sysdeps/nptl/fork.c

   1 /* Copyright (C) 2002-2014 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 <assert.h>
  20 #include <stdlib.h>
  21 #include <unistd.h>
  22 #include <sys/types.h>
  23 #include <sysdep.h>
  24 #include <libio/libioP.h>
  25 #include <tls.h>
  26 #include <hp-timing.h>
  27 #include <ldsodefs.h>
  28 #include <bits/stdio-lock.h>
  29 #include <atomic.h>
  30 #include <nptl/pthreadP.h>
  31 #include <fork.h>
  32 #include <arch-fork.h>
  33
  34
  35 unsigned long int *__fork_generation_pointer;
  36
  37
  38
  39 /* The single linked list of all currently registered fork handlers.  */
  40 struct fork_handler *__fork_handlers;
  41
  42
  43 static void
  44 fresetlockfiles (void)
  45 {
  46   _IO_ITER i;
  47
  48   for (i = _IO_iter_begin(); i != _IO_iter_end(); i = _IO_iter_next(i))
  49     _IO_lock_init (*((_IO_lock_t *) _IO_iter_file(i)->_lock));
  50 }
  51
  52
  53 pid_t
  54 __libc_fork (void)
  55 {
  56   pid_t pid;
  57   struct used_handler
  58   {
  59     struct fork_handler *handler;
  60     struct used_handler *next;
  61   } *allp = NULL;
  62
  63   /* Run all the registered preparation handlers.  In reverse order.
  64      While doing this we build up a list of all the entries.  */
  65   struct fork_handler *runp;
  66   while ((runp = __fork_handlers) != NULL)
  67     {
  68       /* Make sure we read from the current RUNP pointer.  */
  69       atomic_full_barrier ();
  70
  71       unsigned int oldval = runp->refcntr;
  72
  73       if (oldval == 0)
  74         /* This means some other thread removed the list just after
  75            the pointer has been loaded.  Try again.  Either the list
  76            is empty or we can retry it.  */
  77         continue;
  78
  79       /* Bump the reference counter.  */
  80       if (atomic_compare_and_exchange_bool_acq (&__fork_handlers->refcntr,
  81                                                 oldval + 1, oldval))
  82         /* The value changed, try again.  */
  83         continue;
  84
  85       /* We bumped the reference counter for the first entry in the
  86          list.  That means that none of the following entries will
  87          just go away.  The unloading code works in the order of the
  88          list.
  89
  90          While executing the registered handlers we are building a
  91          list of all the entries so that we can go backward later on.  */
  92       while (1)
  93         {
  94           /* Execute the handler if there is one.  */
  95           if (runp->prepare_handler != NULL)
  96             runp->prepare_handler ();
  97
  98           /* Create a new element for the list.  */
  99           struct used_handler *newp
 100             = (struct used_handler *) alloca (sizeof (*newp));
 101           newp->handler = runp;
 102           newp->next = allp;
 103           allp = newp;
 104
 105           /* Advance to the next handler.  */
 106           runp = runp->next;
 107           if (runp == NULL)
 108             break;
 109
 110           /* Bump the reference counter for the next entry.  */
 111           atomic_increment (&runp->refcntr);
 112         }
 113
 114       /* We are done.  */
 115       break;
 116     }
 117
 118   _IO_list_lock ();
 119
 120 #ifndef NDEBUG
 121   pid_t ppid = THREAD_GETMEM (THREAD_SELF, tid);
 122 #endif
 123
 124   /* We need to prevent the getpid() code to update the PID field so
 125      that, if a signal arrives in the child very early and the signal
 126      handler uses getpid(), the value returned is correct.  */
 127   pid_t parentpid = THREAD_GETMEM (THREAD_SELF, pid);
 128   THREAD_SETMEM (THREAD_SELF, pid, -parentpid);
 129
 130 #ifdef ARCH_FORK
 131   pid = ARCH_FORK ();
 132 #else
 133 # error "ARCH_FORK must be defined so that the CLONE_SETTID flag is used"
 134   pid = INLINE_SYSCALL (fork, 0);
 135 #endif
