mirror of
https://sourceware.org/git/glibc.git
synced 2024-12-13 06:40:09 +00:00
688903eb3e
* All files with FSF copyright notices: Update copyright dates using scripts/update-copyrights. * locale/programs/charmap-kw.h: Regenerated. * locale/programs/locfile-kw.h: Likewise.
113 lines
4.6 KiB
C
113 lines
4.6 KiB
C
/* Copyright (C) 2002-2018 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>
|
|
#include <stdbool.h>
|
|
#include "pthread_rwlock_common.c"
|
|
|
|
|
|
/* See pthread_rwlock_common.c for an overview. */
|
|
int
|
|
__pthread_rwlock_tryrdlock (pthread_rwlock_t *rwlock)
|
|
{
|
|
/* For tryrdlock, we could speculate that we will succeed and go ahead and
|
|
register as a reader. However, if we misspeculate, we have to do the
|
|
same steps as a timed-out rdlock, which will increase contention.
|
|
Therefore, there is a trade-off between being able to use a combinable
|
|
read-modify-write operation and a CAS loop as used below; we pick the
|
|
latter because it simplifies the code, and should perform better when
|
|
tryrdlock is used in cases where writers are infrequent.
|
|
Because POSIX does not require a failed trylock to "synchronize memory",
|
|
relaxed MO is sufficient here and on the failure path of the CAS
|
|
below. */
|
|
unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
|
|
unsigned int rnew;
|
|
do
|
|
{
|
|
if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
|
|
{
|
|
/* If we are in a read phase, try to acquire unless there is a
|
|
primary writer and we prefer writers and there will be no
|
|
recursive read locks. */
|
|
if (((r & PTHREAD_RWLOCK_WRLOCKED) != 0)
|
|
&& (rwlock->__data.__flags
|
|
== PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP))
|
|
return EBUSY;
|
|
rnew = r + (1 << PTHREAD_RWLOCK_READER_SHIFT);
|
|
}
|
|
else
|
|
{
|
|
/* If there is a writer that has acquired the lock and we are in
|
|
a write phase, fail. */
|
|
if ((r & PTHREAD_RWLOCK_WRLOCKED) != 0)
|
|
return EBUSY;
|
|
else
|
|
{
|
|
/* If we do not care about potentially waiting writers, just
|
|
try to acquire. */
|
|
rnew = (r + (1 << PTHREAD_RWLOCK_READER_SHIFT))
|
|
^ PTHREAD_RWLOCK_WRPHASE;
|
|
}
|
|
}
|
|
/* If we could have caused an overflow or take effect during an
|
|
overflow, we just can / need to return EAGAIN. There is no need to
|
|
have actually modified the number of readers because we could have
|
|
done that and cleaned up immediately. */
|
|
if (rnew >= PTHREAD_RWLOCK_READER_OVERFLOW)
|
|
return EAGAIN;
|
|
}
|
|
/* If the CAS fails, we retry; this prevents that tryrdlock fails spuriously
|
|
(i.e., fails to acquire the lock although there is no writer), which is
|
|
fine for C++14 but not currently allowed by POSIX.
|
|
However, because tryrdlock must not appear to block, we should avoid
|
|
starving this CAS loop due to constant changes to __readers:
|
|
While normal rdlock readers that won't be able to acquire will just block
|
|
(and we expect timeouts on timedrdlock to be longer than one retry of the
|
|
CAS loop), we can have concurrently failing tryrdlock calls due to
|
|
readers or writers that acquire and release in the meantime. Using
|
|
randomized exponential back-off to make a live-lock unlikely should be
|
|
sufficient.
|
|
TODO Back-off.
|
|
Acquire MO so we synchronize with prior writers. */
|
|
while (!atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers,
|
|
&r, rnew));
|
|
|
|
if ((r & PTHREAD_RWLOCK_WRPHASE) != 0)
|
|
{
|
|
/* Same as in __pthread_rwlock_rdlock_full:
|
|
We started the read phase, so we are also responsible for
|
|
updating the write-phase futex. Relaxed MO is sufficient.
|
|
Note that there can be no other reader that we have to wake
|
|
because all other readers will see the read phase started by us
|
|
(or they will try to start it themselves); if a writer started
|
|
the read phase, we cannot have started it. Furthermore, we
|
|
cannot discard a PTHREAD_RWLOCK_FUTEX_USED flag because we will
|
|
overwrite the value set by the most recent writer (or the readers
|
|
before it in case of explicit hand-over) and we know that there
|
|
are no waiting readers. */
|
|
atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 0);
|
|
}
|
|
|
|
return 0;
|
|
|
|
|
|
}
|
|
strong_alias (__pthread_rwlock_tryrdlock, pthread_rwlock_tryrdlock)
|