Re: xen-netback hotplug-status regression bug

[Michael Brown <mcb30@xxxxxxxx>]

On 13/04/2021 08:12, Paul Durrant wrote:
> > If the frontend subsequently disconnects and reconnects (e.g. 
> > transitions through Closed->Initialising->Connected) then:
> > - Nothing recreates "hotplug-status"
> >
> > - When the frontend re-enters Connected state, connect() sets up a 
> > watch on "hotplug-status" again
> > - The callback hotplug_status_changed() is never triggered, and so the 
> > backend device never transitions to Connected state.
> That's not how I read it. Given that "hotplug-status" is removed by the 
> call to hotplug_status_changed() then the next call to connect() should 
> fail to register the watch and 'have_hotplug_status_watch' should be 0. 
> Thus backend_switch_state() should not defer the transition to 
> XenbusStateConnected in any subsequent interaction with the frontend.
Thank you for the reply.  I've tested and confirmed my initial 
hypothesis: the call to xenbus_watch_pathfmt() succeeds even if the node 
does not exist.
I confirmed this with ftrace using:

  cd /sys/kernel/debug/tracing
  echo function_graph > current_tracer
  echo set_backend_state > set_ftrace_filter
  echo xenbus_watch_pathfmt >> set_ftrace_filter
  echo register_xenbus_watch >> set_ftrace_filter
  echo xenbus_dev_fatal >> set_ftrace_filter

On the second time that the frontend transitions to Connected, this 
produced the trace:
  set_backend_state [xen_netback]() {
    register_xenbus_watch();
    register_xenbus_watch();
    xenbus_watch_pathfmt() {
      register_xenbus_watch();
    }
  }

which seems to confirm that the error path in xenbus_watch_path() is 
*not* taken, i.e. that the call to register_xenbus_watch() succeeded 
even though the node did not exist.

Other observations also seem to confirm this behaviour:

- Running "xenstore ls" in dom0 confirms that on the second frontend 
transition to Connected, the frontend state is indeed Connected (4) but 
the backend state remains in InitWait (2)
- Running "xenstore watch 
/local/domain/0/backend/vif/<domU>/0/hotplug-status" *before* starting 
the domU confirms that it is possible to create a watch on a node that 
does not (yet) exist, and that the watch *is* notified when the node is 
later created.
> Are you seeing the watch successfully re-registered even though the node 
> does not exist? Perhaps there has been a change in xenstore behaviour?
So, the TL;DR is that yes, the watch does successfully register even 
though the node does not exist.
From a quick look through the xenstored source, it looks as though the 
only check on the node name is the call to is_valid_nodename(), which 
seems to perform a syntactic validity check only.  I can't immediately 
find any commit that would have changed this behaviour.
Thanks,

Michael