diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 3987132..a730855 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -193,8 +193,10 @@
 
 	atomic_sub(map->count, &pages_mapped);
 
-	if (map->notify.flags & UNMAP_NOTIFY_SEND_EVENT)
+	if (map->notify.flags & UNMAP_NOTIFY_SEND_EVENT) {
 		notify_remote_via_evtchn(map->notify.event);
+		evtchn_put(map->notify.event);
+	}
 
 	if (map->pages) {
 		if (!use_ptemod)
@@ -599,6 +601,8 @@
 	struct ioctl_gntdev_unmap_notify op;
 	struct grant_map *map;
 	int rc;
+	int out_flags;
+	unsigned int out_event;
 
 	if (copy_from_user(&op, u, sizeof(op)))
 		return -EFAULT;
@@ -606,6 +610,21 @@
 	if (op.action & ~(UNMAP_NOTIFY_CLEAR_BYTE|UNMAP_NOTIFY_SEND_EVENT))
 		return -EINVAL;
 
+	/* We need to grab a reference to the event channel we are going to use
+	 * to send the notify before releasing the reference we may already have
+	 * (if someone has called this ioctl twice). This is required so that
+	 * it is possible to change the clear_byte part of the notification
+	 * without disturbing the event channel part, which may now be the last
+	 * reference to that event channel.
+	 */
+	if (op.action & UNMAP_NOTIFY_SEND_EVENT) {
+		if (evtchn_get(op.event_channel_port))
+			return -EINVAL;
+	}
+
+	out_flags = op.action;
+	out_event = op.event_channel_port;
+
 	spin_lock(&priv->lock);
 
 	list_for_each_entry(map, &priv->maps, next) {
@@ -624,12 +643,22 @@
 		goto unlock_out;
 	}
 
+	out_flags = map->notify.flags;
+	out_event = map->notify.event;
+
 	map->notify.flags = op.action;
 	map->notify.addr = op.index - (map->index << PAGE_SHIFT);
 	map->notify.event = op.event_channel_port;
+
 	rc = 0;
+
  unlock_out:
 	spin_unlock(&priv->lock);
+
+	/* Drop the reference to the event channel we did not save in the map */
+	if (out_flags & UNMAP_NOTIFY_SEND_EVENT)
+		evtchn_put(out_event);
+
 	return rc;
 }