diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
index a8d182b584..53c17bd354 100644
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -178,6 +178,13 @@ int evtchn_allocate_port(struct domain *d, evtchn_port_t port)
             return -ENOMEM;
         bucket_from_port(d, port) = chn;
 
+        /*
+         * d->valid_evtchns is used to check whether the bucket can be
+         * accessed without the per-domain lock. Therefore,
+         * d->valid_evtchns should be seen *after* the new bucket has
+         * been setup.
+         */
+        smp_wmb();
         write_atomic(&d->valid_evtchns, d->valid_evtchns + EVTCHNS_PER_BUCKET);
     }
 
diff --git a/xen/include/xen/event.h b/xen/include/xen/event.h
index ce45298377..c35f4b23b6 100644
--- a/xen/include/xen/event.h
+++ b/xen/include/xen/event.h
@@ -107,7 +107,17 @@ void notify_via_xen_event_channel(struct domain *ld, int lport);
 
 static inline bool_t port_is_valid(struct domain *d, unsigned int p)
 {
-    return p < read_atomic(&d->valid_evtchns);
+    if ( p >= read_atomic(&d->valid_evtchns) )
+        return false;
+
+    /*
+     * The caller will usually access the event channel afterwards and
+     * may be done without taking the per-domain lock. The barrier is
+     * going in pair the smp_wmb() barrier in evtchn_allocate_port().
+     */
+    smp_rmb();
+
+    return true;
 }
 
 static inline struct evtchn *evtchn_from_port(struct domain *d, unsigned int p)