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/* Bug 23844: Test for pthread_rwlock_trywrlock stalls.
Copyright (C) 2019-2024 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
/* For a full analysis see comments in tst-rwlock-tryrdlock-stall.c.
Summary for the pthread_rwlock_trywrlock() stall:
The stall is caused by pthread_rwlock_trywrlock setting
__wrphase_futex futex to 1 and losing the
PTHREAD_RWLOCK_FUTEX_USED bit.
The fix for bug 23844 ensures that waiters on __wrphase_futex are
correctly woken. Before the fix the test stalls as readers can
wait forever on __wrphase_futex. */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <support/xthread.h>
#include <errno.h>
/* We need only one lock to reproduce the issue. We will need multiple
threads to get the exact case where we have a read, try, and unlock
all interleaving to produce the case where the readers are waiting
and the try clears the PTHREAD_RWLOCK_FUTEX_USED bit and a
subsequent unlock fails to wake them. */
pthread_rwlock_t onelock;
/* The number of threads is arbitrary but empirically chosen to have
enough threads that we see the condition where waiting readers are
not woken by a successful unlock. */
#define NTHREADS 32
_Atomic int do_exit;
void *
run_loop (void *arg)
{
int i = 0, ret;
while (!do_exit)
{
/* Arbitrarily choose if we are the writer or reader. Choose a
high enough ratio of readers to writers to make it likely
that readers block (and eventually are susceptable to
stalling).
If we are a writer, take the write lock, and then unlock.
If we are a reader, try the lock, then lock, then unlock. */
if ((i % 8) != 0)
{
if ((ret = pthread_rwlock_trywrlock (&onelock)) != 0)
{
if (ret == EBUSY)
xpthread_rwlock_wrlock (&onelock);
else
exit (EXIT_FAILURE);
}
}
else
xpthread_rwlock_rdlock (&onelock);
/* Thread does some work and then unlocks. */
xpthread_rwlock_unlock (&onelock);
i++;
}
return NULL;
}
int
do_test (void)
{
int i;
pthread_t tids[NTHREADS];
xpthread_rwlock_init (&onelock, NULL);
for (i = 0; i < NTHREADS; i++)
tids[i] = xpthread_create (NULL, run_loop, NULL);
/* Run for some amount of time. The pthread_rwlock_tryrwlock stall
is very easy to trigger and happens in seconds under the test
conditions. */
sleep (10);
/* Then exit. */
printf ("INFO: Exiting...\n");
do_exit = 1;
/* If any readers stalled then we will timeout waiting for them. */
for (i = 0; i < NTHREADS; i++)
xpthread_join (tids[i]);
printf ("INFO: Done.\n");
xpthread_rwlock_destroy (&onelock);
printf ("PASS: No pthread_rwlock_tryrwlock stalls detected.\n");
return 0;
}
#include <support/test-driver.c>
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