Race condition on device add hanling in xl devd

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

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Hi,

I've found a race condition with handling new devices in driver domain.
xl devd calls hotplug script when new device is detected in xenstore. At
the same time, asynchronously, kernel create actual backend device (vif
in my case). In rare circumstances (especially under high system load)
it may happen that hotplug script is executed before kernel create the
device, and the hotplug script fails. When hotplug scripts were called
by udev, that race didn't existed as udev was informed about the device
by the kernel.
I'm not sure if the race applies to backend in dom0 - haven't happened
to me, but that doesn't really prove anything.

Can you remind me why in driver domain xl devd is used now, instead of
udev?

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)?

The setup:
 - Xen 4.8.4, but I believe the same would happen in xen-unstable
 - Linux 4.19.2 (dom0), Linux 4.14.74 (domU)
 - problem happens when starting a domU with network backend in another
   domU
 - happen more often when Xen run nested in KVM (-> slow), but happened
   to me on bare metal too

-- 
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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