powerpc: Add smp_mb()s to arch_spin_unlock_wait()

Similar to the previous commit which described why we need to add a
barrier to arch_spin_is_locked(), we have a similar problem with
spin_unlock_wait().

We need a barrier on entry to ensure any spinlock we have previously
taken is visibly locked prior to the load of lock->slock.

It's also not clear if spin_unlock_wait() is intended to have ACQUIRE
semantics. For now be conservative and add a barrier on exit to give it
ACQUIRE semantics.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

diff --git a/arch/powerpc/lib/locks.c b/arch/powerpc/lib/locks.c
index 0c9c8d7..170a034 100644 (file)
--- a/arch/powerpc/lib/locks.c
+++ b/arch/powerpc/lib/locks.c
@@ -70,12 +70,16 @@ void __rw_yield(arch_rwlock_t *rw)
 
 void arch_spin_unlock_wait(arch_spinlock_t *lock)
 {
+       smp_mb();
+
        while (lock->slock) {
                HMT_low();
                if (SHARED_PROCESSOR)
                        __spin_yield(lock);
        }
        HMT_medium();
+
+       smp_mb();
 }
 
 EXPORT_SYMBOL(arch_spin_unlock_wait);