mirror of
https://sourceware.org/git/glibc.git
synced 2024-12-04 19:00:09 +00:00
126 lines
4.7 KiB
C
126 lines
4.7 KiB
C
/* Elided pthread mutex lock.
|
|
Copyright (C) 2014-2020 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>
|
|
|
|
#if !defined(LLL_LOCK) && !defined(EXTRAARG)
|
|
/* Make sure the configuration code is always linked in for static
|
|
libraries. */
|
|
#include "elision-conf.c"
|
|
#endif
|
|
|
|
#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);
|
|
}
|