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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>
This commit is contained in:
Michael Brown 2021-04-13 16:25:12 +01:00 committed by David S. Miller
parent 38ec4944b5
commit 2afeec08ab
1 changed files with 8 additions and 4 deletions
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12
drivers/net/xen-netback/xenbus.c
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@ -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);