/* Elided pthread mutex lock.

   Copyright (C) 2014-2018 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 <htm.h>

#include <elision-conf.h>

#include <stdint.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 && aconf.try_tbegin > 0)

    {

      /* Start a transaction and retry it automatically if it aborts with

	 _HTM_TBEGIN_TRANSIENT.  This macro calls tbegin at most retry_cnt

	 + 1 times.  The second argument is considered as retry_cnt.  */

      int status = __libc_tbegin_retry ((void *) 0, aconf.try_tbegin - 1);

      if (__glibc_likely (status == _HTM_TBEGIN_STARTED))

	{

	  /* Check the futex to make sure nobody has touched it in the

	     mean time.  This forces the futex into the cache and makes

	     sure the transaction aborts if another thread acquires the lock

	     concurrently.  */

	  if (__glibc_likely (atomic_load_relaxed (futex) == 0))

	    /* Lock was free.  Return to user code in a transaction.  */

	    return 0;

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

	     This can be the case if e.g. adapt_count was decremented to zero

	     by a former release and another thread has been waken up and

	     acquired it.  */

	  if (__glibc_likely (__libc_tx_nesting_depth () <= 1))

	    {

	      /* In a non-nested transaction there is no need to abort,

		 which is expensive.  Simply end the started transaction.  */

	      __libc_tend ();

	      /* Don't try to use transactions for the next couple of times.

		 See above for why relaxed MO is sufficient.  */

	      if (aconf.skip_lock_busy > 0)

		atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);

	    }

	  else /* nesting depth is > 1 */

	    {

	      /* A nested transaction will abort eventually because it

		 cannot make any progress before *futex changes back to 0.

		 So we may as well abort immediately.

		 This persistently aborts the outer transaction to force

		 the outer mutex use the default lock instead of retrying

		 with transactions until the try_tbegin of the outer mutex

		 is zero.

		 The adapt_count of this inner mutex is not changed,

		 because using the default lock with the inner mutex

		 would abort the outer transaction.  */

	      __libc_tabort (_HTM_FIRST_USER_ABORT_CODE | 1);

	      __builtin_unreachable ();

	    }

	}

      else if (status != _HTM_TBEGIN_TRANSIENT)

	{

	  /* A persistent abort (cc 1 or 3) indicates that a retry is

	     probably futile.  Use the normal locking now and for the

	     next couple of calls.

	     Be careful to avoid writing to the lock.  See above for why

	     relaxed MO is sufficient.  */

	  if (aconf.skip_lock_internal_abort > 0)

	    atomic_store_relaxed (adapt_count,

				  aconf.skip_lock_internal_abort);

	}

      else

	{

	  /* The transaction failed for some retries with

	     _HTM_TBEGIN_TRANSIENT.  Use the normal locking now and for the

	     next couple of calls.  */

	  if (aconf.skip_lock_out_of_tbegin_retries > 0)

	    atomic_store_relaxed (adapt_count,

				  aconf.skip_lock_out_of_tbegin_retries);

	}

    }

  /* Use normal locking as fallback path if the transaction does not

     succeed.  */

  return LLL_LOCK ((*futex), private);

}