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/* Elided pthread mutex trylock.
Copyright (C) 2014-2024 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 <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);
}
libc_hidden_def (__lll_trylock_elision)
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