libgomp/config/linux/mutex.c

   1 /* Copyright (C) 2005-2020 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 }