nptl/unregister-atfork.c

   1 /* Copyright (C) 2002-2015 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
   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 <errno.h>
  20 #include <stdlib.h>
  21 #include <fork.h>
  22 #include <atomic.h>
  23 #include <futex-internal.h>
  24
  25
  26 void
  27 __unregister_atfork (dso_handle)
  28      void *dso_handle;
  29 {
  30   /* Check whether there is any entry in the list which we have to
  31      remove.  It is likely that this is not the case so don't bother
  32      getting the lock.
  33
  34      We do not worry about other threads adding entries for this DSO
  35      right this moment.  If this happens this is a race and we can do
  36      whatever we please.  The program will crash anyway seen.  */
  37   struct fork_handler *runp = __fork_handlers;
  38   struct fork_handler *lastp = NULL;
  39
  40   while (runp != NULL)
  41     if (runp->dso_handle == dso_handle)
  42       break;
  43     else
  44       {
  45         lastp = runp;
  46         runp = runp->next;
  47       }
  48
  49   if (runp == NULL)
  50     /* Nothing to do.  */
  51     return;
  52
  53   /* Get the lock to not conflict with additions or deletions.  Note
  54      that there couldn't have been another thread deleting something.
  55      The __unregister_atfork function is only called from the
  56      dlclose() code which itself serializes the operations.  */
  57   lll_lock (__fork_lock, LLL_PRIVATE);
  58
  59   /* We have to create a new list with all the entries we don't remove.  */
  60   struct deleted_handler
  61   {
  62     struct fork_handler *handler;
  63     struct deleted_handler *next;
  64   } *deleted = NULL;
  65
  66   /* Remove the entries for the DSO which is unloaded from the list.
  67      It's a single linked list so readers are.  */
  68   do
  69     {
  70     again:
  71       if (runp->dso_handle == dso_handle)
  72         {
  73           if (lastp == NULL)
  74             {
  75               /* We have to use an atomic operation here because
  76                  __linkin_atfork also uses one.  */
  77               if (catomic_compare_and_exchange_bool_acq (&__fork_handlers,
  78                                                          runp->next, runp)
  79                   != 0)
  80                 {
  81                   runp = __fork_handlers;
  82                   goto again;
  83                 }
  84             }
  85           else
  86             lastp->next = runp->next;
  87
  88           /* We cannot overwrite the ->next element now.  Put the deleted
  89              entries in a separate list.  */
  90           struct deleted_handler *newp = alloca (sizeof (*newp));
  91           newp->handler = runp;
  92           newp->next = deleted;
  93           deleted = newp;
  94         }
  95       else
  96         lastp = runp;
  97
  98       runp = runp->next;
  99     }
 100   while (runp != NULL);
 101
 102   /* Release the lock.  */
 103   lll_unlock (__fork_lock, LLL_PRIVATE);
 104
 105   /* Walk the list of all entries which have to be deleted.  */
 106   while (deleted != NULL)
 107     {
 108       /* We need to be informed by possible current users.  */
 109       deleted->handler->need_signal = 1;
 110       /* Make sure this gets written out first.  */
 111       atomic_write_barrier ();
 112
 113       /* Decrement the reference counter.  If it does not reach zero
 114          wait for the last user.  */
 115       atomic_decrement (&deleted->handler->refcntr);
 116       unsigned int val;
 117       while ((val = deleted->handler->refcntr) != 0)
 118         futex_wait_simple (&deleted->handler->refcntr, val, FUTEX_PRIVATE);
 119
 120       deleted = deleted->next;
 121     }
 122 }