1/* Concurrent allocation and initialization of a pointer.
2 Copyright (C) 2018-2020 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, see
17 <https://www.gnu.org/licenses/>. */
18
19#include <allocate_once.h>
20#include <stdlib.h>
21#include <stdbool.h>
22
23void *
24__libc_allocate_once_slow (void **place, void *(*allocate) (void *closure),
25 void (*deallocate) (void *closure, void *ptr),
26 void *closure)
27{
28 void *result = allocate (closure);
29 if (result == NULL)
30 return NULL;
31
32 /* This loop implements a strong CAS on *place, with acquire-release
33 MO semantics, from a weak CAS with relaxed-release MO. */
34 while (true)
35 {
36 /* Synchronizes with the acquire MO load in allocate_once. */
37 void *expected = NULL;
38 if (atomic_compare_exchange_weak_release (place, &expected, result))
39 return result;
40
41 /* The failed CAS has relaxed MO semantics, so perform another
42 acquire MO load. */
43 void *other_result = atomic_load_acquire (place);
44 if (other_result == NULL)
45 /* Spurious failure. Try again. */
46 continue;
47
48 /* We lost the race. Free what we allocated and return the
49 other result. */
50 if (deallocate == NULL)
51 free (result);
52 else
53 deallocate (closure, result);
54 return other_result;
55 }
56
57 return result;
58}
59libc_hidden_def (__libc_allocate_once_slow)
60