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/* Copyright (C) 2002-2018 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#include <errno.h>
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#include "pthreadP.h"
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#include <atomic.h>
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#include <stdbool.h>
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#include "pthread_rwlock_common.c"
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/* See pthread_rwlock_common.c for an overview. */
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int
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__pthread_rwlock_tryrdlock (pthread_rwlock_t *rwlock)
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{
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/* For tryrdlock, we could speculate that we will succeed and go ahead and
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register as a reader. However, if we misspeculate, we have to do the
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same steps as a timed-out rdlock, which will increase contention.
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Therefore, there is a trade-off between being able to use a combinable
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read-modify-write operation and a CAS loop as used below; we pick the
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latter because it simplifies the code, and should perform better when
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tryrdlock is used in cases where writers are infrequent.
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Because POSIX does not require a failed trylock to "synchronize memory",
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relaxed MO is sufficient here and on the failure path of the CAS
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below. */
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unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
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unsigned int rnew;
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do
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{
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if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
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{
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/* If we are in a read phase, try to acquire unless there is a
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primary writer and we prefer writers and there will be no
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recursive read locks. */
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if (((r & PTHREAD_RWLOCK_WRLOCKED) != 0)
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&& (rwlock->__data.__flags
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== PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP))
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return EBUSY;
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rnew = r + (1 << PTHREAD_RWLOCK_READER_SHIFT);
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}
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else
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{
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/* If there is a writer that has acquired the lock and we are in
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a write phase, fail. */
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if ((r & PTHREAD_RWLOCK_WRLOCKED) != 0)
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return EBUSY;
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else
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{
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/* If we do not care about potentially waiting writers, just
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try to acquire. */
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rnew = (r + (1 << PTHREAD_RWLOCK_READER_SHIFT))
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^ PTHREAD_RWLOCK_WRPHASE;
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}
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}
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/* If we could have caused an overflow or take effect during an
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overflow, we just can / need to return EAGAIN. There is no need to
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have actually modified the number of readers because we could have
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done that and cleaned up immediately. */
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if (rnew >= PTHREAD_RWLOCK_READER_OVERFLOW)
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return EAGAIN;
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}
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/* If the CAS fails, we retry; this prevents that tryrdlock fails spuriously
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(i.e., fails to acquire the lock although there is no writer), which is
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fine for C++14 but not currently allowed by POSIX.
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However, because tryrdlock must not appear to block, we should avoid
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starving this CAS loop due to constant changes to __readers:
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While normal rdlock readers that won't be able to acquire will just block
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(and we expect timeouts on timedrdlock to be longer than one retry of the
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CAS loop), we can have concurrently failing tryrdlock calls due to
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readers or writers that acquire and release in the meantime. Using
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randomized exponential back-off to make a live-lock unlikely should be
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sufficient.
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TODO Back-off.
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Acquire MO so we synchronize with prior writers. */
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while (!atomic_compare_exchange_weak_acquire (&rwlock->__data.__readers,
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&r, rnew));
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if ((r & PTHREAD_RWLOCK_WRPHASE) != 0)
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{
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/* Same as in __pthread_rwlock_rdlock_full:
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We started the read phase, so we are also responsible for
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updating the write-phase futex. Relaxed MO is sufficient.
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Note that there can be no other reader that we have to wake
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because all other readers will see the read phase started by us
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(or they will try to start it themselves); if a writer started
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the read phase, we cannot have started it. Furthermore, we
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cannot discard a PTHREAD_RWLOCK_FUTEX_USED flag because we will
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overwrite the value set by the most recent writer (or the readers
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before it in case of explicit hand-over) and we know that there
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are no waiting readers. */
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atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 0);
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}
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return 0;
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}
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strong_alias (__pthread_rwlock_tryrdlock, pthread_rwlock_tryrdlock)
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