Re: [Xen-devel] Race condition on device add hanling in xl devd

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Dec 17, 2018 at 01:00:01PM +0100, Marek Marczykowski-Górecki wrote:
> On Mon, Dec 17, 2018 at 10:40:59AM +0100, Roger Pau Monné wrote:
> > On Sun, Dec 16, 2018 at 02:47:43AM +0100, Marek Marczykowski-Górecki wrote:
> > > A workaround could be implemented in hotplug script itself - wait for
> > > the device there. I'm not sure how proper solution could look like. Some
> > > synchronization between xl devd and the kernel (like xl devd monitoring
> > > uevents)?
> > 
> > There's already a synchronization mechanism, libxl waits for the
> > backend to switch to state 2 (XenbusStateInitWait) before running the
> > hotplug scripts [0].
> > 
> > Maybe netback sets state 2 before creating the backend device?
> > 
> > It looks to me like the backend needs to be sure everything needed by
> > the hotplug script is in place before switching to state 2.
> 
> I've done some more tests and I think that's something else. I've added
> a loop waiting for /sys/class/net/$vif to a hotplug script, but it timed
> out (5s). I don't see _any_ kernel messages related to the device.
> 
> It may be some bug in nested virtualization in KVM...

In your message you said you have also observed this behavior when
running on bare metal, so it's likely not related to nested
virtualization?

Thanks, Roger.

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