1  /* elision-trylock.c: Lock eliding trylock for pthreads.
       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  #include <pthread.h>
      20  #include <pthreadP.h>
      21  #include <lowlevellock.h>
      22  #include <elision-conf.h>
      23  #include "htm.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 elided trylocks.  */
      34    __libc_tabort (_ABORT_NESTED_TRYLOCK);
      35  
      36    /* Only try a transaction if it's worth it.  */
      37    if (atomic_load_relaxed (adapt_count) > 0)
      38      {
      39        goto use_lock;
      40      }
      41  
      42    if (__libc_tbegin (0))
      43      {
      44        if (*futex == 0)
      45  	return 0;
      46  
      47        /* Lock was busy.  This is never a nested transaction.
      48           End it, and set the adapt count.  */
      49        __libc_tend (0);
      50  
      51        if (aconf.skip_lock_busy > 0)
      52  	atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);
      53      }
      54    else
      55      {
      56        if (_TEXASRU_FAILURE_PERSISTENT (__builtin_get_texasru ()))
      57  	{
      58  	  /* A persistent failure indicates that a retry will probably
      59  	     result in another failure.  Use normal locking now and
      60  	     for the next couple of calls.  */
      61  	  if (aconf.skip_trylock_internal_abort > 0)
      62  	    atomic_store_relaxed (adapt_count,
      63  				aconf.skip_trylock_internal_abort);
      64  	}
      65      }
      66  
      67  use_lock:
      68    return lll_trylock (*futex);
      69  }
      70  libc_hidden_def (__lll_trylock_elision)