Re: xen-netback hotplug-status regression bug

On 13/04/2021 11:48, Michael Brown wrote:

On 13/04/2021 08:12, Paul Durrant wrote:

quoted

quoted

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.

quoted

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.

Ok, so it sound like this was probably my misunderstanding of xenstore 
semantics in the first place (although I'm sure I remember watch 
registration failing for non-existent nodes at some point in the past... 
that may have been with a non-upstream version of oxenstored though).

Anyway... a reasonable fix would therefore be to read the node first and 
only register the watch if it does exist.

   Paul

Thanks,

Michael