1  /* Copyright (C) 2005-2023 Free Software Foundation, Inc.
       2     Contributed by Richard Henderson <rth@redhat.com>.
       3  
       4     This file is part of the GNU Offloading and Multi Processing Library
       5     (libgomp).
       6  
       7     Libgomp is free software; you can redistribute it and/or modify it
       8     under the terms of the GNU General Public License as published by
       9     the Free Software Foundation; either version 3, or (at your option)
      10     any later version.
      11  
      12     Libgomp is distributed in the hope that it will be useful, but WITHOUT ANY
      13     WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
      14     FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
      15     more details.
      16  
      17     Under Section 7 of GPL version 3, you are granted additional
      18     permissions described in the GCC Runtime Library Exception, version
      19     3.1, as published by the Free Software Foundation.
      20  
      21     You should have received a copy of the GNU General Public License and
      22     a copy of the GCC Runtime Library Exception along with this program;
      23     see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
      24     <http://www.gnu.org/licenses/>.  */
      25  
      26  /* This is a Linux specific implementation of a mutex synchronization
      27     mechanism for libgomp.  This type is private to the library.  This
      28     implementation uses atomic instructions and the futex syscall.  */
      29  
      30  #include "wait.h"
      31  
      32  int gomp_futex_wake = FUTEX_WAKE | FUTEX_PRIVATE_FLAG;
      33  int gomp_futex_wait = FUTEX_WAIT | FUTEX_PRIVATE_FLAG;
      34  
      35  void
      36  gomp_mutex_lock_slow (gomp_mutex_t *mutex, int oldval)
      37  {
      38    /* First loop spins a while.  */
      39    while (oldval == 1)
      40      {
      41        if (do_spin (mutex, 1))
      42  	{
      43  	  /* Spin timeout, nothing changed.  Set waiting flag.  */
      44  	  oldval = __atomic_exchange_n (mutex, -1, MEMMODEL_ACQUIRE);
      45  	  if (oldval == 0)
      46  	    return;
      47  	  futex_wait (mutex, -1);
      48  	  break;
      49  	}
      50        else
      51  	{
      52  	  /* Something changed.  If now unlocked, we're good to go.  */
      53  	  oldval = 0;
      54  	  if (__atomic_compare_exchange_n (mutex, &oldval, 1, false,
      55  					   MEMMODEL_ACQUIRE, MEMMODEL_RELAXED))
      56  	    return;
      57  	}
      58      }
      59  
      60    /* Second loop waits until mutex is unlocked.  We always exit this
      61       loop with wait flag set, so next unlock will awaken a thread.  */
      62    while ((oldval = __atomic_exchange_n (mutex, -1, MEMMODEL_ACQUIRE)))
      63      do_wait (mutex, -1);
      64  }
      65  
      66  void
      67  gomp_mutex_unlock_slow (gomp_mutex_t *mutex)
      68  {
      69    futex_wake (mutex, 1);
      70  }