nptl/sysdeps/unix/sysv/linux/x86/elision-trylock.c

   1 /* elision-trylock.c: Lock eliding trylock for pthreads.
   2    Copyright (C) 2013-2014 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 <pthread.h>
  20 #include <pthreadP.h>
  21 #include <lowlevellock.h>
  22 #include "hle.h"
  23 #include <elision-conf.h>
  24
  25 #define aconf __elision_aconf
  26
  27 /* Try to elide a futex trylock.  FUTEX is the futex variable.  ADAPT_COUNT is
  28    the adaptation counter in the mutex.  */
  29
  30 int
  31 __lll_trylock_elision (int *futex, short *adapt_count)
  32 {
  33   /* Implement POSIX semantics by forbiding nesting
  34      trylock.  Sorry.  After the abort the code is re-executed
  35      non transactional and if the lock was already locked
  36      return an error.  */
  37   _xabort (_ABORT_NESTED_TRYLOCK);
  38
  39   /* Only try a transaction if it's worth it.  */
  40   if (*adapt_count <= 0)
  41     {
  42       unsigned status;
  43
  44       if ((status = _xbegin()) == _XBEGIN_STARTED)
  45         {
  46           if (*futex == 0)
  47             return 0;
  48
  49           /* Lock was busy.  Fall back to normal locking.
  50              Could also _xend here but xabort with 0xff code
  51              is more visible in the profiler.  */
  52           _xabort (_ABORT_LOCK_BUSY);
  53         }
  54
  55       if (!(status & _XABORT_RETRY))
  56         {
  57           /* Internal abort.  No chance for retry.  For future
  58              locks don't try speculation for some time.  */
  59           if (*adapt_count != aconf.skip_trylock_internal_abort)
  60             *adapt_count = aconf.skip_trylock_internal_abort;
  61         }
  62       /* Could do some retries here.  */
  63     }
  64   else
  65     {
  66       /* Lost updates are possible, but harmless.  */
  67       (*adapt_count)--;
  68     }
  69
  70   return lll_trylock (*futex);
  71 }