sysdeps/nacl/exit-thread.h

   1 /* Call to terminate the current thread.  NaCl version.
   2    Copyright (C) 2015-2016 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   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 <atomic.h>
  21 #include <futex-internal.h>
  22 #include <nacl-interfaces.h>
  23 #include <nptl/pthreadP.h>
  24
  25 /* This causes the current thread to exit, without affecting other
  26    threads in the process if there are any.  If there are no other
  27    threads left, then this has the effect of _exit (0).  */
  28
  29 static inline void __attribute__ ((noreturn, always_inline, unused))
  30 __exit_thread (void)
  31 {
  32   struct pthread *pd = THREAD_SELF;
  33
  34   /* The generic logic for pthread_join and stack/descriptor reuse is
  35      based on the Linux kernel feature that will clear and futex-wake
  36      a designated address as a final part of thread teardown.  Correct
  37      synchronization relies on the fact that these happen only after
  38      there is no possibility of user code touching or examining the
  39      late thread's stack.
  40
  41      The NaCl system interface implements half of this: it clears a
  42      word after the thread's user stack is safely dead, but it does
  43      not futex-wake the location.  So, some shenanigans are required.
  44      We change and futex-wake the location here, so as to wake up any
  45      blocked pthread_join (i.e. lll_wait_tid) or pthread_timedjoin_np
  46      (i.e. lll_timedwait_tid).  However, that's before we have safely
  47      vacated the stack.  So instead of clearing the location, we set
  48      it to a special magic value, NACL_EXITING_TID.  This counts as a
  49      "live thread" value for all the generic logic, but is recognized
  50      specially in lll_wait_tid and lll_timedwait_tid (lowlevellock.h).
  51      Once it has this value, lll_wait_tid will busy-wait for the
  52      location to be cleared to zero by the NaCl system code.  Only then
  53      is the stack actually safe to reuse.  */
  54
  55   if (!IS_DETACHED (pd))
  56     {
  57       /* The magic value must not be one that could ever be a valid
  58          TID value.  See pthread-pids.h about the low bit.  */
  59       assert (NACL_EXITING_TID & 1);
  60
  61       /* The magic value must not be one that has the "free" flag
  62          (i.e. sign bit) set.  If that bit is set, then the
  63          descriptor could be reused for a new thread.  */
  64       assert (NACL_EXITING_TID > 0);
  65
  66       atomic_store_relaxed (&pd->tid, NACL_EXITING_TID);
  67       futex_wake ((unsigned int *) &pd->tid, 1, FUTEX_PRIVATE);
  68     }
  69
  70   /* This clears PD->tid some time after the thread stack can never
  71      be touched again.  Unfortunately, it does not also do a
  72      futex-wake at that time (as Linux does via CLONE_CHILD_CLEARTID
  73      and set_tid_address).  So lll_wait_tid does some busy-waiting.  */
  74   __nacl_irt_thread.thread_exit (&pd->tid);
  75
  76   /* That never returns unless something is severely and unrecoverably wrong.
  77      If it ever does, try to make sure we crash.  */
  78   while (1)
  79     __builtin_trap ();
  80 }