sysdeps/hppa/nptl/pthread_spin_unlock.c

   1 /* Copyright (C) 2005-2017 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library.  If not, see
  16    <http://www.gnu.org/licenses/>.  */
  17
  18 #include "pthreadP.h"
  19
  20 int
  21 pthread_spin_unlock (pthread_spinlock_t *lock)
  22 {
  23   /* CONCURRENCTY NOTES:
  24
  25      The atomic_exchange_rel synchronizes-with the atomic_exhange_acq in
  26      pthread_spin_lock.
  27
  28      On hppa we must not use a plain `stw` to reset the guard lock.  This
  29      has to do with the kernel compare-and-swap helper that is used to
  30      implement all of the atomic operations.
  31
  32      The kernel CAS helper uses its own internal locks and that means that
  33      to create a true happens-before relationship between any two threads,
  34      the second thread must observe the internal lock having a value of 0
  35      (it must attempt to take the lock with ldcw).  This creates the
  36      ordering required for a second thread to observe the effects of the
  37      RMW of the kernel CAS helper in any other thread.
  38
  39      Therefore if a variable is used in an atomic macro it must always be
  40      manipulated with atomic macros in order for memory ordering rules to
  41      be preserved.  */
  42   atomic_exchange_rel (lock, 0);
  43   return 0;
  44 }