mirror of
https://sourceware.org/git/glibc.git
synced 2024-12-13 06:40:09 +00:00
688903eb3e
* All files with FSF copyright notices: Update copyright dates using scripts/update-copyrights. * locale/programs/charmap-kw.h: Regenerated. * locale/programs/locfile-kw.h: Likewise.
98 lines
3.9 KiB
C
98 lines
3.9 KiB
C
/* Elided pthread mutex trylock.
|
|
Copyright (C) 2014-2018 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 <htm.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 elided trylocks.
|
|
Sorry. After the abort the code is re-executed
|
|
non transactional and if the lock was already locked
|
|
return an error. */
|
|
if (__libc_tx_nesting_depth () > 0)
|
|
{
|
|
/* Note that this abort may terminate an outermost transaction that
|
|
was created outside glibc.
|
|
This persistently aborts the current transactions to force
|
|
them to use the default lock instead of retrying transactions
|
|
until their try_tbegin is zero.
|
|
*/
|
|
__libc_tabort (_HTM_FIRST_USER_ABORT_CODE | 1);
|
|
__builtin_unreachable ();
|
|
}
|
|
|
|
/* adapt_count can be accessed concurrently; these accesses can be both
|
|
inside of transactions (if critical sections are nested and the outer
|
|
critical section uses lock elision) and outside of transactions. Thus,
|
|
we need to use atomic accesses to avoid data races. However, the
|
|
value of adapt_count is just a hint, so relaxed MO accesses are
|
|
sufficient. */
|
|
if (atomic_load_relaxed (adapt_count) <= 0 && aconf.try_tbegin > 0)
|
|
{
|
|
int status = __libc_tbegin ((void *) 0);
|
|
if (__glibc_likely (status == _HTM_TBEGIN_STARTED))
|
|
{
|
|
/* Check the futex to make sure nobody has touched it in the
|
|
mean time. This forces the futex into the cache and makes
|
|
sure the transaction aborts if another thread acquires the lock
|
|
concurrently. */
|
|
if (__glibc_likely (atomic_load_relaxed (futex) == 0))
|
|
/* Lock was free. Return to user code in a transaction. */
|
|
return 0;
|
|
|
|
/* Lock was busy. Fall back to normal locking.
|
|
This can be the case if e.g. adapt_count was decremented to zero
|
|
by a former release and another thread has been waken up and
|
|
acquired it.
|
|
Since we are in a non-nested transaction there is no need to abort,
|
|
which is expensive. Simply end the started transaction. */
|
|
__libc_tend ();
|
|
/* Note: Changing the adapt_count here might abort a transaction on a
|
|
different CPU, but that could happen anyway when the futex is
|
|
acquired, so there's no need to check the nesting depth here.
|
|
See above for why relaxed MO is sufficient. */
|
|
if (aconf.skip_lock_busy > 0)
|
|
atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);
|
|
}
|
|
else if (status != _HTM_TBEGIN_TRANSIENT)
|
|
{
|
|
/* A persistent abort (cc 1 or 3) indicates that a retry is
|
|
probably futile. Use the normal locking now and for the
|
|
next couple of calls.
|
|
Be careful to avoid writing to the lock. */
|
|
if (aconf.skip_trylock_internal_abort > 0)
|
|
*adapt_count = aconf.skip_trylock_internal_abort;
|
|
}
|
|
/* Could do some retries here. */
|
|
}
|
|
|
|
/* Use normal locking as fallback path if the transaction does not
|
|
succeed. */
|
|
return lll_trylock (*futex);
|
|
}
|