misc/allocate_once.c

   1 /* Concurrent allocation and initialization of a pointer.
   2    Copyright (C) 2018-2022 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
  23 void *
  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 }
  59 libc_hidden_def (__libc_allocate_once_slow)