sysdeps/unix/sysv/linux/arm/nptl/pthread_once.c

   1 /* Copyright (C) 2004-2012 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library.  If not, see
  16    <http://www.gnu.org/licenses/>.  */
  17
  18 #include "pthreadP.h"
  19 #include <lowlevellock.h>
  20
  21 unsigned long int __fork_generation attribute_hidden;
  22
  23 static void
  24 clear_once_control (void *arg)
  25 {
  26   pthread_once_t *once_control = (pthread_once_t *) arg;
  27
  28   *once_control = 0;
  29   lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE);
  30 }
  31
  32 int
  33 __pthread_once (pthread_once_t *once_control, void (*init_routine) (void))
  34 {
  35   for (;;)
  36     {
  37       int oldval;
  38       int newval;
  39
  40       /* Pseudo code:
  41          newval = __fork_generation | 1;
  42          oldval = *once_control;
  43          if ((oldval & 2) == 0)
  44            *once_control = newval;
  45          Do this atomically.
  46       */
  47       do
  48         {
  49           newval = __fork_generation | 1;
  50           oldval = *once_control;
  51           if (oldval & 2)
  52             break;
  53         } while (atomic_compare_and_exchange_val_acq (once_control, newval, oldval) != oldval);
  54
  55       /* Check if the initializer has already been done.  */
  56       if ((oldval & 2) != 0)
  57         return 0;
  58
  59       /* Check if another thread already runs the initializer.  */
  60       if ((oldval & 1) == 0)
  61         break;
  62
  63       /* Check whether the initializer execution was interrupted by a fork.  */
  64       if (oldval != newval)
  65         break;
  66
  67       /* Same generation, some other thread was faster. Wait.  */
  68       lll_futex_wait (once_control, oldval, LLL_PRIVATE);
  69     }
  70
  71   /* This thread is the first here.  Do the initialization.
  72      Register a cleanup handler so that in case the thread gets
  73      interrupted the initialization can be restarted.  */
  74   pthread_cleanup_push (clear_once_control, once_control);
  75
  76   init_routine ();
  77
  78   pthread_cleanup_pop (0);
  79
  80   /* Say that the initialisation is done.  */
  81   *once_control = __fork_generation | 2;
  82
  83   /* Wake up all other threads.  */
  84   lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE);
  85
  86   return 0;
  87 }
  88 weak_alias (__pthread_once, pthread_once)
  89 hidden_def (__pthread_once)