Re: Race condition on device add hanling in xl devd

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

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On Mon, Dec 17, 2018 at 01:18:55PM +0100, Roger Pau Monné wrote:
> 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?

Yes, but on bare metal is so hard to reproduce (like 0.1% or even less
startups), I'm not really sure if that was the same problem, as the
problem doesn't leave that much logs...

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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