/* pthread_spin_lock -- lock a spin lock.  Generic version.

   Copyright (C) 2012-2018 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 <atomic.h>

#include "pthreadP.h"

int

pthread_spin_lock (pthread_spinlock_t *lock)

{

  int val = 0;

  /* We assume that the first try mostly will be successful, thus we use

     atomic_exchange if it is not implemented by a CAS loop (we also assume

     that atomic_exchange can be faster if it succeeds, see

     ATOMIC_EXCHANGE_USES_CAS).  Otherwise, we use a weak CAS and not an

     exchange so we bail out after the first failed attempt to change the

     state.  For the subsequent attempts we use atomic_compare_and_exchange

     after we observe that the lock is not acquired.

     See also comment in pthread_spin_trylock.

     We use acquire MO to synchronize-with the release MO store in

     pthread_spin_unlock, and thus ensure that prior critical sections

     happen-before this critical section.  */

#if ! ATOMIC_EXCHANGE_USES_CAS

  /* Try to acquire the lock with an exchange instruction as this architecture

     has such an instruction and we assume it is faster than a CAS.

     The acquisition succeeds if the lock is not in an acquired state.  */

  if (__glibc_likely (atomic_exchange_acquire (lock, 1) == 0))

    return 0;

#else

  /* Try to acquire the lock with a CAS instruction as this architecture

     has no exchange instruction.  The acquisition succeeds if the lock is not

     acquired.  */

  if (__glibc_likely (atomic_compare_exchange_weak_acquire (lock, &val, 1)))

    return 0;

#endif

  do

    {

      /* The lock is contended and we need to wait.  Going straight back

	 to cmpxchg is not a good idea on many targets as that will force

	 expensive memory synchronizations among processors and penalize other

	 running threads.

	 There is no technical reason for throwing in a CAS every now and then,

	 and so far we have no evidence that it can improve performance.

	 If that would be the case, we have to adjust other spin-waiting loops

	 elsewhere, too!

	 Thus we use relaxed MO reads until we observe the lock to not be

	 acquired anymore.  */

      do

	{

	  /* TODO Back-off.  */

	  atomic_spin_nop ();

	  val = atomic_load_relaxed (lock);

	}

      while (val != 0);

      /* We need acquire memory order here for the same reason as mentioned

	 for the first try to lock the spinlock.  */

    }

  while (!atomic_compare_exchange_weak_acquire (lock, &val, 1));

  return 0;

}