On Tue, Apr 13, 2021 at 04:25:12PM +0100, Michael Brown wrote:
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.
This change is wrong. The 'hotplug-status' node is created _only_ by a
hotplug script and done so when it's executed. When kernel waits for
hotplug script to be executed it waits for the node to _appear_, not
_change_. So, this change basically made the kernel not waiting for the
hotplug script at all.
