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76 lines
2.5 KiB
C
76 lines
2.5 KiB
C
/* elision-trylock.c: Lock eliding trylock for pthreads.
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Copyright (C) 2013-2017 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
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the 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. See __lll_lock_elision for
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why we need atomic accesses. Relaxed MO is sufficient because this is
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just a hint. */
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if (atomic_load_relaxed (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. See above for MO. */
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if (atomic_load_relaxed (adapt_count)
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!= aconf.skip_lock_internal_abort)
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atomic_store_relaxed (adapt_count, aconf.skip_lock_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 (see above). */
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atomic_store_relaxed (adapt_count,
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atomic_load_relaxed (adapt_count) - 1);
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}
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return lll_trylock (*futex);
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}
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