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e8c659d74e
Add elision paths to the basic mutex locks. The normal path has a check for RTM and upgrades the lock to RTM when available. Trylocks cannot automatically upgrade, so they check for elision every time. We use a 4 byte value in the mutex to store the lock elision adaptation state. This is separate from the adaptive spin state and uses a separate field. Condition variables currently do not support elision. Recursive mutexes and condition variables may be supported at some point, but are not in the current implementation. Also "trylock" will not automatically enable elision unless some other lock call has been already called on the lock. This version does not use IFUNC, so it means every lock has one additional check for elision. Benchmarking showed the overhead to be negligible.
72 lines
2.2 KiB
C
72 lines
2.2 KiB
C
/* elision-trylock.c: Lock eliding trylock for pthreads.
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Copyright (C) 2013 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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<http://www.gnu.org/licenses/>. */
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#include <pthread.h>
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#include <pthreadP.h>
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#include <lowlevellock.h>
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#include "hle.h"
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#include <elision-conf.h>
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#define aconf __elision_aconf
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/* Try to elide a futex trylock. FUTEX is the futex variable. ADAPT_COUNT is the
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adaptation counter in the mutex. */
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int
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__lll_trylock_elision (int *futex, short *adapt_count)
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{
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/* Implement POSIX semantics by forbiding nesting
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trylock. Sorry. After the abort the code is re-executed
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non transactional and if the lock was already locked
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return an error. */
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_xabort (_ABORT_NESTED_TRYLOCK);
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/* Only try a transaction if it's worth it. */
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if (*adapt_count <= 0)
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{
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unsigned status;
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if ((status = _xbegin()) == _XBEGIN_STARTED)
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{
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if (*futex == 0)
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return 0;
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/* Lock was busy. Fall back to normal locking.
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Could also _xend here but xabort with 0xff code
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is more visible in the profiler. */
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_xabort (_ABORT_LOCK_BUSY);
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}
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if (!(status & _XABORT_RETRY))
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{
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/* Internal abort. No chance for retry. For future
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locks don't try speculation for some time. */
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if (*adapt_count != aconf.skip_trylock_internal_abort)
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*adapt_count = aconf.skip_trylock_internal_abort;
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}
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/* Could do some retries here. */
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}
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else
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{
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/* Lost updates are possible, but harmless. */
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(*adapt_count)--;
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}
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return lll_trylock (*futex);
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}
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