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

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Date:   Tue, 11 May 2021 12:40:54 +0200
From:   Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
To:     "Durrant, Paul" <pdurrant@...zon.co.uk>
Cc:     Michael Brown <mbrown@...systems.co.uk>,
        "paul@....org" <paul@....org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>
Subject: Re: [PATCH] xen-netback: Check for hotplug-status existence before
 watching

On Tue, May 11, 2021 at 07:06:55AM +0000, Durrant, Paul wrote:
> > -----Original Message-----
> > From: Marek Marczykowski-Górecki <marmarek@...isiblethingslab.com>
> > Sent: 10 May 2021 20:43
> > To: Michael Brown <mbrown@...systems.co.uk>; paul@....org
> > Cc: paul@....org; xen-devel@...ts.xenproject.org; netdev@...r.kernel.org; wei.liu@...nel.org; Durrant,
> > Paul <pdurrant@...zon.co.uk>
> > Subject: RE: [EXTERNAL] [PATCH] xen-netback: Check for hotplug-status existence before watching
> > 
> > On Mon, May 10, 2021 at 08:06:55PM +0100, Michael Brown wrote:
> > > If you have a suggested patch, I'm happy to test that it doesn't reintroduce
> > > the regression bug that was fixed by this commit.
> > 
> > Actually, I've just tested with a simple reloading xen-netfront module. It
> > seems in this case, the hotplug script is not re-executed. In fact, I
> > think it should not be re-executed at all, since the vif interface
> > remains in place (it just gets NO-CARRIER flag).
> > 
> > This brings a question, why removing hotplug-status in the first place?
> > The interface remains correctly configured by the hotplug script after
> > all. From the commit message:
> > 
> >     xen-netback: remove 'hotplug-status' once it has served its purpose
> > 
> >     Removing the 'hotplug-status' node in netback_remove() is wrong; the script
> >     may not have completed. Only remove the node once the watch has fired and
> >     has been unregistered.
> > 
> > I think the intention was to remove 'hotplug-status' node _later_ in
> > case of quickly adding and removing the interface. Is that right, Paul?
> 
> The removal was done to allow unbind/bind to function correctly. IIRC before the original patch doing a bind would stall forever waiting for the hotplug status to change, which would never happen.

Hmm, in that case maybe don't remove it at all in the backend, and let
it be cleaned up by the toolstack, when it removes other backend-related
nodes?

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

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