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/* Copyright (C) 2005-2021 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library. If not, see
<https://www.gnu.org/licenses/>. */
#include "pthreadP.h"
int
pthread_spin_unlock (pthread_spinlock_t *lock)
{
/* CONCURRENCTY NOTES:
The atomic_exchange_rel synchronizes-with the atomic_exhange_acq in
pthread_spin_lock.
On hppa we must not use a plain `stw` to reset the guard lock. This
has to do with the kernel compare-and-swap helper that is used to
implement all of the atomic operations.
The kernel CAS helper uses its own internal locks and that means that
to create a true happens-before relationship between any two threads,
the second thread must observe the internal lock having a value of 0
(it must attempt to take the lock with ldcw). This creates the
ordering required for a second thread to observe the effects of the
RMW of the kernel CAS helper in any other thread.
Therefore if a variable is used in an atomic macro it must always be
manipulated with atomic macros in order for memory ordering rules to
be preserved. */
atomic_exchange_rel (lock, 0);
return 0;
}
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