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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. */ | |
43 | pthread_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 | ||
52 | void * | |
53 | run_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 | ||
84 | int | |
85 | do_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> |