1  /* elide.h: Generic lock elision support for powerpc.
       2     Copyright (C) 2015-2023 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