/* elision-trylock.c: Lock eliding trylock for pthreads.

   Copyright (C) 2013-2018 Free Software Foundation, Inc.

   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or

   modify it under the terms of the GNU Lesser General Public

   License as published by the Free Software Foundation; either

   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,

   but WITHOUT ANY WARRANTY; without even the implied warranty of

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public

   License along with the GNU C Library; if not, see

   <http://www.gnu.org/licenses/>.  */

#include <pthread.h>

#include <pthreadP.h>

#include <lowlevellock.h>

#include "hle.h"

#include <elision-conf.h>

#define aconf __elision_aconf

/* Try to elide a futex trylock.  FUTEX is the futex variable.  ADAPT_COUNT is

   the adaptation counter in the mutex.  */

int

__lll_trylock_elision (int *futex, short *adapt_count)

{

  /* Implement POSIX semantics by forbiding nesting

     trylock.  Sorry.  After the abort the code is re-executed

     non transactional and if the lock was already locked

     return an error.  */

  _xabort (_ABORT_NESTED_TRYLOCK);

  /* Only try a transaction if it's worth it.  See __lll_lock_elision for

     why we need atomic accesses.  Relaxed MO is sufficient because this is

     just a hint.  */

  if (atomic_load_relaxed (adapt_count) <= 0)

    {

      unsigned status;

      if ((status = _xbegin()) == _XBEGIN_STARTED)

	{

	  if (*futex == 0)

	    return 0;

	  /* Lock was busy.  Fall back to normal locking.

	     Could also _xend here but xabort with 0xff code

	     is more visible in the profiler.  */

	  _xabort (_ABORT_LOCK_BUSY);

	}

      if (!(status & _XABORT_RETRY))

        {

          /* Internal abort.  No chance for retry.  For future

             locks don't try speculation for some time.  See above for MO.  */

          if (atomic_load_relaxed (adapt_count)

              != aconf.skip_lock_internal_abort)

            atomic_store_relaxed (adapt_count, aconf.skip_lock_internal_abort);

        }

      /* Could do some retries here.  */

    }

  else

    {

      /* Lost updates are possible but harmless (see above).  */

      atomic_store_relaxed (adapt_count,

	  atomic_load_relaxed (adapt_count) - 1);

    }

  return lll_trylock (*futex);

}