/* Concurrent allocation and initialization of a pointer.

   Copyright (C) 2018 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

   <http://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)