sysdeps/nptl/fork.c

   1 /* Copyright (C) 2002-2017 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 #include <malloc/malloc-internal.h>
  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   /* Determine if we are running multiple threads.  We skip some fork
  58      handlers in the single-thread case, to make fork safer to use in
  59      signal handlers.  POSIX requires that fork is async-signal-safe,
  60      but our current fork implementation is not.  */
  61   bool multiple_threads = THREAD_GETMEM (THREAD_SELF, header.multiple_threads);
  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   /* If we are not running multiple threads, we do not have to
 119      preserve lock state.  If fork runs from a signal handler, only
 120      async-signal-safe functions can be used in the child.  These data
 121      structures are only used by unsafe functions, so their state does
 122      not matter if fork was called from a signal handler.  */
 123   if (multiple_threads)
 124     {
 125       _IO_list_lock ();
 126
 127       /* Acquire malloc locks.  This needs to come last because fork
 128          handlers may use malloc, and the libio list lock has an
 129          indirect malloc dependency as well (via the getdelim
 130          function).  */
 131       call_function_static_weak (__malloc_fork_lock_parent);
 132     }
 133
 134 #ifndef NDEBUG
 135   pid_t ppid = THREAD_GETMEM (THREAD_SELF, tid);
 136 #endif
 137
 138 #ifdef ARCH_FORK
 139   pid = ARCH_FORK ();
 140 #else
 141 # error "ARCH_FORK must be defined so that the CLONE_SETTID flag is used"
 142   pid = INLINE_SYSCALL (fork, 0);
 143 #endif
 144
 145
 146   if (pid == 0)
 147     {
 148       struct pthread *self = THREAD_SELF;
 149
 150       assert (THREAD_GETMEM (self, tid) != ppid);
 151
 152       /* See __pthread_once.  */
 153       if (__fork_generation_pointer != NULL)
 154         *__fork_generation_pointer += __PTHREAD_ONCE_FORK_GEN_INCR;
 155
 156 #if HP_TIMING_AVAIL
 157       /* The CPU clock of the thread and process have to be set to zero.  */
 158       hp_timing_t now;
 159       HP_TIMING_NOW (now);
 160       THREAD_SETMEM (self, cpuclock_offset, now);
 161       GL(dl_cpuclock_offset) = now;
 162 #endif
 163
 164 #ifdef __NR_set_robust_list
 165       /* Initialize the robust mutex list which has been reset during
 166          the fork.  We do not check for errors since if it fails here
 167          it failed at process start as well and noone could have used
 168          robust mutexes.  We also do not have to set
 169          self->robust_head.futex_offset since we inherit the correct
 170          value from the parent.  */
 171 # ifdef SHARED
 172       if (__builtin_expect (__libc_pthread_functions_init, 0))
 173         PTHFCT_CALL (ptr_set_robust, (self));
 174 # else
 175       extern __typeof (__nptl_set_robust) __nptl_set_robust
 176         __attribute__((weak));
 177       if (__builtin_expect (__nptl_set_robust != NULL, 0))
 178         __nptl_set_robust (self);
 179 # endif
 180 #endif
 181
 182       /* Reset the lock state in the multi-threaded case.  */
 183       if (multiple_threads)
 184         {
 185           /* Release malloc locks.  */
 186           call_function_static_weak (__malloc_fork_unlock_child);
 187
 188           /* Reset the file list.  These are recursive mutexes.  */
 189           fresetlockfiles ();
 190
 191           /* Reset locks in the I/O code.  */
 192           _IO_list_resetlock ();
 193         }
 194
 195       /* Reset the lock the dynamic loader uses to protect its data.  */
 196       __rtld_lock_initialize (GL(dl_load_lock));
 197
 198       /* Run the handlers registered for the child.  */
 199       while (allp != NULL)
 200         {
 201           if (allp->handler->child_handler != NULL)
 202             allp->handler->child_handler ();
 203
 204           /* Note that we do not have to wake any possible waiter.
 205              This is the only thread in the new process.  The count
 206              may have been bumped up by other threads doing a fork.
 207              We reset it to 1, to avoid waiting for non-existing
 208              thread(s) to release the count.  */
 209           allp->handler->refcntr = 1;
 210
 211           /* XXX We could at this point look through the object pool
 212              and mark all objects not on the __fork_handlers list as
 213              unused.  This is necessary in case the fork() happened
 214              while another thread called dlclose() and that call had
 215              to create a new list.  */
 216
 217           allp = allp->next;
 218         }
 219
 220       /* Initialize the fork lock.  */
 221       __fork_lock = LLL_LOCK_INITIALIZER;
 222     }
 223   else
 224     {
 225       assert (THREAD_GETMEM (THREAD_SELF, tid) == ppid);
 226
 227       /* Release acquired locks in the multi-threaded case.  */
 228       if (multiple_threads)
 229         {
 230           /* Release malloc locks, parent process variant.  */
 231           call_function_static_weak (__malloc_fork_unlock_parent);
 232
 233           /* We execute this even if the 'fork' call failed.  */
 234           _IO_list_unlock ();
 235         }
 236
 237       /* Run the handlers registered for the parent.  */
 238       while (allp != NULL)
 239         {
 240           if (allp->handler->parent_handler != NULL)
 241             allp->handler->parent_handler ();
 242
 243           if (atomic_decrement_and_test (&allp->handler->refcntr)
 244               && allp->handler->need_signal)
 245             futex_wake (&allp->handler->refcntr, 1, FUTEX_PRIVATE);
 246
 247           allp = allp->next;
 248         }
 249     }
 250
 251   return pid;
 252 }
 253 weak_alias (__libc_fork, __fork)
 254 libc_hidden_def (__fork)
 255 weak_alias (__libc_fork, fork)