Re: [PATCH] xen-netback: Check for hotplug-status existence before watching

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Mon, May 10, 2021 at 07:47:01PM +0100, Michael Brown wrote:
> On 10/05/2021 19:32, Marek Marczykowski-Górecki wrote:
> > 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.
> 
> That doesn't sound plausible to me.  In the setup as you describe, how is
> the kernel expected to differentiate between "hotplug script has not yet
> created the node" and "hotplug script does not exist and will therefore
> never create any node"?

Is the later valid at all? From what I can see in the toolstack, it
always sets some hotplug script (if not specified explicitly - then
"vif-bridge"),

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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