1 /* Bug 23844: Test for pthread_rwlock_trywrlock stalls.
2 Copyright (C) 2019-2022 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
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.
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.
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/>. */
19 /* For a full analysis see comments in tst-rwlock-tryrdlock-stall.c.
21 Summary for the pthread_rwlock_trywrlock() stall:
23 The stall is caused by pthread_rwlock_trywrlock setting
24 __wrphase_futex futex to 1 and loosing the
25 PTHREAD_RWLOCK_FUTEX_USED bit.
27 The fix for bug 23844 ensures that waiters on __wrphase_futex are
28 correctly woken. Before the fix the test stalls as readers can
29 wait forever on __wrphase_futex. */
35 #include <support/xthread.h>
38 /* We need only one lock to reproduce the issue. We will need multiple
39 threads to get the exact case where we have a read, try, and unlock
40 all interleaving to produce the case where the readers are waiting
41 and the try clears the PTHREAD_RWLOCK_FUTEX_USED bit and a
42 subsequent unlock fails to wake them. */
43 pthread_rwlock_t onelock
;
45 /* The number of threads is arbitrary but empirically chosen to have
46 enough threads that we see the condition where waiting readers are
47 not woken by a successful unlock. */
58 /* Arbitrarily choose if we are the writer or reader. Choose a
59 high enough ratio of readers to writers to make it likely
60 that readers block (and eventually are susceptable to
63 If we are a writer, take the write lock, and then unlock.
64 If we are a reader, try the lock, then lock, then unlock. */
67 if ((ret
= pthread_rwlock_trywrlock (&onelock
)) != 0)
70 xpthread_rwlock_wrlock (&onelock
);
76 xpthread_rwlock_rdlock (&onelock
);
77 /* Thread does some work and then unlocks. */
78 xpthread_rwlock_unlock (&onelock
);
88 pthread_t tids
[NTHREADS
];
89 xpthread_rwlock_init (&onelock
, NULL
);
90 for (i
= 0; i
< NTHREADS
; i
++)
91 tids
[i
] = xpthread_create (NULL
, run_loop
, NULL
);
92 /* Run for some amount of time. The pthread_rwlock_tryrwlock stall
93 is very easy to trigger and happens in seconds under the test
97 printf ("INFO: Exiting...\n");
99 /* If any readers stalled then we will timeout waiting for them. */
100 for (i
= 0; i
< NTHREADS
; i
++)
101 xpthread_join (tids
[i
]);
102 printf ("INFO: Done.\n");
103 xpthread_rwlock_destroy (&onelock
);
104 printf ("PASS: No pthread_rwlock_tryrwlock stalls detected.\n");
108 #include <support/test-driver.c>