1  /* Copyright (C) 2005-2023 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