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Diffstat (limited to 'nptl/pthread_rwlock_tryrdlock.c')
| -rw-r--r-- | nptl/pthread_rwlock_tryrdlock.c | 112 |
1 files changed, 0 insertions, 112 deletions
diff --git a/nptl/pthread_rwlock_tryrdlock.c b/nptl/pthread_rwlock_tryrdlock.c deleted file mode 100644 index 6c3014ce53..0000000000 --- a/nptl/pthread_rwlock_tryrdlock.c +++ /dev/null @@ -1,112 +0,0 @@ -/* Copyright (C) 2002-2017 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) |