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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; therefore, we add support for shorter-size atomic load and stores too. * include/atomic.h (__atomic_check_size_ls): New. (atomic_load_relaxed, atomic_load_acquire, atomic_store_relaxed, atomic_store_release): Use it. * sysdeps/x86/elide.h (ACCESS_ONCE): Remove. (elision_adapt, ELIDE_LOCK): Use atomics. * sysdeps/unix/sysv/linux/x86/elision-lock.c (__lll_lock_elision): Use atomics and improve code comments. * sysdeps/unix/sysv/linux/x86/elision-trylock.c (__lll_trylock_elision): Likewise.
108 lines
3.4 KiB
C
108 lines
3.4 KiB
C
/* elision-lock.c: Elided pthread mutex lock.
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Copyright (C) 2011-2016 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 "hle.h"
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#include <elision-conf.h>
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#if !defined(LLL_LOCK) && !defined(EXTRAARG)
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/* Make sure the configuration code is always linked in for static
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libraries. */
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#include "elision-conf.c"
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#endif
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#ifndef EXTRAARG
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#define EXTRAARG
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#endif
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#ifndef LLL_LOCK
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#define LLL_LOCK(a,b) lll_lock(a,b), 0
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#endif
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#define aconf __elision_aconf
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/* Adaptive lock using transactions.
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By default the lock region is run as a transaction, and when it
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aborts or the lock is busy the lock adapts itself. */
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int
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__lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
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{
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/* adapt_count can be accessed concurrently; these accesses can be both
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inside of transactions (if critical sections are nested and the outer
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critical section uses lock elision) and outside of transactions. Thus,
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we need to use atomic accesses to avoid data races. However, the
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value of adapt_count is just a hint, so relaxed MO accesses are
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sufficient. */
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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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int try_xbegin;
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for (try_xbegin = aconf.retry_try_xbegin;
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try_xbegin > 0;
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try_xbegin--)
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{
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if ((status = _xbegin()) == _XBEGIN_STARTED)
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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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Could also _xend here but xabort with 0xff code
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is more visible in the profiler. */
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_xabort (_ABORT_LOCK_BUSY);
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}
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if (!(status & _XABORT_RETRY))
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{
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if ((status & _XABORT_EXPLICIT)
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&& _XABORT_CODE (status) == _ABORT_LOCK_BUSY)
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{
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/* Right now we skip here. Better would be to wait a bit
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and retry. This likely needs some spinning. See
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above for why relaxed MO is sufficient. */
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if (atomic_load_relaxed (adapt_count)
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!= aconf.skip_lock_busy)
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atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);
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}
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/* Internal abort. There is no chance for retry.
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Use the normal locking and next time use lock.
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Be careful to avoid writing to the lock. See above for why
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relaxed MO is sufficient. */
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else if (atomic_load_relaxed (adapt_count)
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!= aconf.skip_lock_internal_abort)
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atomic_store_relaxed (adapt_count,
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aconf.skip_lock_internal_abort);
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break;
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}
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}
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}
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else
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{
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/* Use a normal lock until the threshold counter runs out.
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Lost updates possible. */
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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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/* Use a normal lock here. */
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return LLL_LOCK ((*futex), private);
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}
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