1  /* elide.h: Generic lock elision support.
       2     Copyright (C) 2014-2023 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  #ifndef ELIDE_H
      19  #define ELIDE_H 1
      20  
      21  #include <hle.h>
      22  #include <elision-conf.h>
      23  #include <atomic.h>
      24  
      25  
      26  /* Adapt elision with ADAPT_COUNT and STATUS and decide retries.  */
      27  
      28  static inline bool
      29  elision_adapt(signed char *adapt_count, unsigned int status)
      30  {
      31    if (status & _XABORT_RETRY)
      32      return false;
      33    if ((status & _XABORT_EXPLICIT)
      34        && _XABORT_CODE (status) == _ABORT_LOCK_BUSY)
      35      {
      36        /* Right now we skip here.  Better would be to wait a bit
      37  	 and retry.  This likely needs some spinning. Be careful
      38  	 to avoid writing the lock.
      39  	 Using relaxed MO and separate atomic accesses is sufficient because
      40  	 adapt_count is just a hint.  */
      41        if (atomic_load_relaxed (adapt_count) != __elision_aconf.skip_lock_busy)
      42  	atomic_store_relaxed (adapt_count, __elision_aconf.skip_lock_busy);
      43      }
      44    /* Internal abort.  There is no chance for retry.
      45       Use the normal locking and next time use lock.
      46       Be careful to avoid writing to the lock.  See above for MO.  */
      47    else if (atomic_load_relaxed (adapt_count)
      48        != __elision_aconf.skip_lock_internal_abort)
      49      atomic_store_relaxed (adapt_count,
      50  	__elision_aconf.skip_lock_internal_abort);
      51    return true;
      52  }
      53  
      54  /* is_lock_free must be executed inside the transaction */
      55  
      56  /* Returns true if lock defined by IS_LOCK_FREE was elided.
      57     ADAPT_COUNT is a per-lock state variable; it must be accessed atomically
      58     to avoid data races but is just a hint, so using relaxed MO and separate
      59     atomic loads and stores instead of atomic read-modify-write operations is
      60     sufficient.  */
      61  
      62  #define ELIDE_LOCK(adapt_count, is_lock_free)			\
      63    ({								\
      64      int ret = 0;						\
      65  								\
      66      if (atomic_load_relaxed (&(adapt_count)) <= 0)		\
      67        {								\
      68          for (int i = __elision_aconf.retry_try_xbegin; i > 0; i--) \
      69            {							\
      70              unsigned int status;				\
      71  	    if ((status = _xbegin ()) == _XBEGIN_STARTED)	\
      72  	      {							\
      73  	        if (is_lock_free)				\
      74  	          {						\
      75  		    ret = 1;					\
      76  		    break;					\
      77  	          }						\
      78  	        _xabort (_ABORT_LOCK_BUSY);			\
      79  	      }							\
      80  	    if (!elision_adapt (&(adapt_count), status))	\
      81  	      break;						\
      82            }							\
      83        }								\
      84      else 							\
      85        atomic_store_relaxed (&(adapt_count),			\
      86  	  atomic_load_relaxed (&(adapt_count)) - 1);		\
      87      ret;							\
      88    })
      89  
      90  /* Returns true if lock defined by IS_LOCK_FREE was try-elided.
      91     ADAPT_COUNT is a per-lock state variable.  */
      92  
      93  #define ELIDE_TRYLOCK(adapt_count, is_lock_free, write) ({	\
      94    int ret = 0;						\
      95    if (__elision_aconf.retry_try_xbegin > 0)		\
      96      {  							\
      97        if (write)					\
      98          _xabort (_ABORT_NESTED_TRYLOCK);		\
      99        ret = ELIDE_LOCK (adapt_count, is_lock_free);     \
     100      }							\
     101      ret;						\
     102      })
     103  
     104  /* Returns true if lock defined by IS_LOCK_FREE was elided.  The call
     105     to _xend crashes if the application incorrectly tries to unlock a
     106     lock which has not been locked.  */
     107  
     108  #define ELIDE_UNLOCK(is_lock_free)		\
     109    ({						\
     110    int ret = 0;					\
     111    if (is_lock_free)				\
     112      {						\
     113        _xend ();					\
     114        ret = 1;					\
     115      }						\
     116    ret;						\
     117    })
     118  
     119  #endif