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The elision interfaces are closely aligned between the targets that implement them, so declare them in the generic <lowlevellock.h> file. Empty .c stubs are provided, so that fewer makefile updates under sysdeps are needed. Also simplify initialization via __libc_early_init. The symbols __lll_clocklock_elision, __lll_lock_elision, __lll_trylock_elision, __lll_unlock_elision, __pthread_force_elision move into libc. For the time being, non-hidden references are used from libpthread to access them, but once that part of libpthread is moved into libc, hidden symbols will be used again. (Hidden references seem desirable to reduce the likelihood of transactions aborts.)
103 lines
3.3 KiB
C
103 lines
3.3 KiB
C
/* elision-lock.c: Elided pthread mutex lock.
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Copyright (C) 2011-2021 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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<https://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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#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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libc_hidden_def (__lll_lock_elision)
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