summaryrefslogtreecommitdiff
path: root/REORG.TODO/nptl/pthread_rwlock_tryrdlock.c
diff options
context:
space:
mode:
Diffstat (limited to 'REORG.TODO/nptl/pthread_rwlock_tryrdlock.c')
-rw-r--r--REORG.TODO/nptl/pthread_rwlock_tryrdlock.c112
1 files changed, 112 insertions, 0 deletions
diff --git a/REORG.TODO/nptl/pthread_rwlock_tryrdlock.c b/REORG.TODO/nptl/pthread_rwlock_tryrdlock.c
new file mode 100644
index 0000000000..6c3014ce53
--- /dev/null
+++ b/REORG.TODO/nptl/pthread_rwlock_tryrdlock.c
@@ -0,0 +1,112 @@
+/* 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)