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70 lines
2.5 KiB
C
70 lines
2.5 KiB
C
/* pthread_spin_lock -- lock a spin lock. Generic version.
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Copyright (C) 2012 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 <atomic.h>
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#include "pthreadP.h"
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/* A machine-specific version can define SPIN_LOCK_READS_BETWEEN_CMPXCHG
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to the number of plain reads that it's optimal to spin on between uses
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of atomic_compare_and_exchange_val_acq. If spinning forever is optimal
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then use -1. If no plain reads here would ever be optimal, use 0. */
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#ifndef SPIN_LOCK_READS_BETWEEN_CMPXCHG
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# warning machine-dependent file should define SPIN_LOCK_READS_BETWEEN_CMPXCHG
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# define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
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#endif
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int
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pthread_spin_lock (pthread_spinlock_t *lock)
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{
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/* atomic_exchange usually takes less instructions than
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atomic_compare_and_exchange. On the other hand,
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atomic_compare_and_exchange potentially generates less bus traffic
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when the lock is locked.
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We assume that the first try mostly will be successful, and we use
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atomic_exchange. For the subsequent tries we use
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atomic_compare_and_exchange. */
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if (atomic_exchange_acq (lock, 1) == 0)
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return 0;
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do
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{
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/* The lock is contended and we need to wait. Going straight back
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to cmpxchg is not a good idea on many targets as that will force
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expensive memory synchronizations among processors and penalize other
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running threads.
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On the other hand, we do want to update memory state on the local core
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once in a while to avoid spinning indefinitely until some event that
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will happen to update local memory as a side-effect. */
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if (SPIN_LOCK_READS_BETWEEN_CMPXCHG >= 0)
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{
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int wait = SPIN_LOCK_READS_BETWEEN_CMPXCHG;
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while (*lock != 0 && wait > 0)
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--wait;
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}
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else
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{
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while (*lock != 0)
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;
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}
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}
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while (atomic_compare_and_exchange_val_acq (lock, 1, 0) != 0);
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return 0;
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}
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