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/* elision-trylock.c: Lock eliding trylock for pthreads.
Copyright (C) 2013-2020 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/>. */
#include <pthread.h>
#include <pthreadP.h>
#include <lowlevellock.h>
#include "hle.h"
#include <elision-conf.h>
#define aconf __elision_aconf
/* Try to elide a futex trylock. FUTEX is the futex variable. ADAPT_COUNT is
the adaptation counter in the mutex. */
int
__lll_trylock_elision (int *futex, short *adapt_count)
{
/* Implement POSIX semantics by forbiding nesting
trylock. Sorry. After the abort the code is re-executed
non transactional and if the lock was already locked
return an error. */
_xabort (_ABORT_NESTED_TRYLOCK);
/* Only try a transaction if it's worth it. See __lll_lock_elision for
why we need atomic accesses. Relaxed MO is sufficient because this is
just a hint. */
if (atomic_load_relaxed (adapt_count) <= 0)
{
unsigned status;
if ((status = _xbegin()) == _XBEGIN_STARTED)
{
if (*futex == 0)
return 0;
/* Lock was busy. Fall back to normal locking.
Could also _xend here but xabort with 0xff code
is more visible in the profiler. */
_xabort (_ABORT_LOCK_BUSY);
}
if (!(status & _XABORT_RETRY))
{
/* Internal abort. No chance for retry. For future
locks don't try speculation for some time. See above for MO. */
if (atomic_load_relaxed (adapt_count)
!= aconf.skip_lock_internal_abort)
atomic_store_relaxed (adapt_count, aconf.skip_lock_internal_abort);
}
/* Could do some retries here. */
}
else
{
/* Lost updates are possible but harmless (see above). */
atomic_store_relaxed (adapt_count,
atomic_load_relaxed (adapt_count) - 1);
}
return lll_trylock (*futex);
}
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