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1/* Bug 23844: Test for pthread_rwlock_trywrlock stalls.
2 Copyright (C) 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/* For a full analysis see comments in tst-rwlock-tryrdlock-stall.c.
20
21 Summary for the pthread_rwlock_trywrlock() stall:
22
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.
26
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. */
30
31#include <stdio.h>
32#include <stdlib.h>
33#include <unistd.h>
34#include <pthread.h>
35#include <support/xthread.h>
36#include <errno.h>
37
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. */
43pthread_rwlock_t onelock;
44
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. */
48#define NTHREADS 32
49
50_Atomic int do_exit;
51
52void *
53run_loop (void *arg)
54{
55 int i = 0, ret;
56 while (!do_exit)
57 {
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
61 stalling).
62
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. */
65 if ((i % 8) != 0)
66 {
67 if ((ret = pthread_rwlock_trywrlock (&onelock)) != 0)
68 {
69 if (ret == EBUSY)
70 xpthread_rwlock_wrlock (&onelock);
71 else
72 exit (EXIT_FAILURE);
73 }
74 }
75 else
76 xpthread_rwlock_rdlock (&onelock);
77 /* Thread does some work and then unlocks. */
78 xpthread_rwlock_unlock (&onelock);
79 i++;
80 }
81 return NULL;
82}
83
84int
85do_test (void)
86{
87 int i;
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
94 conditions. */
95 sleep (10);
96 /* Then exit. */
97 printf ("INFO: Exiting...\n");
98 do_exit = 1;
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");
105 return 0;
106}
107
108#include <support/test-driver.c>
109

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