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

   1 /* Copyright (C) 2003-2012 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
  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       int tmp;
  41
  42       /* Pseudo code:
  43          newval = __fork_generation | 1;
  44          oldval = *once_control;
  45          if ((oldval & 2) == 0)
  46            *once_control = newval;
  47          Do this atomically.
  48       */
  49       newval = __fork_generation | 1;
  50       __asm __volatile (
  51                 "1:     ldl_l   %0, %2\n"
  52                 "       and     %0, 2, %1\n"
  53                 "       bne     %1, 2f\n"
  54                 "       mov     %3, %1\n"
  55                 "       stl_c   %1, %2\n"
  56                 "       beq     %1, 1b\n"
  57                 "2:     mb"
  58                 : "=&r" (oldval), "=&r" (tmp), "=m" (*once_control)
  59                 : "r" (newval), "m" (*once_control));
  60
  61       /* Check if the initializer has already been done.  */
  62       if ((oldval & 2) != 0)
  63         return 0;
  64
  65       /* Check if another thread already runs the initializer.  */
  66       if ((oldval & 1) == 0)
  67         break;
  68
  69       /* Check whether the initializer execution was interrupted by a fork.  */
  70       if (oldval != newval)
  71         break;
  72
  73       /* Same generation, some other thread was faster. Wait.  */
  74       lll_futex_wait (once_control, oldval, LLL_PRIVATE);
  75     }
  76
  77   /* This thread is the first here.  Do the initialization.
  78      Register a cleanup handler so that in case the thread gets
  79      interrupted the initialization can be restarted.  */
  80   pthread_cleanup_push (clear_once_control, once_control);
  81
  82   init_routine ();
  83
  84   pthread_cleanup_pop (0);
  85
  86   /* Add one to *once_control to take the bottom 2 bits from 01 to 10.  */
  87   atomic_increment (once_control);
  88
  89   /* Wake up all other threads.  */
  90   lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE);
  91
  92   return 0;
  93 }
  94 weak_alias (__pthread_once, pthread_once)
  95 hidden_def (__pthread_once)