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 in progress
  44    (__fork_generation | __PTHREAD_ONCE_INPROGRESS), and initialization
  45    finished (__PTHREAD_ONCE_DONE); __fork_generation does not use the bits
  46    that are used for __PTHREAD_ONCE_INPROGRESS and __PTHREAD_ONCE_DONE (which
  47    is what __PTHREAD_ONCE_FORK_GEN_INCR is used for).  If in the first state,
  48    threads will try to run the initialization by moving to the second state;
  49    the first thread to do so via a CAS on once_control runs init_routine,
  50    other threads block.
  51    When forking the process, some threads can be interrupted during the second
  52    state; they won't be present in the forked child, so we need to restart
  53    initialization in the child.  To distinguish an in-progress initialization
  54    from an interrupted initialization (in which case we need to reclaim the
  55    lock), we look at the fork generation that's part of the second state: We
  56    can reclaim iff it differs from the current fork generation.
  57    XXX: This algorithm has an ABA issue on the fork generation: If an
  58    initialization is interrupted, we then fork 2^30 times (30 bits of
  59    once_control are used for the fork generation), and try to initialize
  60    again, we can deadlock because we can't distinguish the in-progress and
  61    interrupted cases anymore.
  62    XXX: We split out this slow path because current compilers do not generate
  63    as efficient code when the fast path in __pthread_once below is not in a
  64    separate function.  */
  65 static int
  66 __attribute__ ((noinline))
  67 __pthread_once_slow (pthread_once_t *once_control, void (*init_routine) (void))
  68 {
  69   while (1)
  70     {
  71       int oldval, val, newval;
  72
  73       /* We need acquire memory order for this load because if the value
  74          signals that initialization has finished, we need to see any
  75          data modifications done during initialization.  */
  76       val = *once_control;
  77       atomic_read_barrier ();
  78       do
  79         {
  80           /* Check if the initialization has already been done.  */
  81           if (__glibc_likely ((val & __PTHREAD_ONCE_DONE) != 0))
  82             return 0;
  83
  84           oldval = val;
  85           /* We try to set the state to in-progress and having the current
  86              fork generation.  We don't need atomic accesses for the fork
  87              generation because it's immutable in a particular process, and
  88              forked child processes start with a single thread that modified
  89              the generation.  */
  90           newval = __fork_generation | __PTHREAD_ONCE_INPROGRESS;
  91           /* We need acquire memory order here for the same reason as for the
  92              load from once_control above.  */
  93           val = atomic_compare_and_exchange_val_acq (once_control, newval,
  94                                                      oldval);
  95         }
  96       while (__glibc_unlikely (val != oldval));
  97
  98       /* Check if another thread already runs the initializer.  */
  99       if ((oldval & __PTHREAD_ONCE_INPROGRESS) != 0)
 100         {
 101           /* Check whether the initializer execution was interrupted by a
 102              fork.  We know that for both values, __PTHREAD_ONCE_INPROGRESS
 103              is set and __PTHREAD_ONCE_DONE is not.  */
 104           if (oldval == newval)
 105             {
 106               /* Same generation, some other thread was faster. Wait.  */
 107               lll_futex_wait (once_control, newval, LLL_PRIVATE);
 108               continue;
 109             }
 110         }
 111
 112       /* This thread is the first here.  Do the initialization.
 113          Register a cleanup handler so that in case the thread gets
 114          interrupted the initialization can be restarted.  */
 115       pthread_cleanup_push (clear_once_control, once_control);
 116
 117       init_routine ();
 118
 119       pthread_cleanup_pop (0);
 120
 121
 122       /* Mark *once_control as having finished the initialization.  We need
 123          release memory order here because we need to synchronize with other
 124          threads that want to use the initialized data.  */
 125       atomic_write_barrier ();
 126       *once_control = __PTHREAD_ONCE_DONE;
 127
 128       /* Wake up all other threads.  */
 129       lll_futex_wake (once_control, INT_MAX, LLL_PRIVATE);
 130       break;
 131     }
 132
 133   return 0;
 134 }
 135
 136 int
 137 __pthread_once (pthread_once_t *once_control, void (*init_routine) (void))
 138 {
 139   /* Fast path.  See __pthread_once_slow.  */
 140   int val;
 141   val = *once_control;
 142   atomic_read_barrier ();
 143   if (__glibc_likely ((val & __PTHREAD_ONCE_DONE) != 0))
 144     return 0;
 145   else
 146     return __pthread_once_slow (once_control, init_routine);
 147 }
 148 weak_alias (__pthread_once, pthread_once)
 149 hidden_def (__pthread_once)