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c813dae5d8
This uses atomic operations to access lock elision metadata that is accessed
concurrently (ie, adapt_count fields). The size of the data is less than a
word but accessed only with atomic loads and stores.
See also x86 commit ca6e601a9d:
"Use C11-like atomics instead of plain memory accesses in x86 lock elision."
ChangeLog:
* sysdeps/unix/sysv/linux/s390/elision-lock.c
(__lll_lock_elision): Use atomics to load / store adapt_count.
* sysdeps/unix/sysv/linux/s390/elision-trylock.c
(__lll_trylock_elision): Likewise.
99 lines
3.4 KiB
C
99 lines
3.4 KiB
C
/* Elided pthread mutex trylock.
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Copyright (C) 2014-2016 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#include <pthread.h>
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#include <pthreadP.h>
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#include <lowlevellock.h>
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#include <htmintrin.h>
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#include <elision-conf.h>
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#define aconf __elision_aconf
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/* Try to elide a futex trylock. FUTEX is the futex variable. ADAPT_COUNT is
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the adaptation counter in the mutex. */
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int
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__lll_trylock_elision (int *futex, short *adapt_count)
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{
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__asm__ __volatile__ (".machinemode \"zarch_nohighgprs\"\n\t"
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".machine \"all\""
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: : : "memory");
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/* Implement POSIX semantics by forbiding nesting elided trylocks.
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Sorry. After the abort the code is re-executed
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non transactional and if the lock was already locked
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return an error. */
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if (__builtin_tx_nesting_depth () > 0)
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{
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/* Note that this abort may terminate an outermost transaction that
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was created outside glibc.
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This persistently aborts the current transactions to force
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them to use the default lock instead of retrying transactions
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until their try_tbegin is zero.
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*/
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__builtin_tabort (_HTM_FIRST_USER_ABORT_CODE | 1);
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}
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/* Only try a transaction if it's worth it. See __lll_lock_elision for
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why we need atomic accesses. Relaxed MO is sufficient because this is
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just a hint. */
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if (atomic_load_relaxed (adapt_count) <= 0)
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{
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unsigned status;
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if (__builtin_expect
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((status = __builtin_tbegin ((void *)0)) == _HTM_TBEGIN_STARTED, 1))
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{
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if (*futex == 0)
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return 0;
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/* Lock was busy. Fall back to normal locking. */
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/* Since we are in a non-nested transaction there is no need to abort,
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which is expensive. */
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__builtin_tend ();
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/* Note: Changing the adapt_count here might abort a transaction on a
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different cpu, but that could happen anyway when the futex is
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acquired, so there's no need to check the nesting depth here.
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See above for why relaxed MO is sufficient. */
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if (aconf.skip_lock_busy > 0)
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atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);
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}
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else
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{
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if (status != _HTM_TBEGIN_TRANSIENT)
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{
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/* A persistent abort (cc 1 or 3) indicates that a retry is
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probably futile. Use the normal locking now and for the
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next couple of calls.
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Be careful to avoid writing to the lock. */
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if (aconf.skip_trylock_internal_abort > 0)
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*adapt_count = aconf.skip_trylock_internal_abort;
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}
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}
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/* Could do some retries here. */
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}
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else
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{
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/* Lost updates are possible, but harmless. Due to races this might lead
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to *adapt_count becoming less than zero. */
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atomic_store_relaxed (adapt_count,
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atomic_load_relaxed (adapt_count) - 1);
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}
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return lll_trylock (*futex);
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}
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