sysdeps/hppa/nptl/pthread_spin_unlock.c

   1 /* Copyright (C) 2005-2024 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    <https://www.gnu.org/licenses/>.  */
  17
  18 #include "pthreadP.h"
  19 #include <shlib-compat.h>
  20
  21 int
  22 __pthread_spin_unlock (pthread_spinlock_t *lock)
  23 {
  24   /* CONCURRENCY NOTES:
  25
  26      The atomic_exchange_release synchronizes-with the atomic_exhange_acq in
  27      pthread_spin_lock.
  28
  29      On hppa we must not use a plain `stw` to reset the guard lock.  This
  30      has to do with the kernel compare-and-swap helper that is used to
  31      implement all of the atomic operations.
  32
  33      The kernel CAS helper uses its own internal locks and that means that
  34      to create a true happens-before relationship between any two threads,
  35      the second thread must observe the internal lock having a value of 0
  36      (it must attempt to take the lock with ldcw).  This creates the
  37      ordering required for a second thread to observe the effects of the
  38      RMW of the kernel CAS helper in any other thread.
  39
  40      Therefore if a variable is used in an atomic macro it must always be
  41      manipulated with atomic macros in order for memory ordering rules to
  42      be preserved.  */
  43   atomic_exchange_release (lock, 0);
  44   return 0;
  45 }
  46 versioned_symbol (libc, __pthread_spin_unlock, pthread_spin_unlock,
  47                   GLIBC_2_34);
  48
  49 #if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_2, GLIBC_2_34)
  50 compat_symbol (libpthread, __pthread_spin_unlock, pthread_spin_unlock,
  51                GLIBC_2_2);
  52 #endif