mirror of
https://sourceware.org/git/glibc.git
synced 2024-11-09 23:00:07 +00:00
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)
69 lines
3.0 KiB
C
69 lines
3.0 KiB
C
/* Copyright (C) 2002-2019 Free Software Foundation, Inc.
|
|
This file is part of the GNU C Library.
|
|
Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
|
|
|
|
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
|
|
<http://www.gnu.org/licenses/>. */
|
|
|
|
#include <errno.h>
|
|
#include "pthreadP.h"
|
|
#include <atomic.h>
|
|
|
|
/* See pthread_rwlock_common.c for an overview. */
|
|
int
|
|
__pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock)
|
|
{
|
|
/* When in a trywrlock, we can acquire the write lock if it is in states
|
|
#1 (idle and read phase) and #5 (idle and write phase), and also in #6
|
|
(readers waiting, write phase) if we prefer writers.
|
|
If we observe any other state, we are allowed to fail and do not need to
|
|
"synchronize memory" as specified by POSIX (hence relaxed MO is
|
|
sufficient for the first load and the CAS failure path).
|
|
We face a similar issue as in tryrdlock in that we need to both avoid
|
|
live-locks / starvation and must not fail spuriously (see there for
|
|
further comments) -- and thus must loop until we get a definitive
|
|
observation or state change. */
|
|
unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
|
|
bool prefer_writer =
|
|
(rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP);
|
|
while (((r & PTHREAD_RWLOCK_WRLOCKED) == 0)
|
|
&& (((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0)
|
|
|| (prefer_writer && ((r & PTHREAD_RWLOCK_WRPHASE) != 0))))
|
|
{
|
|
/* Try to transition to states #7 or #8 (i.e., acquire the lock). */
|
|
if (atomic_compare_exchange_weak_acquire (
|
|
&rwlock->__data.__readers, &r,
|
|
r | PTHREAD_RWLOCK_WRPHASE | PTHREAD_RWLOCK_WRLOCKED))
|
|
{
|
|
/* We have become the primary writer and we cannot have shared
|
|
the PTHREAD_RWLOCK_FUTEX_USED flag with someone else, so we
|
|
can simply enable blocking (see full wrlock code). */
|
|
atomic_store_relaxed (&rwlock->__data.__writers_futex, 1);
|
|
/* If we started a write phase, we need to enable readers to
|
|
wait. If we did not, we must not change it because other threads
|
|
may have set the PTHREAD_RWLOCK_FUTEX_USED in the meantime. */
|
|
if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
|
|
atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1);
|
|
atomic_store_relaxed (&rwlock->__data.__cur_writer,
|
|
THREAD_GETMEM (THREAD_SELF, tid));
|
|
return 0;
|
|
}
|
|
/* TODO Back-off. */
|
|
/* See above. */
|
|
}
|
|
return EBUSY;
|
|
}
|
|
|
|
strong_alias (__pthread_rwlock_trywrlock, pthread_rwlock_trywrlock)
|