diff options
Diffstat (limited to 'sysdeps/hppa/nptl/pthread_spin_unlock.c')
| -rw-r--r-- | sysdeps/hppa/nptl/pthread_spin_unlock.c | 26 |
1 files changed, 21 insertions, 5 deletions
diff --git a/sysdeps/hppa/nptl/pthread_spin_unlock.c b/sysdeps/hppa/nptl/pthread_spin_unlock.c index a183fed35e..6e4d71ecf1 100644 --- a/sysdeps/hppa/nptl/pthread_spin_unlock.c +++ b/sysdeps/hppa/nptl/pthread_spin_unlock.c @@ -1,4 +1,4 @@ -/* Copyright (C) 2005-2015 Free Software Foundation, Inc. +/* Copyright (C) 2005-2016 Free Software Foundation, Inc. This file is part of the GNU C Library. The GNU C Library is free software; you can redistribute it and/or @@ -20,9 +20,25 @@ int pthread_spin_unlock (pthread_spinlock_t *lock) { - int tmp = 0; - /* This should be a memory barrier to newer compilers */ - __asm__ __volatile__ ("stw,ma %1,0(%0)" - : : "r" (lock), "r" (tmp) : "memory"); + /* CONCURRENCTY NOTES: + + The atomic_exchange_rel synchronizes-with the atomic_exhange_acq in + pthread_spin_lock. + + On hppa we must not use a plain `stw` to reset the guard lock. This + has to do with the kernel compare-and-swap helper that is used to + implement all of the atomic operations. + + The kernel CAS helper uses its own internal locks and that means that + to create a true happens-before relationship between any two threads, + the second thread must observe the internal lock having a value of 0 + (it must attempt to take the lock with ldcw). This creates the + ordering required for a second thread to observe the effects of the + RMW of the kernel CAS helper in any other thread. + + Therefore if a variable is used in an atomic macro it must always be + manipulated with atomic macros in order for memory ordering rules to + be preserved. */ + atomic_exchange_rel (lock, 0); return 0; } |