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The elision interfaces are closely aligned between the targets that implement them, so declare them in the generic <lowlevellock.h> file. Empty .c stubs are provided, so that fewer makefile updates under sysdeps are needed. Also simplify initialization via __libc_early_init. The symbols __lll_clocklock_elision, __lll_lock_elision, __lll_trylock_elision, __lll_unlock_elision, __pthread_force_elision move into libc. For the time being, non-hidden references are used from libpthread to access them, but once that part of libpthread is moved into libc, hidden symbols will be used again. (Hidden references seem desirable to reduce the likelihood of transactions aborts.)
77 lines
2.5 KiB
C
77 lines
2.5 KiB
C
/* elision-trylock.c: Lock eliding trylock for pthreads.
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Copyright (C) 2013-2021 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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#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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libc_hidden_def (__lll_trylock_elision)
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