glibc/nptl/sysdeps/unix/sysv/linux/x86/elision-trylock.c
Andi Kleen e8c659d74e Add elision to pthread_mutex_{try,timed,un}lock
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.
2013-07-02 08:46:55 -07:00

72 lines
2.2 KiB
C

/* 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. 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. */
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);
}