 136
 137
 138   if (pid == 0)
 139     {
 140       struct pthread *self = THREAD_SELF;
 141
 142       assert (THREAD_GETMEM (self, tid) != ppid);
 143
 144       /* See __pthread_once.  */
 145       if (__fork_generation_pointer != NULL)
 146         *__fork_generation_pointer += __PTHREAD_ONCE_FORK_GEN_INCR;
 147
 148       /* Adjust the PID field for the new process.  */
 149       THREAD_SETMEM (self, pid, THREAD_GETMEM (self, tid));
 150
 151 #if HP_TIMING_AVAIL
 152       /* The CPU clock of the thread and process have to be set to zero.  */
 153       hp_timing_t now;
 154       HP_TIMING_NOW (now);
 155       THREAD_SETMEM (self, cpuclock_offset, now);
 156       GL(dl_cpuclock_offset) = now;
 157 #endif
 158
 159 #ifdef __NR_set_robust_list
 160       /* Initialize the robust mutex list which has been reset during
 161          the fork.  We do not check for errors since if it fails here
 162          it failed at process start as well and noone could have used
 163          robust mutexes.  We also do not have to set
 164          self->robust_head.futex_offset since we inherit the correct
 165          value from the parent.  */
 166 # ifdef SHARED
 167       if (__builtin_expect (__libc_pthread_functions_init, 0))
 168         PTHFCT_CALL (ptr_set_robust, (self));
 169 # else
 170       extern __typeof (__nptl_set_robust) __nptl_set_robust
 171         __attribute__((weak));
 172       if (__builtin_expect (__nptl_set_robust != NULL, 0))
 173         __nptl_set_robust (self);
 174 # endif
 175 #endif
 176
 177       /* Reset the file list.  These are recursive mutexes.  */
 178       fresetlockfiles ();
 179
 180       /* Reset locks in the I/O code.  */
 181       _IO_list_resetlock ();
 182
 183       /* Reset the lock the dynamic loader uses to protect its data.  */
 184       __rtld_lock_initialize (GL(dl_load_lock));
 185
 186       /* Run the handlers registered for the child.  */
 187       while (allp != NULL)
 188         {
 189           if (allp->handler->child_handler != NULL)
 190             allp->handler->child_handler ();
 191
 192           /* Note that we do not have to wake any possible waiter.
 193              This is the only thread in the new process.  The count
 194              may have been bumped up by other threads doing a fork.
 195              We reset it to 1, to avoid waiting for non-existing
 196              thread(s) to release the count.  */
 197           allp->handler->refcntr = 1;
 198
 199           /* XXX We could at this point look through the object pool
 200              and mark all objects not on the __fork_handlers list as
 201              unused.  This is necessary in case the fork() happened
 202              while another thread called dlclose() and that call had
 203              to create a new list.  */
 204
 205           allp = allp->next;
 206         }
 207
 208       /* Initialize the fork lock.  */
 209       __fork_lock = LLL_LOCK_INITIALIZER;
 210     }
 211   else
 212     {
 213       assert (THREAD_GETMEM (THREAD_SELF, tid) == ppid);
 214
 215       /* Restore the PID value.  */
 216       THREAD_SETMEM (THREAD_SELF, pid, parentpid);
 217
 218       /* We execute this even if the 'fork' call failed.  */
 219       _IO_list_unlock ();
 220
 221       /* Run the handlers registered for the parent.  */
 222       while (allp != NULL)
 223         {
 224           if (allp->handler->parent_handler != NULL)
 225             allp->handler->parent_handler ();
 226
 227           if (atomic_decrement_and_test (&allp->handler->refcntr)
 228               && allp->handler->need_signal)
 229             lll_futex_wake (allp->handler->refcntr, 1, LLL_PRIVATE);
 230
 231           allp = allp->next;
 232         }
 233     }
 234
 235   return pid;
 236 }
 237 weak_alias (__libc_fork, __fork)
 238 libc_hidden_def (__fork)
 239 weak_alias (__libc_fork, fork)