1 /* Copyright (C) 2002-2022 Free Software Foundation, Inc.
2 This file is part of the GNU C Library.
3
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.
8
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
11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
12 Lesser General Public License for more details.
13
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/>. */
17
18 #include <errno.h>
19 #include "pthreadP.h"
20 #include <atomic.h>
21 #include <shlib-compat.h>
22
23 /* See pthread_rwlock_common.c for an overview. */
24 int
___pthread_rwlock_trywrlock(pthread_rwlock_t * rwlock)25 ___pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock)
26 {
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))))
43 {
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 /* We have become the primary writer and we cannot have shared
50 the PTHREAD_RWLOCK_FUTEX_USED flag with someone else, so we
51 can simply enable blocking (see full wrlock code). */
52 atomic_store_relaxed (&rwlock->__data.__writers_futex, 1);
53 /* If we started a write phase, we need to enable readers to
54 wait. If we did not, we must not change it because other threads
55 may have set the PTHREAD_RWLOCK_FUTEX_USED in the meantime. */
56 if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
57 atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1);
58 atomic_store_relaxed (&rwlock->__data.__cur_writer,
59 THREAD_GETMEM (THREAD_SELF, tid));
60 return 0;
61 }
62 /* TODO Back-off. */
63 /* See above. */
64 }
65 return EBUSY;
66 }
67 versioned_symbol (libc, ___pthread_rwlock_trywrlock,
68 pthread_rwlock_trywrlock, GLIBC_2_34);
69 libc_hidden_ver (___pthread_rwlock_trywrlock, __pthread_rwlock_trywrlock)
70
71 #if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_1, GLIBC_2_34)
72 compat_symbol (libpthread, ___pthread_rwlock_trywrlock,
73 pthread_rwlock_trywrlock, GLIBC_2_1);
74 #endif
75 #if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_2, GLIBC_2_34)
76 compat_symbol (libpthread, ___pthread_rwlock_trywrlock,
77 __pthread_rwlock_trywrlock, GLIBC_2_2);
78 #endif
79