sysdeps/powerpc/nptl/elide.h

   1 /* elide.h: Generic lock elision support for powerpc.
   2    Copyright (C) 2015-2024 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    <https://www.gnu.org/licenses/>.  */
  18
  19 #ifndef ELIDE_PPC_H
  20 # define ELIDE_PPC_H
  21
  22 # include <htm.h>
  23 # include <elision-conf.h>
  24
  25 /* Get the new value of adapt_count according to the elision
  26    configurations.  Returns true if the system should retry again or false
  27    otherwise.  */
  28 static inline bool
  29 __get_new_count (uint8_t *adapt_count, int attempt)
  30 {
  31   /* A persistent failure indicates that a retry will probably
  32      result in another failure.  Use normal locking now and
  33      for the next couple of calls.  */
  34   if (_TEXASRU_FAILURE_PERSISTENT (__builtin_get_texasru ()))
  35     {
  36       if (__elision_aconf.skip_lock_internal_abort > 0)
  37         *adapt_count = __elision_aconf.skip_lock_internal_abort;
  38       return false;
  39     }
  40   /* Same logic as above, but for a number of temporary failures in a
  41      a row.  */
  42   else if (attempt <= 1 && __elision_aconf.skip_lock_out_of_tbegin_retries > 0
  43            && __elision_aconf.try_tbegin > 0)
  44     *adapt_count = __elision_aconf.skip_lock_out_of_tbegin_retries;
  45   return true;
  46 }
  47
  48 /* CONCURRENCY NOTES:
  49
  50    The evaluation of the macro expression is_lock_free encompasses one or
  51    more loads from memory locations that are concurrently modified by other
  52    threads.  For lock elision to work, this evaluation and the rest of the
  53    critical section protected by the lock must be atomic because an
  54    execution with lock elision must be equivalent to an execution in which
  55    the lock would have been actually acquired and released.  Therefore, we
  56    evaluate is_lock_free inside of the transaction that represents the
  57    critical section for which we want to use lock elision, which ensures
  58    the atomicity that we require.  */
  59
  60 /* Returns 0 if the lock defined by is_lock_free was elided.
  61    ADAPT_COUNT is a per-lock state variable.  */
  62 # define ELIDE_LOCK(adapt_count, is_lock_free)                          \
  63   ({                                                                    \
  64     int ret = 0;                                                        \
  65     if (adapt_count > 0)                                                \
  66       (adapt_count)--;                                                  \
  67     else                                                                \
  68       for (int i = __elision_aconf.try_tbegin; i > 0; i--)              \
  69         {                                                               \
  70           if (__libc_tbegin (0))                                        \
  71             {                                                           \
  72               if (is_lock_free)                                         \
  73                 {                                                       \
  74                   ret = 1;                                              \
  75                   break;                                                \
  76                 }                                                       \
  77               __libc_tabort (_ABORT_LOCK_BUSY);                         \
  78             }                                                           \
  79           else                                                          \
  80             if (!__get_new_count (&adapt_count,i))                      \
  81               break;                                                    \
  82         }                                                               \
  83     ret;                                                                \
  84   })
  85
  86 # define ELIDE_TRYLOCK(adapt_count, is_lock_free, write)        \
  87   ({                                                            \
  88     int ret = 0;                                                \
  89     if (__elision_aconf.try_tbegin > 0)                         \
  90       {                                                         \
  91         if (write)                                              \
  92           __libc_tabort (_ABORT_NESTED_TRYLOCK);                \
  93         ret = ELIDE_LOCK (adapt_count, is_lock_free);           \
  94       }                                                         \
  95     ret;                                                        \
  96   })
  97
  98
  99 static inline bool
 100 __elide_unlock (int is_lock_free)
 101 {
 102   if (is_lock_free)
 103     {
 104       /* This code is expected to crash when trying to unlock a lock not
 105          held by this thread.  More information is available in the
 106          __pthread_rwlock_unlock() implementation.  */
 107       __libc_tend (0);
 108       return true;
 109     }
 110   return false;
 111 }
 112
 113 # define ELIDE_UNLOCK(is_lock_free) \
 114   __elide_unlock (is_lock_free)
 115
 116 #endif