diff --git a/drivers/xen/events/events_2l.c b/drivers/xen/events/events_2l.c
index 5478c761cd418f21e610f4a972f70905d9819821..f026624898e7a3ce116b3ad8defae08259410ace 100644
--- a/drivers/xen/events/events_2l.c
+++ b/drivers/xen/events/events_2l.c
@@ -91,6 +91,8 @@ static void evtchn_2l_unmask(unsigned port)
 
 	BUG_ON(!irqs_disabled());
 
+	smp_wmb();	/* All writes before unmask must be visible. */
+
 	if (unlikely((cpu != cpu_from_evtchn(port))))
 		do_hypercall = 1;
 	else {