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

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Message-ID: <9edd6873034f474baafd70b1df693001@EX13D32EUC003.ant.amazon.com>
Date:   Tue, 11 May 2021 12:46:38 +0000
From:   "Durrant, Paul" <pdurrant@...zon.co.uk>
To:     Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
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

> -----Original Message-----
> From: Marek Marczykowski-Górecki <marmarek@...isiblethingslab.com>
> Sent: 11 May 2021 11:45
> To: Durrant, Paul <pdurrant@...zon.co.uk>
> Cc: Michael Brown <mbrown@...systems.co.uk>; paul@....org; xen-devel@...ts.xenproject.org;
> netdev@...r.kernel.org; wei.liu@...nel.org
> Subject: RE: [EXTERNAL] [PATCH] xen-netback: Check for hotplug-status existence before watching
> 
> On Tue, May 11, 2021 at 12:40:54PM +0200, Marek Marczykowski-Górecki wrote:
> > 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?
> 
> No, unbind/bind _does_ require hotplug script to be called again.
> 

Yes, sorry I was misremembering. My memory is hazy but there was definitely a problem at the time with leaving the node in place.

> When exactly vif interface appears in the system (starts to be available
> for the hotplug script)? Maybe remove 'hotplug-status' just before that
> point?
> 

I really can't remember any detail. Perhaps try reverting both patches then and check that the unbind/rmmod/modprobe/bind sequence still works (and the backend actually makes it into connected state).

  Paul