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bfff8b1b | 1 | /* Copyright (C) 2002-2017 Free Software Foundation, Inc. |
76a50749 UD |
2 | This file is part of the GNU C Library. |
3 | Contributed by Ulrich Drepper <drepper@redhat.com>, 2002. | |
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 | |
59ba27a6 PE |
16 | License along with the GNU C Library; if not, see |
17 | <http://www.gnu.org/licenses/>. */ | |
76a50749 UD |
18 | |
19 | #include <errno.h> | |
20 | #include "pthreadP.h" | |
cc25c8b4 | 21 | #include <atomic.h> |
76a50749 | 22 | |
cc25c8b4 | 23 | /* See pthread_rwlock_common.c for an overview. */ |
76a50749 | 24 | int |
c93c5dec | 25 | __pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock) |
76a50749 | 26 | { |
cc25c8b4 TR |
27 | /* When in a trywrlock, we can acquire the write lock if it is in states |
28 | #1 (idle and read phase) and #5 (idle and write phase), and also in #6 | |
29 | (readers waiting, write phase) if we prefer writers. | |
30 | If we observe any other state, we are allowed to fail and do not need to | |
31 | "synchronize memory" as specified by POSIX (hence relaxed MO is | |
32 | sufficient for the first load and the CAS failure path). | |
33 | We face a similar issue as in tryrdlock in that we need to both avoid | |
34 | live-locks / starvation and must not fail spuriously (see there for | |
35 | further comments) -- and thus must loop until we get a definitive | |
36 | observation or state change. */ | |
37 | unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers); | |
38 | bool prefer_writer = | |
39 | (rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP); | |
40 | while (((r & PTHREAD_RWLOCK_WRLOCKED) == 0) | |
41 | && (((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0) | |
42 | || (prefer_writer && ((r & PTHREAD_RWLOCK_WRPHASE) != 0)))) | |
76a50749 | 43 | { |
cc25c8b4 TR |
44 | /* Try to transition to states #7 or #8 (i.e., acquire the lock). */ |
45 | if (atomic_compare_exchange_weak_acquire ( | |
46 | &rwlock->__data.__readers, &r, | |
47 | r | PTHREAD_RWLOCK_WRPHASE | PTHREAD_RWLOCK_WRLOCKED)) | |
48 | { | |
49 | atomic_store_relaxed (&rwlock->__data.__writers_futex, 1); | |
50 | atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1); | |
51 | atomic_store_relaxed (&rwlock->__data.__cur_writer, | |
52 | THREAD_GETMEM (THREAD_SELF, tid)); | |
53 | return 0; | |
54 | } | |
55 | /* TODO Back-off. */ | |
56 | /* See above. */ | |
76a50749 | 57 | } |
cc25c8b4 | 58 | return EBUSY; |
76a50749 | 59 | } |
cc25c8b4 | 60 | |
76a50749 | 61 | strong_alias (__pthread_rwlock_trywrlock, pthread_rwlock_trywrlock) |