1  /* elision-lock.c: Elided pthread mutex lock.
       2     Copyright (C) 2011-2023 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  #include <pthread.h>
      20  #include "pthreadP.h"
      21  #include "lowlevellock.h"
      22  #include "hle.h"
      23  #include <elision-conf.h>
      24  
      25  #ifndef EXTRAARG
      26  #define EXTRAARG
      27  #endif
      28  #ifndef LLL_LOCK
      29  #define LLL_LOCK(a,b) lll_lock(a,b), 0
      30  #endif
      31  
      32  #define aconf __elision_aconf
      33  
      34  /* Adaptive lock using transactions.
      35     By default the lock region is run as a transaction, and when it
      36     aborts or the lock is busy the lock adapts itself.  */
      37  
      38  int
      39  __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
      40  {
      41    /* adapt_count can be accessed concurrently; these accesses can be both
      42       inside of transactions (if critical sections are nested and the outer
      43       critical section uses lock elision) and outside of transactions.  Thus,
      44       we need to use atomic accesses to avoid data races.  However, the
      45       value of adapt_count is just a hint, so relaxed MO accesses are
      46       sufficient.  */
      47    if (atomic_load_relaxed (adapt_count) <= 0)
      48      {
      49        unsigned status;
      50        int try_xbegin;
      51  
      52        for (try_xbegin = aconf.retry_try_xbegin;
      53  	   try_xbegin > 0;
      54  	   try_xbegin--)
      55  	{
      56  	  if ((status = _xbegin()) == _XBEGIN_STARTED)
      57  	    {
      58  	      if (*futex == 0)
      59  		return 0;
      60  
      61  	      /* Lock was busy.  Fall back to normal locking.
      62  		 Could also _xend here but xabort with 0xff code
      63  		 is more visible in the profiler.  */
      64  	      _xabort (_ABORT_LOCK_BUSY);
      65  	    }
      66  
      67  	  if (!(status & _XABORT_RETRY))
      68  	    {
      69  	      if ((status & _XABORT_EXPLICIT)
      70  			&& _XABORT_CODE (status) == _ABORT_LOCK_BUSY)
      71  	        {
      72  		  /* Right now we skip here.  Better would be to wait a bit
      73  		     and retry.  This likely needs some spinning.  See
      74  		     above for why relaxed MO is sufficient.  */
      75  		  if (atomic_load_relaxed (adapt_count)
      76  		      != aconf.skip_lock_busy)
      77  		    atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);
      78  		}
      79  	      /* Internal abort.  There is no chance for retry.
      80  		 Use the normal locking and next time use lock.
      81  		 Be careful to avoid writing to the lock.  See above for why
      82  		 relaxed MO is sufficient.  */
      83  	      else if (atomic_load_relaxed (adapt_count)
      84  		  != aconf.skip_lock_internal_abort)
      85  		atomic_store_relaxed (adapt_count,
      86  		    aconf.skip_lock_internal_abort);
      87  	      break;
      88  	    }
      89  	}
      90      }
      91    else
      92      {
      93        /* Use a normal lock until the threshold counter runs out.
      94  	 Lost updates possible.  */
      95        atomic_store_relaxed (adapt_count,
      96  	  atomic_load_relaxed (adapt_count) - 1);
      97      }
      98  
      99    /* Use a normal lock here.  */
     100    return LLL_LOCK ((*futex), private);
     101  }
     102  libc_hidden_def (__lll_lock_elision)