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/* force-elision.h: Automatic enabling of elision for mutexes
Copyright (C) 2013-2019 Free Software Foundation, Inc.
This file is part of the GNU C Library.
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
<https://www.gnu.org/licenses/>. */
/* Automatically enable elision for existing user lock kinds. */
#define FORCE_ELISION(m, s) \
if (__pthread_force_elision) \
{ \
/* See concurrency notes regarding __kind in \
struct __pthread_mutex_s in \
sysdeps/nptl/bits/thread-shared-types.h. \
\
There are the following cases for the kind of a mutex \
(The mask PTHREAD_MUTEX_ELISION_FLAGS_NP covers the flags \
PTHREAD_MUTEX_ELISION_NP and PTHREAD_MUTEX_NO_ELISION_NP where \
only one of both flags can be set): \
- both flags are not set: \
This is the first lock operation for this mutex. Enable \
elision as it is not enabled so far. \
Note: It can happen that multiple threads are calling e.g. \
pthread_mutex_lock at the same time as the first lock \
operation for this mutex. Then elision is enabled for this \
mutex by multiple threads. Storing with relaxed MO is enough \
as all threads will store the same new value for the kind of \
the mutex. But we have to ensure that we always use the \
elision path regardless if this thread has enabled elision or \
another one. \
\
- PTHREAD_MUTEX_ELISION_NP flag is set: \
Elision was already enabled for this mutex by a previous lock \
operation. See case above. Just use the elision path. \
\
- PTHREAD_MUTEX_NO_ELISION_NP flag is set: \
Elision was explicitly disabled by pthread_mutexattr_settype. \
Do not use the elision path. \
Note: The flag PTHREAD_MUTEX_NO_ELISION_NP will never be \
changed after mutex initialization. */ \
int mutex_kind = atomic_load_relaxed (&((m)->__data.__kind)); \
if ((mutex_kind & PTHREAD_MUTEX_ELISION_FLAGS_NP) == 0) \
{ \
mutex_kind |= PTHREAD_MUTEX_ELISION_NP; \
atomic_store_relaxed (&((m)->__data.__kind), mutex_kind); \
} \
if ((mutex_kind & PTHREAD_MUTEX_ELISION_NP) != 0) \
{ \
s; \
} \
}
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