|
Packit |
6c4009 |
/* Copyright (C) 2002-2018 Free Software Foundation, Inc.
|
|
Packit |
6c4009 |
This file is part of the GNU C Library.
|
|
Packit |
6c4009 |
Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
|
|
Packit |
6c4009 |
|
|
Packit |
6c4009 |
The GNU C Library is free software; you can redistribute it and/or
|
|
Packit |
6c4009 |
modify it under the terms of the GNU Lesser General Public
|
|
Packit |
6c4009 |
License as published by the Free Software Foundation; either
|
|
Packit |
6c4009 |
version 2.1 of the License, or (at your option) any later version.
|
|
Packit |
6c4009 |
|
|
Packit |
6c4009 |
The GNU C Library is distributed in the hope that it will be useful,
|
|
Packit |
6c4009 |
but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
Packit |
6c4009 |
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
|
|
Packit |
6c4009 |
Lesser General Public License for more details.
|
|
Packit |
6c4009 |
|
|
Packit |
6c4009 |
You should have received a copy of the GNU Lesser General Public
|
|
Packit |
6c4009 |
License along with the GNU C Library; if not, see
|
|
Packit |
6c4009 |
<http://www.gnu.org/licenses/>. */
|
|
Packit |
6c4009 |
|
|
Packit |
6c4009 |
#include <errno.h>
|
|
Packit |
6c4009 |
#include "pthreadP.h"
|
|
Packit |
6c4009 |
#include <atomic.h>
|
|
Packit |
6c4009 |
|
|
Packit |
6c4009 |
/* See pthread_rwlock_common.c for an overview. */
|
|
Packit |
6c4009 |
int
|
|
Packit |
6c4009 |
__pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock)
|
|
Packit |
6c4009 |
{
|
|
Packit |
6c4009 |
/* When in a trywrlock, we can acquire the write lock if it is in states
|
|
Packit |
6c4009 |
#1 (idle and read phase) and #5 (idle and write phase), and also in #6
|
|
Packit |
6c4009 |
(readers waiting, write phase) if we prefer writers.
|
|
Packit |
6c4009 |
If we observe any other state, we are allowed to fail and do not need to
|
|
Packit |
6c4009 |
"synchronize memory" as specified by POSIX (hence relaxed MO is
|
|
Packit |
6c4009 |
sufficient for the first load and the CAS failure path).
|
|
Packit |
6c4009 |
We face a similar issue as in tryrdlock in that we need to both avoid
|
|
Packit |
6c4009 |
live-locks / starvation and must not fail spuriously (see there for
|
|
Packit |
6c4009 |
further comments) -- and thus must loop until we get a definitive
|
|
Packit |
6c4009 |
observation or state change. */
|
|
Packit |
6c4009 |
unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
|
|
Packit |
6c4009 |
bool prefer_writer =
|
|
Packit |
6c4009 |
(rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP);
|
|
Packit |
6c4009 |
while (((r & PTHREAD_RWLOCK_WRLOCKED) == 0)
|
|
Packit |
6c4009 |
&& (((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0)
|
|
Packit |
6c4009 |
|| (prefer_writer && ((r & PTHREAD_RWLOCK_WRPHASE) != 0))))
|
|
Packit |
6c4009 |
{
|
|
Packit |
6c4009 |
/* Try to transition to states #7 or #8 (i.e., acquire the lock). */
|
|
Packit |
6c4009 |
if (atomic_compare_exchange_weak_acquire (
|
|
Packit |
6c4009 |
&rwlock->__data.__readers, &r,
|
|
Packit |
6c4009 |
r | PTHREAD_RWLOCK_WRPHASE | PTHREAD_RWLOCK_WRLOCKED))
|
|
Packit |
6c4009 |
{
|
|
Packit Service |
2ead18 |
/* We have become the primary writer and we cannot have shared
|
|
Packit Service |
2ead18 |
the PTHREAD_RWLOCK_FUTEX_USED flag with someone else, so we
|
|
Packit Service |
2ead18 |
can simply enable blocking (see full wrlock code). */
|
|
Packit |
6c4009 |
atomic_store_relaxed (&rwlock->__data.__writers_futex, 1);
|
|
Packit Service |
2ead18 |
/* If we started a write phase, we need to enable readers to
|
|
Packit Service |
2ead18 |
wait. If we did not, we must not change it because other threads
|
|
Packit Service |
2ead18 |
may have set the PTHREAD_RWLOCK_FUTEX_USED in the meantime. */
|
|
Packit Service |
2ead18 |
if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
|
|
Packit Service |
2ead18 |
atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1);
|
|
Packit |
6c4009 |
atomic_store_relaxed (&rwlock->__data.__cur_writer,
|
|
Packit |
6c4009 |
THREAD_GETMEM (THREAD_SELF, tid));
|
|
Packit |
6c4009 |
return 0;
|
|
Packit |
6c4009 |
}
|
|
Packit |
6c4009 |
/* TODO Back-off. */
|
|
Packit |
6c4009 |
/* See above. */
|
|
Packit |
6c4009 |
}
|
|
Packit |
6c4009 |
return EBUSY;
|
|
Packit |
6c4009 |
}
|
|
Packit |
6c4009 |
|
|
Packit |
6c4009 |
strong_alias (__pthread_rwlock_trywrlock, pthread_rwlock_trywrlock)
|