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

   1 /* Copyright (C) 2003, 2004 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, write to the Free
  16    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  17    02111-1307 USA.  */
  18
  19 #include "pthreadP.h"
  20 #include <lowlevellock.h>
  21
  22
  23 unsigned long int __fork_generation attribute_hidden;
  24
  25 static void
  26 clear_once_control (void *arg)
  27 {
  28   pthread_once_t *once_control = (pthread_once_t *) arg;
  29
  30   *once_control = 0;
  31   lll_futex_wake (once_control, INT_MAX);
  32 }
  33
  34 int
  35 __pthread_once (pthread_once_t *once_control, void (*init_routine) (void))
  36 {
  37   for (;;)
  38     {
  39       int oldval;
  40       int newval;
  41       int tmp;
  42
  43       /* Pseudo code:
  44          newval = __fork_generation | 1;
  45          oldval = *once_control;
  46          if ((oldval & 2) == 0)
  47            *once_control = newval;
  48          Do this atomically.
  49       */
  50       newval = __fork_generation | 1;
  51       __asm __volatile (
  52                 "1:     ldl_l   %0, %2\n"
  53                 "       and     %0, 2, %1\n"
  54                 "       bne     %1, 2f\n"
  55                 "       mov     %3, %1\n"
  56                 "       stl_c   %1, %2\n"
  57                 "       beq     %1, 1b\n"
  58                 "2:     mb"
  59                 : "=&r" (oldval), "=&r" (tmp), "=m" (*once_control)
  60                 : "r" (newval), "m" (*once_control));
  61
  62       /* Check if the initializer has already been done.  */
  63       if ((oldval & 2) != 0)
  64         return 0;
  65
  66       /* Check if another thread already runs the initializer.  */
  67       if ((oldval & 1) == 0)
  68         break;
  69
  70       /* Check whether the initializer execution was interrupted by a fork.  */
  71       if (oldval != newval)
  72         break;
  73
  74       /* Same generation, some other thread was faster. Wait.  */
  75       lll_futex_wait (once_control, oldval);
  76     }
  77
  78   /* This thread is the first here.  Do the initialization.
  79      Register a cleanup handler so that in case the thread gets
  80      interrupted the initialization can be restarted.  */
  81   pthread_cleanup_push (clear_once_control, once_control);
  82
  83   init_routine ();
  84
  85   pthread_cleanup_pop (0);
  86
  87   /* Add one to *once_control to take the bottom 2 bits from 01 to 10.  */
  88   atomic_increment (once_control);
  89
  90   /* Wake up all other threads.  */
  91   lll_futex_wake (once_control, INT_MAX);
  92
  93   return 0;
  94 }
  95 weak_alias (__pthread_once, pthread_once)
  96 strong_alias (__pthread_once, __pthread_once_internal)