nptl/pthread_once.c

   1 /* Copyright (C) 2003-2014 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contributed by Jakub Jelinek <jakub@redhat.com>, 2003.
   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
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the 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 #include <atomic.h>
  22
  23
  24 unsigned long int __fork_generation attribute_hidden;
  25
  26
  27 static void
  28 clear_once_control (void *arg)
  29 {
  30   pthread_once_t *once_control = (pthread_once_t *) arg;
  31
  32   /* Reset to the uninitialized state here.  We don't need a stronger memory
  33      order because we do not need to make any other of our writes visible to
  34      other threads that see this value: This function will be called if we
  35      get interrupted (see __pthread_once), so all we need to relay to other
  36      threads is the state being reset again.  */
  37   *once_control = 0;
  38   lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE);
  39 }
  40
  41
  42 /* This is similar to a lock implementation, but we distinguish between three
  43    states: not yet initialized (0), initialization finished (2), and
  44    initialization in progress (__fork_generation | 1).  If in the first state,
  45    threads will try to run the initialization by moving to the second state;
  46    the first thread to do so via a CAS on once_control runs init_routine,
  47    other threads block.
  48    When forking the process, some threads can be interrupted during the second
  49    state; they won't be present in the forked child, so we need to restart
  50    initialization in the child.  To distinguish an in-progress initialization
  51    from an interrupted initialization (in which case we need to reclaim the
  52    lock), we look at the fork generation that's part of the second state: We
  53    can reclaim iff it differs from the current fork generation.
  54    XXX: This algorithm has an ABA issue on the fork generation: If an
  55    initialization is interrupted, we then fork 2^30 times (30 bits of
  56    once_control are used for the fork generation), and try to initialize
  57    again, we can deadlock because we can't distinguish the in-progress and
  58    interrupted cases anymore.  */
  59 int
  60 __pthread_once (once_control, init_routine)
  61      pthread_once_t *once_control;
  62      void (*init_routine) (void);
  63 {
  64   while (1)
  65     {
  66       int oldval, val, newval;
  67
  68       /* We need acquire memory order for this load because if the value
  69          signals that initialization has finished, we need to be see any
  70          data modifications done during initialization.  */
  71       val = *once_control;
  72       atomic_read_barrier();
  73       do
  74         {
  75           /* Check if the initialization has already been done.  */
  76           if (__glibc_likely ((val & 2) != 0))
  77             return 0;
  78
  79           oldval = val;
  80           /* We try to set the state to in-progress and having the current
  81              fork generation.  We don't need atomic accesses for the fork
  82              generation because it's immutable in a particular process, and
  83              forked child processes start with a single thread that modified
  84              the generation.  */
  85           newval = __fork_generation | 1;
  86           /* We need acquire memory order here for the same reason as for the
  87              load from once_control above.  */
  88           val = atomic_compare_and_exchange_val_acq (once_control, newval,
  89                                                      oldval);
  90         }
  91       while (__glibc_unlikely (val != oldval));
  92
  93       /* Check if another thread already runs the initializer.  */
  94       if ((oldval & 1) != 0)
  95         {
  96           /* Check whether the initializer execution was interrupted by a
  97              fork.  We know that for both values, bit 0 is set and bit 1 is
  98              not.  */
  99           if (oldval == newval)
 100             {
 101               /* Same generation, some other thread was faster. Wait.  */
 102               lll_futex_wait (once_control, newval, LLL_PRIVATE);
 103               continue;
 104             }
 105         }
 106
 107       /* This thread is the first here.  Do the initialization.
 108          Register a cleanup handler so that in case the thread gets
 109          interrupted the initialization can be restarted.  */
 110       pthread_cleanup_push (clear_once_control, once_control);
 111
 112       init_routine ();
 113
 114       pthread_cleanup_pop (0);
 115
 116
 117       /* Mark *once_control as having finished the initialization.  We need
 118          release memory order here because we need to synchronize with other
 119          threads that want to use the initialized data.  */
 120       atomic_write_barrier();
 121       *once_control = 2;
 122
 123       /* Wake up all other threads.  */
 124       lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE);
 125       break;
 126     }
 127
 128   return 0;
 129 }
 130 weak_alias (__pthread_once, pthread_once)
 131 hidden_def (__pthread_once)