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