ports/sysdeps/unix/sysv/linux/aarch64/nptl/pthread_once.c

   1 /* Copyright (C) 2004-2014 Free Software Foundation, Inc.
   2
   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 License as
   7    published by the Free Software Foundation; either version 2.1 of the
   8    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    <http://www.gnu.org/licenses/>.  */
  18
  19 #include "pthreadP.h"
  20 #include <lowlevellock.h>
  21
  22 unsigned long int __fork_generation attribute_hidden;
  23
  24 static void
  25 clear_once_control (void *arg)
  26 {
  27   pthread_once_t *once_control = (pthread_once_t *) arg;
  28
  29   *once_control = 0;
  30   lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE);
  31 }
  32
  33 int
  34 __pthread_once (pthread_once_t *once_control, void (*init_routine) (void))
  35 {
  36   for (;;)
  37     {
  38       int oldval;
  39       int newval;
  40
  41       /* Pseudo code:
  42          newval = __fork_generation | 1;
  43          oldval = *once_control;
  44          if ((oldval & 2) == 0)
  45            *once_control = newval;
  46          Do this atomically.
  47       */
  48       do
  49         {
  50           newval = __fork_generation | 1;
  51           oldval = *once_control;
  52           if (oldval & 2)
  53             break;
  54         } while (atomic_compare_and_exchange_val_acq (once_control, newval, oldval) != oldval);
  55
  56       /* Check if the initializer has already been done.  */
  57       if ((oldval & 2) != 0)
  58         return 0;
  59
  60       /* Check if another thread already runs the initializer.  */
  61       if ((oldval & 1) == 0)
  62         break;
  63
  64       /* Check whether the initializer execution was interrupted by a fork.  */
  65       if (oldval != newval)
  66         break;
  67
  68       /* Same generation, some other thread was faster. Wait.  */
  69       lll_futex_wait (once_control, oldval, LLL_PRIVATE);
  70     }
  71
  72   /* This thread is the first here.  Do the initialization.
  73      Register a cleanup handler so that in case the thread gets
  74      interrupted the initialization can be restarted.  */
  75   pthread_cleanup_push (clear_once_control, once_control);
  76
  77   init_routine ();
  78
  79   pthread_cleanup_pop (0);
  80
  81   /* Say that the initialisation is done.  */
  82   *once_control = __fork_generation | 2;
  83
  84   /* Wake up all other threads.  */
  85   lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE);
  86
  87   return 0;
  88 }
  89 weak_alias (__pthread_once, pthread_once)
  90 hidden_def (__pthread_once)