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/* Concurrent allocation and initialization of a pointer.
Copyright (C) 2018-2020 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 <allocate_once.h>
#include <stdlib.h>
#include <stdbool.h>
void *
__libc_allocate_once_slow (void **place, void *(*allocate) (void *closure),
void (*deallocate) (void *closure, void *ptr),
void *closure)
{
void *result = allocate (closure);
if (result == NULL)
return NULL;
/* This loop implements a strong CAS on *place, with acquire-release
MO semantics, from a weak CAS with relaxed-release MO. */
while (true)
{
/* Synchronizes with the acquire MO load in allocate_once. */
void *expected = NULL;
if (atomic_compare_exchange_weak_release (place, &expected, result))
return result;
/* The failed CAS has relaxed MO semantics, so perform another
acquire MO load. */
void *other_result = atomic_load_acquire (place);
if (other_result == NULL)
/* Spurious failure. Try again. */
continue;
/* We lost the race. Free what we allocated and return the
other result. */
if (deallocate == NULL)
free (result);
else
deallocate (closure, result);
return other_result;
}
return result;
}
libc_hidden_def (__libc_allocate_once_slow)
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