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

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

On Sun, Dec 16, 2018 at 02:47:43AM +0100, Marek Marczykowski-Górecki wrote:
> 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?

udev is Linux specific, while the current code works for Linux, NetBSD
and FreeBSD.

> 
> 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.

Thanks, Roger.

[0] 
http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=tools/libxl/libxl_device.c;h=a4a8e9ac323e9d3804d36573181c74b7b5c63bc6;hb=refs/heads/staging#l934

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