xen-netback: Check for hotplug-status existence before watching

The logic in connect() is currently written with the assumption that
xenbus_watch_pathfmt() will return an error for a node that does not
exist.  This assumption is incorrect: xenstore does allow a watch to
be registered for a nonexistent node (and will send notifications
should the node be subsequently created).

As of commit 1f2565780 ("xen-netback: remove 'hotplug-status' once it
has served its purpose"), this leads to a failure when a domU
transitions into XenbusStateConnected more than once.  On the first
domU transition into Connected state, the "hotplug-status" node will
be deleted by the hotplug_status_changed() callback in dom0.  On the
second or subsequent domU transition into Connected state, the
hotplug_status_changed() callback will therefore never be invoked, and
so the backend will remain stuck in InitWait.

This failure prevents scenarios such as reloading the xen-netfront
module within a domU, or booting a domU via iPXE.  There is
unfortunately no way for the domU to work around this dom0 bug.

Fix by explicitly checking for existence of the "hotplug-status" node,
thereby creating the behaviour that was previously assumed to exist.

Signed-off-by: Michael Brown <mbrown@fensystems.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/xen-netback/xenbus.c b/drivers/net/xen-netback/xenbus.c
index a5439c1..d24b7a7 100644 (file)
--- a/drivers/net/xen-netback/xenbus.c
+++ b/drivers/net/xen-netback/xenbus.c
@@ -824,11 +824,15 @@ static void connect(struct backend_info *be)
        xenvif_carrier_on(be->vif);
 
        unregister_hotplug_status_watch(be);
-       err = xenbus_watch_pathfmt(dev, &be->hotplug_status_watch, NULL,
-                                  hotplug_status_changed,
-                                  "%s/%s", dev->nodename, "hotplug-status");
-       if (!err)
+       if (xenbus_exists(XBT_NIL, dev->nodename, "hotplug-status")) {
+               err = xenbus_watch_pathfmt(dev, &be->hotplug_status_watch,
+                                          NULL, hotplug_status_changed,
+                                          "%s/%s", dev->nodename,
+                                          "hotplug-status");
+               if (err)
+                       goto err;
                be->have_hotplug_status_watch = 1;
+       }
 
        netif_tx_wake_all_queues(be->vif->dev);