nptl/sysdeps/unix/sysv/linux/fork.c

   1 /* Copyright (C) 2002, 2003, 2007 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, write to the Free
  17    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  18    02111-1307 USA.  */
  19
  20 #include <assert.h>
  21 #include <stdlib.h>
  22 #include <unistd.h>
  23 #include <sys/types.h>
  24 #include <sysdep.h>
  25 #include <libio/libioP.h>
  26 #include <tls.h>
  27 #include "fork.h"
  28 #include <hp-timing.h>
  29 #include <ldsodefs.h>
  30 #include <bits/stdio-lock.h>
  31 #include <atomic.h>
  32
  33
  34 unsigned long int *__fork_generation_pointer;
  35
  36
  37
  38 /* The single linked list of all currently registered for handlers.  */
  39 struct fork_handler *__fork_handlers;
  40
  41
  42 static void
  43 fresetlockfiles (void)
  44 {
  45   _IO_ITER i;
  46
  47   for (i = _IO_iter_begin(); i != _IO_iter_end(); i = _IO_iter_next(i))
  48     _IO_lock_init (*((_IO_lock_t *) _IO_iter_file(i)->_lock));
  49 }
  50
  51
  52 pid_t
  53 __libc_fork (void)
  54 {
  55   pid_t pid;
  56   struct used_handler
  57   {
  58     struct fork_handler *handler;
  59     struct used_handler *next;
  60   } *allp = NULL;
  61
  62   /* Run all the registered preparation handlers.  In reverse order.
  63      While doing this we build up a list of all the entries.  */
  64   struct fork_handler *runp;
  65   while ((runp = __fork_handlers) != NULL)
  66     {
  67       unsigned int oldval = runp->refcntr;
  68
  69       if (oldval == 0)
  70         /* This means some other thread removed the list just after
  71            the pointer has been loaded.  Try again.  Either the list
  72            is empty or we can retry it.  */
  73         continue;
  74
  75       /* Bump the reference counter.  */
  76       if (atomic_compare_and_exchange_bool_acq (&__fork_handlers->refcntr,
  77                                                 oldval + 1, oldval))
  78         /* The value changed, try again.  */
  79         continue;
  80
  81       /* We bumped the reference counter for the first entry in the
  82          list.  That means that none of the following entries will
  83          just go away.  The unloading code works in the order of the
  84          list.
  85
  86          While executing the registered handlers we are building a
  87          list of all the entries so that we can go backward later on.  */
  88       while (1)
  89         {
  90           /* Execute the handler if there is one.  */
  91           if (runp->prepare_handler != NULL)
  92             runp->prepare_handler ();
  93
  94           /* Create a new element for the list.  */
  95           struct used_handler *newp
  96             = (struct used_handler *) alloca (sizeof (*newp));
  97           newp->handler = runp;
  98           newp->next = allp;
  99           allp = newp;
 100
 101           /* Advance to the next handler.  */
 102           runp = runp->next;
 103           if (runp == NULL)
 104             break;
 105
 106           /* Bump the reference counter for the next entry.  */
 107           atomic_increment (&runp->refcntr);
 108         }
 109
 110       /* We are done.  */
 111       break;
 112     }
 113
 114   _IO_list_lock ();
 115
 116 #ifndef NDEBUG
 117   pid_t ppid = THREAD_GETMEM (THREAD_SELF, tid);
 118 #endif
 119
 120   /* We need to prevent the getpid() code to update the PID field so
 121      that, if a signal arrives in the child very early and the signal
 122      handler uses getpid(), the value returned is correct.  */
 123   pid_t parentpid = THREAD_GETMEM (THREAD_SELF, pid);
 124   THREAD_SETMEM (THREAD_SELF, pid, -parentpid);
 125
 126 #ifdef ARCH_FORK
 127   pid = ARCH_FORK ();
 128 #else
 129 # error "ARCH_FORK must be defined so that the CLONE_SETTID flag is used"
 130   pid = INLINE_SYSCALL (fork, 0);
 131 #endif
 132
 133
 134   if (pid == 0)
 135     {
 136       struct pthread *self = THREAD_SELF;
 137
 138       assert (THREAD_GETMEM (self, tid) != ppid);
 139
 140       if (__fork_generation_pointer != NULL)
 141         *__fork_generation_pointer += 4;
 142
 143       /* Adjust the PID field for the new process.  */
 144       THREAD_SETMEM (self, pid, THREAD_GETMEM (self, tid));
 145
 146 #if HP_TIMING_AVAIL
 147       /* The CPU clock of the thread and process have to be set to zero.  */
 148       hp_timing_t now;
 149       HP_TIMING_NOW (now);
 150       THREAD_SETMEM (self, cpuclock_offset, now);
 151       GL(dl_cpuclock_offset) = now;
 152 #endif
 153
 154       /* Reset the file list.  These are recursive mutexes.  */
 155       fresetlockfiles ();
 156
 157       /* Reset locks in the I/O code.  */
 158       _IO_list_resetlock ();
 159
 160       /* Reset the lock the dynamic loader uses to protect its data.  */
 161       __rtld_lock_initialize (GL(dl_load_lock));
 162
 163       /* Run the handlers registered for the child.  */
 164       while (allp != NULL)
 165         {
 166           if (allp->handler->child_handler != NULL)
 167             allp->handler->child_handler ();
 168
 169           /* Note that we do not have to wake any possible waiter.
 170              This is the only thread in the new process.  The count
 171              may have been bumped up by other threads doing a fork.
 172              We reset it to 1, to avoid waiting for non-existing
 173              thread(s) to release the count.  */
 174           allp->handler->refcntr = 1;
 175
 176           /* XXX We could at this point look through the object pool
 177              and mark all objects not on the __fork_handlers list as
 178              unused.  This is necessary in case the fork() happened
 179              while another thread called dlclose() and that call had
 180              to create a new list.  */
 181
 182           allp = allp->next;
 183         }
 184
 185       /* Initialize the fork lock.  */
 186       __fork_lock = LLL_LOCK_INITIALIZER;
 187     }
 188   else
 189     {
 190       assert (THREAD_GETMEM (THREAD_SELF, tid) == ppid);
 191
 192       /* Restore the PID value.  */
 193       THREAD_SETMEM (THREAD_SELF, pid, parentpid);
 194
 195       /* We execute this even if the 'fork' call failed.  */
 196       _IO_list_unlock ();
 197
 198       /* Run the handlers registered for the parent.  */
 199       while (allp != NULL)
 200         {
 201           if (allp->handler->parent_handler != NULL)
 202             allp->handler->parent_handler ();
 203
 204           if (atomic_decrement_and_test (&allp->handler->refcntr)
 205               && allp->handler->need_signal)
 206             lll_futex_wake (allp->handler->refcntr, 1, LLL_PRIVATE);
 207
 208           allp = allp->next;
 209         }
 210     }
 211
 212   return pid;
 213 }
 214 weak_alias (__libc_fork, __fork)
 215 libc_hidden_def (__fork)
 216 weak_alias (__libc_fork, fork)