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/* Test program for a read-phase / write-phase explicit hand-over.
Copyright (C) 2017-2025 Free Software Foundation, Inc.
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; see the file COPYING.LIB. If
not, see <https://www.gnu.org/licenses/>. */
#include <errno.h>
#include <error.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdint.h>
#include <time.h>
#include <atomic.h>
#include <support/xthread.h>
/* We really want to set threads to 2 to reproduce this issue. The goal
is to have one primary writer and a single reader, and to hit the
bug that happens in the interleaving of those two phase transitions.
However, on most hardware, adding a second writer seems to help the
interleaving happen slightly more often, say 20% of the time. On a
16 core ppc64 machine this fails 100% of the time with an unpatched
glibc. On a 8 core x86_64 machine this fails ~93% of the time, but
it doesn't fail at all on a 4 core system, so having available
unloaded cores makes a big difference in reproducibility. On an 8
core qemu/kvm guest the reproducer reliability drops to ~10%. */
#define THREADS 3
#define KIND PTHREAD_RWLOCK_PREFER_READER_NP
static pthread_rwlock_t lock;
static int done = 0;
static void*
tf (void* arg)
{
while (atomic_load_relaxed (&done) == 0)
{
int rcnt = 0;
int wcnt = 100;
if ((uintptr_t) arg == 0)
{
rcnt = 1;
wcnt = 1;
}
do
{
if (wcnt)
{
xpthread_rwlock_wrlock (&lock);
xpthread_rwlock_unlock (&lock);
wcnt--;
}
if (rcnt)
{
xpthread_rwlock_rdlock (&lock);
xpthread_rwlock_unlock (&lock);
rcnt--;
}
}
while ((atomic_load_relaxed (&done) == 0) && (rcnt + wcnt > 0));
}
return NULL;
}
static int
do_test (void)
{
pthread_t thr[THREADS];
int n;
pthread_rwlockattr_t attr;
xpthread_rwlockattr_init (&attr);
xpthread_rwlockattr_setkind_np (&attr, KIND);
xpthread_rwlock_init (&lock, &attr);
/* Make standard error the same as standard output. */
dup2 (1, 2);
/* Make sure we see all message, even those on stdout. */
setvbuf (stdout, NULL, _IONBF, 0);
for (n = 0; n < THREADS; ++n)
thr[n] = xpthread_create (NULL, tf, (void *) (uintptr_t) n);
struct timespec delay;
delay.tv_sec = 10;
delay.tv_nsec = 0;
nanosleep (&delay, NULL);
atomic_store_relaxed (&done, 1);
/* Wait for all the threads. */
for (n = 0; n < THREADS; ++n)
xpthread_join (thr[n]);
return 0;
}
#include <support/test-driver.c>
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