1  /* Concurrent allocation and initialization of a pointer.
       2     Copyright (C) 2018-2023 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)