1  /* elision-trylock.c: Lock eliding trylock for pthreads.
       2     Copyright (C) 2013-2023 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  #define aconf __elision_aconf
      26  
      27  /* Try to elide a futex trylock.  FUTEX is the futex variable.  ADAPT_COUNT is
      28     the adaptation counter in the mutex.  */
      29  
      30  int
      31  __lll_trylock_elision (int *futex, short *adapt_count)
      32  {
      33    /* Implement POSIX semantics by forbiding nesting
      34       trylock.  Sorry.  After the abort the code is re-executed
      35       non transactional and if the lock was already locked
      36       return an error.  */
      37    _xabort (_ABORT_NESTED_TRYLOCK);
      38  
      39    /* Only try a transaction if it's worth it.  See __lll_lock_elision for
      40       why we need atomic accesses.  Relaxed MO is sufficient because this is
      41       just a hint.  */
      42    if (atomic_load_relaxed (adapt_count) <= 0)
      43      {
      44        unsigned status;
      45  
      46        if ((status = _xbegin()) == _XBEGIN_STARTED)
      47  	{
      48  	  if (*futex == 0)
      49  	    return 0;
      50  
      51  	  /* Lock was busy.  Fall back to normal locking.
      52  	     Could also _xend here but xabort with 0xff code
      53  	     is more visible in the profiler.  */
      54  	  _xabort (_ABORT_LOCK_BUSY);
      55  	}
      56  
      57        if (!(status & _XABORT_RETRY))
      58          {
      59            /* Internal abort.  No chance for retry.  For future
      60               locks don't try speculation for some time.  See above for MO.  */
      61            if (atomic_load_relaxed (adapt_count)
      62                != aconf.skip_lock_internal_abort)
      63              atomic_store_relaxed (adapt_count, aconf.skip_lock_internal_abort);
      64          }
      65        /* Could do some retries here.  */
      66      }
      67    else
      68      {
      69        /* Lost updates are possible but harmless (see above).  */
      70        atomic_store_relaxed (adapt_count,
      71  	  atomic_load_relaxed (adapt_count) - 1);
      72      }
      73  
      74    return lll_trylock (*futex);
      75  }
      76  libc_hidden_def (__lll_trylock_elision)