quoted

+	/* Ignoring the return value from the unregister
+	 * call.  A failed return value simply says there
+	 * was no registered event.  Hence there will not
+	 * be any calls to process a switch event that
+	 * could cause a problem.
+	 */
+	spu_switch_event_unregister(&spu_active);

Better to use this here, to show the return value is ignored:

       (void)spu_switch_event_unregister(...)

Actually Linux style is to not add those casts.  But I don't think we
all the notifier_chain_unregister funtion should return error at all,
we should probably fix that up in the core code.