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86013ef5ce
For a full analysis of both the pthread_rwlock_tryrdlock() stall
and the pthread_rwlock_trywrlock() stall see:
https://sourceware.org/bugzilla/show_bug.cgi?id=23844#c14
In the pthread_rwlock_trydlock() function we fail to inspect for
PTHREAD_RWLOCK_FUTEX_USED in __wrphase_futex and wake the waiting
readers.
In the pthread_rwlock_trywrlock() function we write 1 to
__wrphase_futex and loose the setting of the PTHREAD_RWLOCK_FUTEX_USED
bit, again failing to wake waiting readers during unlock.
The fix in the case of pthread_rwlock_trydlock() is to check for
PTHREAD_RWLOCK_FUTEX_USED and wake the readers.
The fix in the case of pthread_rwlock_trywrlock() is to only write
1 to __wrphase_futex if we installed the write phase, since all other
readers would be spinning waiting for this step.
We add two new tests, one exercises the stall for
pthread_rwlock_trywrlock() which is easy to exercise, and one exercises
the stall for pthread_rwlock_trydlock() which is harder to exercise.
The pthread_rwlock_trywrlock() test fails consistently without the fix,
and passes after. The pthread_rwlock_tryrdlock() test fails roughly
5-10% of the time without the fix, and passes all the time after.
Signed-off-by: Carlos O'Donell <carlos@redhat.com>
Signed-off-by: Torvald Riegel <triegel@redhat.com>
Signed-off-by: Rik Prohaska <prohaska7@gmail.com>
Co-authored-by: Torvald Riegel <triegel@redhat.com>
Co-authored-by: Rik Prohaska <prohaska7@gmail.com>
(cherry picked from commit 5fc9ed4c40)
109 lines
3.4 KiB
C
109 lines
3.4 KiB
C
/* Bug 23844: Test for pthread_rwlock_trywrlock stalls.
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Copyright (C) 2019 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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/* For a full analysis see comments in tst-rwlock-tryrdlock-stall.c.
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Summary for the pthread_rwlock_trywrlock() stall:
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The stall is caused by pthread_rwlock_trywrlock setting
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__wrphase_futex futex to 1 and loosing the
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PTHREAD_RWLOCK_FUTEX_USED bit.
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The fix for bug 23844 ensures that waiters on __wrphase_futex are
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correctly woken. Before the fix the test stalls as readers can
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wait forever on __wrphase_futex. */
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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#include <pthread.h>
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#include <support/xthread.h>
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#include <errno.h>
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/* We need only one lock to reproduce the issue. We will need multiple
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threads to get the exact case where we have a read, try, and unlock
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all interleaving to produce the case where the readers are waiting
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and the try clears the PTHREAD_RWLOCK_FUTEX_USED bit and a
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subsequent unlock fails to wake them. */
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pthread_rwlock_t onelock;
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/* The number of threads is arbitrary but empirically chosen to have
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enough threads that we see the condition where waiting readers are
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not woken by a successful unlock. */
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#define NTHREADS 32
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_Atomic int do_exit;
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void *
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run_loop (void *arg)
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{
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int i = 0, ret;
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while (!do_exit)
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{
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/* Arbitrarily choose if we are the writer or reader. Choose a
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high enough ratio of readers to writers to make it likely
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that readers block (and eventually are susceptable to
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stalling).
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If we are a writer, take the write lock, and then unlock.
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If we are a reader, try the lock, then lock, then unlock. */
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if ((i % 8) != 0)
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{
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if ((ret = pthread_rwlock_trywrlock (&onelock)) != 0)
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{
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if (ret == EBUSY)
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xpthread_rwlock_wrlock (&onelock);
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else
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exit (EXIT_FAILURE);
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}
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}
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else
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xpthread_rwlock_rdlock (&onelock);
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/* Thread does some work and then unlocks. */
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xpthread_rwlock_unlock (&onelock);
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i++;
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}
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return NULL;
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}
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int
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do_test (void)
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{
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int i;
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pthread_t tids[NTHREADS];
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xpthread_rwlock_init (&onelock, NULL);
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for (i = 0; i < NTHREADS; i++)
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tids[i] = xpthread_create (NULL, run_loop, NULL);
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/* Run for some amount of time. The pthread_rwlock_tryrwlock stall
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is very easy to trigger and happens in seconds under the test
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conditions. */
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sleep (10);
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/* Then exit. */
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printf ("INFO: Exiting...\n");
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do_exit = 1;
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/* If any readers stalled then we will timeout waiting for them. */
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for (i = 0; i < NTHREADS; i++)
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xpthread_join (tids[i]);
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printf ("INFO: Done.\n");
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xpthread_rwlock_destroy (&onelock);
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printf ("PASS: No pthread_rwlock_tryrwlock stalls detected.\n");
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return 0;
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}
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#include <support/test-driver.c>
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