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glibc/nptl/sysdeps/unix/sysv/linux/x86/elision-trylock.c
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/* elision-trylock.c: Lock eliding trylock for pthreads. | |
Copyright (C) 2013 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 | |
<http://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. TRY_LOCK is the | |
adaptation counter in the mutex. UPGRADED is != 0 when this is for an | |
automatically upgraded lock. */ | |
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. */ | |
if (*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. */ | |
if (*adapt_count != aconf.skip_trylock_internal_abort) | |
*adapt_count = aconf.skip_trylock_internal_abort; | |
} | |
/* Could do some retries here. */ | |
} | |
else | |
{ | |
/* Lost updates are possible, but harmless. */ | |
(*adapt_count)--; | |
} | |
return lll_trylock (*futex); | |
} |