kref: fix CPU ordering with respect to krefs

some atomic operations are only atomic, not ordered. Thus a CPU is allowed
to reorder memory references to an object to before the reference is
obtained. This fixes it.
Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

lib/kref.c

@@ -21,6 +21,7 @@
 void kref_init(struct kref *kref)
 {
 	atomic_set(&kref->refcount,1);
+	smp_mb();
 }
 
 /**
@@ -31,6 +32,7 @@ void kref_get(struct kref *kref)
 {
 	WARN_ON(!atomic_read(&kref->refcount));
 	atomic_inc(&kref->refcount);
+	smp_mb__after_atomic_inc();
 }
 
 /**