sysdeps/powerpc/nptl/elide.h

   1 /* elide.h: Generic lock elision support for powerpc.
   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 #ifndef ELIDE_PPC_H
  20 # define ELIDE_PPC_H
  21
  22 #ifdef ENABLE_LOCK_ELISION
  23 # include <htm.h>
  24 # include <elision-conf.h>
  25
  26 /* Get the new value of adapt_count according to the elision
  27    configurations.  Returns true if the system should retry again or false
  28    otherwise.  */
  29 static inline bool
  30 __get_new_count (uint8_t *adapt_count, int attempt)
  31 {
  32   /* A persistent failure indicates that a retry will probably
  33      result in another failure.  Use normal locking now and
  34      for the next couple of calls.  */
  35   if (_TEXASRU_FAILURE_PERSISTENT (__builtin_get_texasru ()))
  36     {
  37       if (__elision_aconf.skip_lock_internal_abort > 0)
  38         *adapt_count = __elision_aconf.skip_lock_internal_abort;
  39       return false;
  40     }
  41   /* Same logic as above, but for a number of temporary failures in a
  42      a row.  */
  43   else if (attempt <= 1 && __elision_aconf.skip_lock_out_of_tbegin_retries > 0
  44            && __elision_aconf.try_tbegin > 0)
  45     *adapt_count = __elision_aconf.skip_lock_out_of_tbegin_retries;
  46   return true;
  47 }
  48
  49 /* CONCURRENCY NOTES:
  50
  51    The evaluation of the macro expression is_lock_free encompasses one or
  52    more loads from memory locations that are concurrently modified by other
  53    threads.  For lock elision to work, this evaluation and the rest of the
  54    critical section protected by the lock must be atomic because an
  55    execution with lock elision must be equivalent to an execution in which
  56    the lock would have been actually acquired and released.  Therefore, we
  57    evaluate is_lock_free inside of the transaction that represents the
  58    critical section for which we want to use lock elision, which ensures
  59    the atomicity that we require.  */
  60
  61 /* Returns 0 if the lock defined by is_lock_free was elided.
  62    ADAPT_COUNT is a per-lock state variable.  */
  63 # define ELIDE_LOCK(adapt_count, is_lock_free)                          \
  64   ({                                                                    \
  65     int ret = 0;                                                        \
  66     if (adapt_count > 0)                                                \
  67       (adapt_count)--;                                                  \
  68     else                                                                \
  69       for (int i = __elision_aconf.try_tbegin; i > 0; i--)              \
  70         {                                                               \
  71           if (__libc_tbegin (0))                                        \
  72             {                                                           \
  73               if (is_lock_free)                                         \
  74                 {                                                       \
  75                   ret = 1;                                              \
  76                   break;                                                \
  77                 }                                                       \
  78               __libc_tabort (_ABORT_LOCK_BUSY);                         \
  79             }                                                           \
  80           else                                                          \
  81             if (!__get_new_count (&adapt_count,i))                      \
  82               break;                                                    \
  83         }                                                               \
  84     ret;                                                                \
  85   })
  86
  87 # define ELIDE_TRYLOCK(adapt_count, is_lock_free, write)        \
  88   ({                                                            \
  89     int ret = 0;                                                \
  90     if (__elision_aconf.try_tbegin > 0)                         \
  91       {                                                         \
  92         if (write)                                              \
  93           __libc_tabort (_ABORT_NESTED_TRYLOCK);                \
  94         ret = ELIDE_LOCK (adapt_count, is_lock_free);           \
  95       }                                                         \
  96     ret;                                                        \
  97   })
  98
  99
 100 static inline bool
 101 __elide_unlock (int is_lock_free)
 102 {
 103   if (is_lock_free)
 104     {
 105       __libc_tend (0);
 106       return true;
 107     }
 108   return false;
 109 }
 110
 111 # define ELIDE_UNLOCK(is_lock_free) \
 112   __elide_unlock (is_lock_free)
 113
 114 # else
 115
 116 # define ELIDE_LOCK(adapt_count, is_lock_free) 0
 117 # define ELIDE_TRYLOCK(adapt_count, is_lock_free, write) 0
 118 # define ELIDE_UNLOCK(is_lock_free) 0
 119
 120 #endif /* ENABLE_LOCK_ELISION  */
 121
 122 #endif