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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.)
121 lines
4.6 KiB
C
121 lines
4.6 KiB
C
/* Elided pthread mutex lock.
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Copyright (C) 2014-2021 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 <htm.h>
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#include <elision-conf.h>
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#include <stdint.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 && aconf.try_tbegin > 0)
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{
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/* Start a transaction and retry it automatically if it aborts with
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_HTM_TBEGIN_TRANSIENT. This macro calls tbegin at most retry_cnt
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+ 1 times. The second argument is considered as retry_cnt. */
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int status = __libc_tbegin_retry ((void *) 0, aconf.try_tbegin - 1);
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if (__glibc_likely (status == _HTM_TBEGIN_STARTED))
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{
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/* Check the futex to make sure nobody has touched it in the
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mean time. This forces the futex into the cache and makes
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sure the transaction aborts if another thread acquires the lock
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concurrently. */
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if (__glibc_likely (atomic_load_relaxed (futex) == 0))
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/* Lock was free. Return to user code in a transaction. */
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return 0;
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/* Lock was busy. Fall back to normal locking.
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This can be the case if e.g. adapt_count was decremented to zero
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by a former release and another thread has been waken up and
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acquired it. */
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if (__glibc_likely (__libc_tx_nesting_depth () <= 1))
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{
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/* In a non-nested transaction there is no need to abort,
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which is expensive. Simply end the started transaction. */
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__libc_tend ();
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/* Don't try to use transactions for the next couple of times.
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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 /* nesting depth is > 1 */
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{
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/* A nested transaction will abort eventually because it
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cannot make any progress before *futex changes back to 0.
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So we may as well abort immediately.
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This persistently aborts the outer transaction to force
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the outer mutex use the default lock instead of retrying
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with transactions until the try_tbegin of the outer mutex
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is zero.
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The adapt_count of this inner mutex is not changed,
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because using the default lock with the inner mutex
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would abort the outer transaction. */
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__libc_tabort (_HTM_FIRST_USER_ABORT_CODE | 1);
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__builtin_unreachable ();
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}
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}
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else 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. See above for why
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relaxed MO is sufficient. */
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if (aconf.skip_lock_internal_abort > 0)
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atomic_store_relaxed (adapt_count,
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aconf.skip_lock_internal_abort);
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}
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else
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{
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/* The transaction failed for some retries with
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_HTM_TBEGIN_TRANSIENT. Use the normal locking now and for the
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next couple of calls. */
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if (aconf.skip_lock_out_of_tbegin_retries > 0)
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atomic_store_relaxed (adapt_count,
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aconf.skip_lock_out_of_tbegin_retries);
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}
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}
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/* Use normal locking as fallback path if the transaction does not
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succeed. */
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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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