nptl/unregister-atfork.c

   1 /* Copyright (C) 2002-2017 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 (void *dso_handle)
  28 {
  29   /* Check whether there is any entry in the list which we have to
  30      remove.  It is likely that this is not the case so don't bother
  31      getting the lock.
  32
  33      We do not worry about other threads adding entries for this DSO
  34      right this moment.  If this happens this is a race and we can do
  35      whatever we please.  The program will crash anyway seen.  */
  36   struct fork_handler *runp = __fork_handlers;
  37   struct fork_handler *lastp = NULL;
  38
  39   while (runp != NULL)
  40     if (runp->dso_handle == dso_handle)
  41       break;
  42     else
  43       {
  44         lastp = runp;
  45         runp = runp->next;
  46       }
  47
  48   if (runp == NULL)
  49     /* Nothing to do.  */
  50     return;
  51
  52   /* Get the lock to not conflict with additions or deletions.  Note
  53      that there couldn't have been another thread deleting something.
  54      The __unregister_atfork function is only called from the
  55      dlclose() code which itself serializes the operations.  */
  56   lll_lock (__fork_lock, LLL_PRIVATE);
  57
  58   /* We have to create a new list with all the entries we don't remove.  */
  59   struct deleted_handler
  60   {
  61     struct fork_handler *handler;
  62     struct deleted_handler *next;
  63   } *deleted = NULL;
  64
  65   /* Remove the entries for the DSO which is unloaded from the list.
  66      It's a single linked list so readers are.  */
  67   do
  68     {
  69     again:
  70       if (runp->dso_handle == dso_handle)
  71         {
  72           if (lastp == NULL)
  73             {
  74               /* We have to use an atomic operation here because
  75                  __linkin_atfork also uses one.  */
  76               if (catomic_compare_and_exchange_bool_acq (&__fork_handlers,
  77                                                          runp->next, runp)
  78                   != 0)
  79                 {
  80                   runp = __fork_handlers;
  81                   goto again;
  82                 }
  83             }
  84           else
  85             lastp->next = runp->next;
  86
  87           /* We cannot overwrite the ->next element now.  Put the deleted
  88              entries in a separate list.  */
  89           struct deleted_handler *newp = alloca (sizeof (*newp));
  90           newp->handler = runp;
  91           newp->next = deleted;
  92           deleted = newp;
  93         }
  94       else
  95         lastp = runp;
  96
  97       runp = runp->next;
  98     }
  99   while (runp != NULL);
 100
 101   /* Release the lock.  */
 102   lll_unlock (__fork_lock, LLL_PRIVATE);
 103
 104   /* Walk the list of all entries which have to be deleted.  */
 105   while (deleted != NULL)
 106     {
 107       /* We need to be informed by possible current users.  */
 108       deleted->handler->need_signal = 1;
 109       /* Make sure this gets written out first.  */
 110       atomic_write_barrier ();
 111
 112       /* Decrement the reference counter.  If it does not reach zero
 113          wait for the last user.  */
 114       atomic_decrement (&deleted->handler->refcntr);
 115       unsigned int val;
 116       while ((val = deleted->handler->refcntr) != 0)
 117         futex_wait_simple (&deleted->handler->refcntr, val, FUTEX_PRIVATE);
 118
 119       deleted = deleted->next;
 120     }
 121 }