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/* elision-lock.c: Elided pthread mutex lock.
Copyright (C) 2011-2021 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>
#ifndef EXTRAARG
#define EXTRAARG
#endif
#ifndef LLL_LOCK
#define LLL_LOCK(a,b) lll_lock(a,b), 0
#endif
#define aconf __elision_aconf
/* Adaptive lock using transactions.
By default the lock region is run as a transaction, and when it
aborts or the lock is busy the lock adapts itself. */
int
__lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
{
/* 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)
{
unsigned status;
int try_xbegin;
for (try_xbegin = aconf.retry_try_xbegin;
try_xbegin > 0;
try_xbegin--)
{
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))
{
if ((status & _XABORT_EXPLICIT)
&& _XABORT_CODE (status) == _ABORT_LOCK_BUSY)
{
/* Right now we skip here. Better would be to wait a bit
and retry. This likely needs some spinning. See
above for why relaxed MO is sufficient. */
if (atomic_load_relaxed (adapt_count)
!= aconf.skip_lock_busy)
atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);
}
/* Internal abort. There is no chance for retry.
Use the normal locking and next time use lock.
Be careful to avoid writing to the lock. See above for why
relaxed MO is sufficient. */
else if (atomic_load_relaxed (adapt_count)
!= aconf.skip_lock_internal_abort)
atomic_store_relaxed (adapt_count,
aconf.skip_lock_internal_abort);
break;
}
}
}
else
{
/* Use a normal lock until the threshold counter runs out.
Lost updates possible. */
atomic_store_relaxed (adapt_count,
atomic_load_relaxed (adapt_count) - 1);
}
/* Use a normal lock here. */
return LLL_LOCK ((*futex), private);
}
libc_hidden_def (__lll_lock_elision)
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