diff --git a/drivers/xen/events/events_2l.c b/drivers/xen/events/events_2l.c
index 64df919a2111..e1af5e093ff4 100644
--- a/drivers/xen/events/events_2l.c
+++ b/drivers/xen/events/events_2l.c
@@ -91,6 +91,8 @@ static void evtchn_2l_unmask(evtchn_port_t 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 {