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99 lines
3.9 KiB
C
99 lines
3.9 KiB
C
/* Elided pthread mutex trylock.
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Copyright (C) 2014-2024 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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#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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/* 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 (__libc_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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__libc_tabort (_HTM_FIRST_USER_ABORT_CODE | 1);
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__builtin_unreachable ();
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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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int status = __libc_tbegin ((void *) 0);
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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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Since we are 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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/* 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 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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/* Could do some retries here. */
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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_trylock (*futex);
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}
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libc_hidden_def (__lll_trylock_elision)
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