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/* Elided pthread mutex lock.
Copyright (C) 2014-2023 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>
#include <stdint.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 && aconf.try_tbegin > 0)
{
/* Start a transaction and retry it automatically if it aborts with
_HTM_TBEGIN_TRANSIENT. This macro calls tbegin at most retry_cnt
+ 1 times. The second argument is considered as retry_cnt. */
int status = __libc_tbegin_retry ((void *) 0, aconf.try_tbegin - 1);
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. */
if (__glibc_likely (__libc_tx_nesting_depth () <= 1))
{
/* In a non-nested transaction there is no need to abort,
which is expensive. Simply end the started transaction. */
__libc_tend ();
/* Don't try to use transactions for the next couple of times.
See above for why relaxed MO is sufficient. */
if (aconf.skip_lock_busy > 0)
atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);
}
else /* nesting depth is > 1 */
{
/* A nested transaction will abort eventually because it
cannot make any progress before *futex changes back to 0.
So we may as well abort immediately.
This persistently aborts the outer transaction to force
the outer mutex use the default lock instead of retrying
with transactions until the try_tbegin of the outer mutex
is zero.
The adapt_count of this inner mutex is not changed,
because using the default lock with the inner mutex
would abort the outer transaction. */
__libc_tabort (_HTM_FIRST_USER_ABORT_CODE | 1);
__builtin_unreachable ();
}
}
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. See above for why
relaxed MO is sufficient. */
if (aconf.skip_lock_internal_abort > 0)
atomic_store_relaxed (adapt_count,
aconf.skip_lock_internal_abort);
}
else
{
/* The transaction failed for some retries with
_HTM_TBEGIN_TRANSIENT. Use the normal locking now and for the
next couple of calls. */
if (aconf.skip_lock_out_of_tbegin_retries > 0)
atomic_store_relaxed (adapt_count,
aconf.skip_lock_out_of_tbegin_retries);
}
}
/* Use normal locking as fallback path if the transaction does not
succeed. */
return LLL_LOCK ((*futex), private);
}
libc_hidden_def (__lll_lock_elision)
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