1 /* Copyright (C) 2002-2023 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 <stdbool.h>
22 #include "pthread_rwlock_common.c"
23
24
25 /* See pthread_rwlock_common.c for an overview. */
26 int
27 ___pthread_rwlock_tryrdlock (pthread_rwlock_t *rwlock)
28 {
29 /* For tryrdlock, we could speculate that we will succeed and go ahead and
30 register as a reader. However, if we misspeculate, we have to do the
31 same steps as a timed-out rdlock, which will increase contention.
32 Therefore, there is a trade-off between being able to use a combinable
33 read-modify-write operation and a CAS loop as used below; we pick the
34 latter because it simplifies the code, and should perform better when
35 tryrdlock is used in cases where writers are infrequent.
36 Because POSIX does not require a failed trylock to "synchronize memory",
37 relaxed MO is sufficient here and on the failure path of the CAS
38 below. */
39 unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
40 unsigned int rnew;
41 do
42 {
43 if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
44 {
45 /* If we are in a read phase, try to acquire unless there is a
46 primary writer and we prefer writers and there will be no
47 recursive read locks. */
48 if (((r & PTHREAD_RWLOCK_WRLOCKED) != 0)
49 && (rwlock->__data.__flags
50 == PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP))
51 return EBUSY;
52 rnew = r + (1 << PTHREAD_RWLOCK_READER_SHIFT);
53 }
54 else
55 {
56 /* If there is a writer that has acquired the lock and we are in
57 a write phase, fail. */
58 if ((r & PTHREAD_RWLOCK_WRLOCKED) != 0)
59 return EBUSY;
60 else
61 {
62 /* If we do not care about potentially waiting writers, just
63 try to acquire. */
64 rnew = (r + (1 << PTHREAD_RWLOCK_READER_SHIFT))
65 ^ PTHREAD_RWLOCK_WRPHASE;
66 }
67 }
68 /* If we could have caused an overflow or take effect during an
69 overflow, we just can / need to return EAGAIN. There is no need to
70 have actually modified the number of readers because we could have
71 done that and cleaned up immediately. */
72 if (rnew >= PTHREAD_RWLOCK_READER_OVERFLOW)
73 return EAGAIN;
74 }
75 /* If the CAS fails, we retry; this prevents that tryrdlock fails spuriously
76 (i.e., fails to acquire the lock although there is no writer), which is
77 fine for C++14 but not currently allowed by POSIX.
78 However, because tryrdlock must not appear to block, we should avoid
79 starving this CAS loop due to constant changes to __readers:
80 While normal rdlock readers that won't be able to acquire will just block
81 (and we expect timeouts on timedrdlock to be longer than one retry of the
82 CAS loop), we can have concurrently failing tryrdlock calls due to
83 readers or writers that acquire and release in the meantime. Using
84 randomized exponential back-off to make a live-lock unlikely should be
85 sufficient.
86 TODO Back-off.
87 Acquire MO so we synchronize with prior writers. */
88 while (!atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers,
89 &r, rnew));
90
91 if ((r & PTHREAD_RWLOCK_WRPHASE) != 0)
92 {
93 /* Same as in __pthread_rwlock_rdlock_full:
94 We started the read phase, so we are also responsible for
95 updating the write-phase futex. Relaxed MO is sufficient.
96 We have to do the same steps as a writer would when handing over the
97 read phase to use because other readers cannot distinguish between
98 us and the writer.
99 Note that __pthread_rwlock_tryrdlock callers will not have to be
100 woken up because they will either see the read phase started by us
101 or they will try to start it themselves; however, callers of
102 __pthread_rwlock_rdlock_full just increase the reader count and then
103 check what state the lock is in, so they cannot distinguish between
104 us and a writer that acquired and released the lock in the
105 meantime. */
106 if ((atomic_exchange_relaxed (&rwlock->__data.__wrphase_futex, 0)
107 & PTHREAD_RWLOCK_FUTEX_USED) != 0)
108 {
109 int private = __pthread_rwlock_get_private (rwlock);
110 futex_wake (&rwlock->__data.__wrphase_futex, INT_MAX, private);
111 }
112 }
113
114 return 0;
115
116
117 }
118 versioned_symbol (libc, ___pthread_rwlock_tryrdlock,
119 pthread_rwlock_tryrdlock, GLIBC_2_34);
120 libc_hidden_ver (___pthread_rwlock_tryrdlock, __pthread_rwlock_tryrdlock)
121
122 #if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_1, GLIBC_2_34)
123 compat_symbol (libpthread, ___pthread_rwlock_tryrdlock,
124 pthread_rwlock_tryrdlock, GLIBC_2_1);
125 #endif
126 #if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_2, GLIBC_2_34)
127 compat_symbol (libpthread, ___pthread_rwlock_tryrdlock,
128 __pthread_rwlock_tryrdlock, GLIBC_2_2);
129 #endif