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