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63 lines
2.8 KiB
C
63 lines
2.8 KiB
C
/* force-elision.h: Automatic enabling of elision for mutexes
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Copyright (C) 2013-2020 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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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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<https://www.gnu.org/licenses/>. */
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/* Automatically enable elision for existing user lock kinds. */
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#define FORCE_ELISION(m, s) \
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if (__pthread_force_elision) \
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{ \
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/* See concurrency notes regarding __kind in \
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struct __pthread_mutex_s in \
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sysdeps/nptl/bits/thread-shared-types.h. \
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\
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There are the following cases for the kind of a mutex \
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(The mask PTHREAD_MUTEX_ELISION_FLAGS_NP covers the flags \
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PTHREAD_MUTEX_ELISION_NP and PTHREAD_MUTEX_NO_ELISION_NP where \
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only one of both flags can be set): \
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- both flags are not set: \
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This is the first lock operation for this mutex. Enable \
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elision as it is not enabled so far. \
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Note: It can happen that multiple threads are calling e.g. \
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pthread_mutex_lock at the same time as the first lock \
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operation for this mutex. Then elision is enabled for this \
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mutex by multiple threads. Storing with relaxed MO is enough \
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as all threads will store the same new value for the kind of \
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the mutex. But we have to ensure that we always use the \
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elision path regardless if this thread has enabled elision or \
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another one. \
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\
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- PTHREAD_MUTEX_ELISION_NP flag is set: \
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Elision was already enabled for this mutex by a previous lock \
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operation. See case above. Just use the elision path. \
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\
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- PTHREAD_MUTEX_NO_ELISION_NP flag is set: \
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Elision was explicitly disabled by pthread_mutexattr_settype. \
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Do not use the elision path. \
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Note: The flag PTHREAD_MUTEX_NO_ELISION_NP will never be \
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changed after mutex initialization. */ \
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int mutex_kind = atomic_load_relaxed (&((m)->__data.__kind)); \
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if ((mutex_kind & PTHREAD_MUTEX_ELISION_FLAGS_NP) == 0) \
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{ \
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mutex_kind |= PTHREAD_MUTEX_ELISION_NP; \
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atomic_store_relaxed (&((m)->__data.__kind), mutex_kind); \
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} \
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if ((mutex_kind & PTHREAD_MUTEX_ELISION_NP) != 0) \
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{ \
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s; \
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} \
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}
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