google3/third_party/grte/v5_src/glibc-2.27/sysdeps/unix/sysv/linux/x86/elision-lock.c

/* elision-lock.c: Elided pthread mutex lock.
   Copyright (C) 2011-2018 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>

#if !defined(LLL_LOCK) && !defined(EXTRAARG)
/* Make sure the configuration code is always linked in for static
   libraries.  */
#include "elision-conf.c"
#endif

#ifndef EXTRAARG
#define EXTRAARG
#endif
#ifndef LLL_LOCK
#define LLL_LOCK(a,b) lll_lock(a,b), 0
#endif

#define aconf __elision_aconf

/* Adaptive lock using transactions.
   By default the lock region is run as a transaction, and when it
   aborts or the lock is busy the lock adapts itself.  */

int
__lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
{
  /* adapt_count can be accessed concurrently; these accesses can be both
     inside of transactions (if critical sections are nested and the outer
     critical section uses lock elision) and outside of transactions.  Thus,
     we need to use atomic accesses to avoid data races.  However, the
     value of adapt_count is just a hint, so relaxed MO accesses are
     sufficient.  */
  if (atomic_load_relaxed (adapt_count) <= 0)
    {
      unsigned status;
      int try_xbegin;

      for (try_xbegin = aconf.retry_try_xbegin;
	   try_xbegin > 0;
	   try_xbegin--)
	{
	  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))
	    {
	      if ((status & _XABORT_EXPLICIT)
			&& _XABORT_CODE (status) == _ABORT_LOCK_BUSY)
	        {
		  /* Right now we skip here.  Better would be to wait a bit
		     and retry.  This likely needs some spinning.  See
		     above for why relaxed MO is sufficient.  */
		  if (atomic_load_relaxed (adapt_count)
		      != aconf.skip_lock_busy)
		    atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);
		}
	      /* Internal abort.  There is no chance for retry.
		 Use the normal locking and next time use lock.
		 Be careful to avoid writing to the lock.  See above for why
		 relaxed MO is sufficient.  */
	      else if (atomic_load_relaxed (adapt_count)
		  != aconf.skip_lock_internal_abort)
		atomic_store_relaxed (adapt_count,
		    aconf.skip_lock_internal_abort);
	      break;
	    }
	}
    }
  else
    {
      /* Use a normal lock until the threshold counter runs out.
	 Lost updates possible.  */
      atomic_store_relaxed (adapt_count,
	  atomic_load_relaxed (adapt_count) - 1);
    }

  /* Use a normal lock here.  */
  return LLL_LOCK ((*futex), private);
}