nptl/sysdeps/unix/sysv/linux/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 "fork.h"
  27 #include <hp-timing.h>
  28 #include <ldsodefs.h>
  29 #include <bits/stdio-lock.h>
  30 #include <atomic.h>
  31 #include <pthreadP.h>
  32
  33
  34 unsigned long int *__fork_generation_pointer;
  35
  36
  37
  38 /* The single linked list of all currently registered fork 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       /* Make sure we read from the current RUNP pointer.  */
  68       atomic_full_barrier ();
  69
  70       unsigned int oldval = runp->refcntr;
  71
  72       if (oldval == 0)
  73         /* This means some other thread removed the list just after
  74            the pointer has been loaded.  Try again.  Either the list
  75            is empty or we can retry it.  */
  76         continue;
  77
  78       /* Bump the reference counter.  */
  79       if (atomic_compare_and_exchange_bool_acq (&__fork_handlers->refcntr,
  80                                                 oldval + 1, oldval))
  81         /* The value changed, try again.  */
  82         continue;
  83
  84       /* We bumped the reference counter for the first entry in the
  85          list.  That means that none of the following entries will
  86          just go away.  The unloading code works in the order of the
  87          list.
  88
  89          While executing the registered handlers we are building a
  90          list of all the entries so that we can go backward later on.  */
  91       while (1)
  92         {
  93           /* Execute the handler if there is one.  */
  94           if (runp->prepare_handler != NULL)
  95             runp->prepare_handler ();
  96
  97           /* Create a new element for the list.  */
  98           struct used_handler *newp
  99             = (struct used_handler *) alloca (sizeof (*newp));
 100           newp->handler = runp;
 101           newp->next = allp;
 102           allp = newp;
 103
 104           /* Advance to the next handler.  */
 105           runp = runp->next;
 106           if (runp == NULL)
 107             break;
 108
 109           /* Bump the reference counter for the next entry.  */
 110           atomic_increment (&runp->refcntr);
 111         }
 112
 113       /* We are done.  */
 114       break;
 115     }
 116
 117   _IO_list_lock ();
 118
 119 #ifndef NDEBUG
 120   pid_t ppid = THREAD_GETMEM (THREAD_SELF, tid);
 121 #endif
 122
 123   /* We need to prevent the getpid() code to update the PID field so
 124      that, if a signal arrives in the child very early and the signal
 125      handler uses getpid(), the value returned is correct.  */
 126   pid_t parentpid = THREAD_GETMEM (THREAD_SELF, pid);
 127   THREAD_SETMEM (THREAD_SELF, pid, -parentpid);
 128
 129 #ifdef ARCH_FORK
 130   pid = ARCH_FORK ();
 131 #else
 132 # error "ARCH_FORK must be defined so that the CLONE_SETTID flag is used"
 133   pid = INLINE_SYSCALL (fork, 0);
 134 #endif
 135
 136
 137   if (pid == 0)
 138     {
 139       struct pthread *self = THREAD_SELF;
 140
 141       assert (THREAD_GETMEM (self, tid) != ppid);
 142
 143       if (__fork_generation_pointer != NULL)
 144         *__fork_generation_pointer += 4;
 145
 146       /* Adjust the PID field for the new process.  */
 147       THREAD_SETMEM (self, pid, THREAD_GETMEM (self, tid));
 148
 149 #if HP_TIMING_AVAIL
 150       /* The CPU clock of the thread and process have to be set to zero.  */
 151       hp_timing_t now;
 152       HP_TIMING_NOW (now);
 153       THREAD_SETMEM (self, cpuclock_offset, now);
 154       GL(dl_cpuclock_offset) = now;
 155 #endif
 156
 157 #ifdef __NR_set_robust_list
 158       /* Initialize the robust mutex list which has been reset during
 159          the fork.  We do not check for errors since if it fails here
 160          it failed at process start as well and noone could have used
 161          robust mutexes.  We also do not have to set
 162          self->robust_head.futex_offset since we inherit the correct
 163          value from the parent.  */
 164 # ifdef SHARED
 165       if (__builtin_expect (__libc_pthread_functions_init, 0))
 166         PTHFCT_CALL (ptr_set_robust, (self));
 167 # else
 168       extern __typeof (__nptl_set_robust) __nptl_set_robust
 169         __attribute__((weak));
 170       if (__builtin_expect (__nptl_set_robust != NULL, 0))
 171         __nptl_set_robust (self);
 172 # endif
 173 #endif
 174
 175       /* Reset the file list.  These are recursive mutexes.  */
 176       fresetlockfiles ();
 177
 178       /* Reset locks in the I/O code.  */
 179       _IO_list_resetlock ();
 180
 181       /* Reset the lock the dynamic loader uses to protect its data.  */
 182       __rtld_lock_initialize (GL(dl_load_lock));
 183
 184       /* Run the handlers registered for the child.  */
 185       while (allp != NULL)
 186         {
 187           if (allp->handler->child_handler != NULL)
 188             allp->handler->child_handler ();
 189
 190           /* Note that we do not have to wake any possible waiter.
 191              This is the only thread in the new process.  The count
 192              may have been bumped up by other threads doing a fork.
 193              We reset it to 1, to avoid waiting for non-existing
 194              thread(s) to release the count.  */
 195           allp->handler->refcntr = 1;
 196
 197           /* XXX We could at this point look through the object pool
 198              and mark all objects not on the __fork_handlers list as
 199              unused.  This is necessary in case the fork() happened
 200              while another thread called dlclose() and that call had
 201              to create a new list.  */
 202
 203           allp = allp->next;
 204         }
 205
 206       /* Initialize the fork lock.  */
 207       __fork_lock = LLL_LOCK_INITIALIZER;
 208     }
 209   else
 210     {
 211       assert (THREAD_GETMEM (THREAD_SELF, tid) == ppid);
 212
 213       /* Restore the PID value.  */
 214       THREAD_SETMEM (THREAD_SELF, pid, parentpid);
 215
 216       /* We execute this even if the 'fork' call failed.  */
 217       _IO_list_unlock ();
 218
 219       /* Run the handlers registered for the parent.  */
 220       while (allp != NULL)
 221         {
 222           if (allp->handler->parent_handler != NULL)
 223             allp->handler->parent_handler ();
 224
 225           if (atomic_decrement_and_test (&allp->handler->refcntr)
 226               && allp->handler->need_signal)
 227             lll_futex_wake (allp->handler->refcntr, 1, LLL_PRIVATE);
 228
 229           allp = allp->next;
 230         }
 231     }
 232
 233   return pid;
 234 }
 235 weak_alias (__libc_fork, __fork)
 236 libc_hidden_def (__fork)
 237 weak_alias (__libc_fork, fork)