed
777 MO is sufficient for futex words; acquire MO on the previous
778 modifications of __readers ensures that this store happens after the
779 store of value 0 by the previous primary writer. */
780 atomic_store_relaxed (&rwlock->__data.__writers_futex,
781 1 | (may_share_futex_used_flag
782 ? PTHREAD_RWLOCK_FUTEX_USED : 0));
783
784 /* If we are in a write phase, we have acquired the lock. */
785 if ((r & PTHREAD_RWLOCK_WRPHASE) != 0)
786 goto done;
787
788 /* If we are in a read phase and there are no readers, try to start a write
789 phase. */
790 while ((r & PTHREAD_RWLOCK_WRPHASE) == 0
791 && (r >> PTHREAD_RWLOCK_READER_SHIFT) == 0)
792 {
793 /* Acquire MO so that we synchronize with prior writers and do
794 not interfere with their updates to __writers_futex, as well
795 as regarding prior readers and their updates to __wrphase_futex,
796 respectively. */
797 if (atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers,
798 &r, r | PTHREAD_RWLOCK_WRPHASE))
799 {
800 /* We have started a write phase, so need to enable readers to wait.
801 See the similar case in __pthread_rwlock_rdlock_full. Unlike in
802 that similar case, we are the (only) primary writer and so do
803 not need to wake another writer. */
804 atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1);
805
806 goto done;
807 }
808 /* TODO Back-off. */
809 }
810
811 /* We became the primary writer in a read phase and there were readers when
812 we did (because of the previous loop). Thus, we have to wait for
813 explicit hand-over from one of these readers.
814 We basically do the same steps as for the similar case in
815 __pthread_rwlock_rdlock_full, except that we additionally might try
816 to directly hand over to another writer and need to wake up
817 other writers or waiting readers (i.e., PTHREAD_RWLOCK_RWAITING). */
818 unsigned int wpf;
819 bool ready = false;
820 for (;;)
821 {
822 while (((wpf = atomic_load_relaxed (&rwlock->__data.__wrphase_futex))
823 | PTHREAD_RWLOCK_FUTEX_USED) == PTHREAD_RWLOCK_FUTEX_USED)
824 {
825 int private = __pthread_rwlock_get_private (rwlock);
826 if ((wpf & PTHREAD_RWLOCK_FUTEX_USED) == 0
827 && (!atomic_compare_exchange_weak_relaxed
828 (&rwlock->__data.__wrphase_futex, &wpf,
829 PTHREAD_RWLOCK_FUTEX_USED)))
830 continue;
831 int err = futex_abstimed_wait (&rwlock->__data.__wrphase_futex,
832 PTHREAD_RWLOCK_FUTEX_USED,
833 clockid, abstime, private);
834 if (err == ETIMEDOUT)
835 {
836 if (rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP)
837 {
838 /* We try writer--writer hand-over. */
839 unsigned int w
840 = atomic_load_relaxed (&rwlock->__data.__writers);
841 if (w != 0)
842 {
843 /* We are about to hand over WRLOCKED, so we must
844 release __writers_futex too; otherwise, we'd have
845 a pending store, which could at least prevent
846 other threads from waiting using the futex
847 because it could interleave with the stores
848 by subsequent writers. In turn, this means that
849 we have to clean up when we do not hand over
850 WRLOCKED.
851 Release MO so that another writer that gets
852 WRLOCKED from us can take over our view of
853 __readers. */
854 unsigned int wf
855 = atomic_exchange_relaxed (&rwlock->__data.__writers_futex, 0);
856 while (w != 0)
857 {
858 if (atomic_compare_exchange_weak_release
859 (&rwlock->__data.__writers, &w,
860 w | PTHREAD_RWLOCK_WRHANDOVER))
861 {
862 /* Wake other writers. */
863 if ((wf & PTHREAD_RWLOCK_FUTEX_USED) != 0)
864 futex_wake (&rwlock->__data.__writers_futex,
865 1, private);
866 return ETIMEDOUT;
867 }
868 /* TODO Back-off. */
869 }
870 /* We still own WRLOCKED and someone else might set
871 a write phase concurrently, so enable waiting
872 again. Make sure we don't loose the flag that
873 signals whether there are threads waiting on
874 this futex. */
875 atomic_store_relaxed (&rwlock->__data.__writers_futex, wf);
876 }
877 }
878 /* If we timed out and we are not in a write phase, we can
879 just stop being a primary writer. Otherwise, we just
880 acquire the lock. */
881 r = atomic_load_relaxed (&rwlock->__data.__readers);
882 if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
883 {
884 /* We are about to release WRLOCKED, so we must release
885 __writers_futex too; see the handling of
886 writer--writer hand-over above. */
887 unsigned int wf
888 = atomic_exchange_relaxed (&rwlock->__data.__writers_futex, 0);
889 while ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
890 {
891 /* While we don't need to make anything from a
892 caller's critical section visible to other
893 threads, we need to ensure that our changes to
894 __writers_futex are properly ordered.
895 Therefore, use release MO to synchronize with
896 subsequent primary writers. Also wake up any
897 waiting readers as they are waiting because of
898 us. */
899 if (atomic_compare_exchange_weak_release
900 (&rwlock->__data.__readers, &r,
901 (r ^ PTHREAD_RWLOCK_WRLOCKED)
902 & ~(unsigned int) PTHREAD_RWLOCK_RWAITING))
903 {
904 /* Wake other writers. */
905 if ((wf & PTHREAD_RWLOCK_FUTEX_USED) != 0)
906 futex_wake (&rwlock->__data.__writers_futex,
907 1, private);
908 /* Wake waiting readers. */
909 if ((r & PTHREAD_RWLOCK_RWAITING) != 0)
910 futex_wake (&rwlock->__data.__readers,
911 INT_MAX, private);
912 return ETIMEDOUT;
913 }
914 }
915 /* We still own WRLOCKED and someone else might set a
916 write phase concurrently, so enable waiting again.
917 Make sure we don't loose the flag that signals
918 whether there are threads waiting on this futex. */
919 atomic_store_relaxed (&rwlock->__data.__writers_futex, wf);
920 }
921 /* Use the acquire MO fence to mirror the steps taken in the
922 non-timeout case. Note that the read can happen both
923 in the atomic_load above as well as in the failure case
924 of the CAS operation. */
925 atomic_thread_fence_acquire ();
926 /* We still need to wait for explicit hand-over, but we must
927 not use futex_wait anymore. */
928 while ((atomic_load_relaxed (&rwlock->__data.__wrphase_futex)
929 | PTHREAD_RWLOCK_FUTEX_USED)
930 == PTHREAD_RWLOCK_FUTEX_USED)
931 {
932 /* TODO Back-off. */
933 }
934 ready = true;
935 break;
936 }
937 /* If we got interrupted (EINTR) or the futex word does not have
938 the expected value (EAGAIN), retry. */
939 }
940 /* See pthread_rwlock_rdlock_full. */
941 if (ready)
942 break;
943 if ((atomic_load_acquire (&rwlock->__data.__readers)
944 & PTHREAD_RWLOCK_WRPHASE) != 0)
945 ready = true;
946 }
947
948 done:
949 atomic_store_relaxed (&rwlock->__data.__cur_writer,
950 THREAD_GETMEM (THREAD_SELF, tid));
951 return 0;
952}
953