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

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

On Mon, Feb 25, 2019 at 12:14:02AM +0100, Marek Marczykowski-Górecki wrote:
> On Mon, Dec 17, 2018 at 05:09:19PM +0100, Roger Pau Monné wrote:
> > On Mon, Dec 17, 2018 at 02:42:23PM +0000, Paul Durrant wrote:
> > > I suspect I must be remembering a XenServer-specific hack^Wpatch then. 
> > > I'd have to dig... it's been a while since I messed with the netif state 
> > > model, which is of course different the blkif state model.
> > 
> > Quite likely. With udev scripts is was feasible to only execute
> > hotplug scripts for vifs with an attached frontend.
> > 
> > With libxl this is not possible, since hotplug scripts are run during
> > domain creation, at which point the guest is completely paused.
> > 
> > I'm not that familiar with bridges and vifs, but maybe the vifs status
> > can be set to offline until there's a frontend attached in order to
> > reduce the bridge distributor load? (if that's not already the case).
> 
> I've found was the problem, and with some definition of "race condition"
> it could be named this way.
> The problem is that for some reason xenstore watch on device add
> sometimes does not fire in xl devd. But then, when libxl in dom0
> timeouts and remove the device, the xenstore watch in xl devd fire and
> hotplug script is called. At this point device is already gone, so
> it fails. xl devd then quickly calls hotplug script the second time, for
> device removal.
> 
> I have no idea why this xenstore watch do not fire, but triggering a
> no-op write into watched path (to trigger the watch again) workarounds
> the problem. I use a xenstore watch in dom0 for that[1] - which works.
> I suspect something related to KVM nested virtualization (lost
> interrupt?)...

That's very weird, could you try to run xenstored in dom0 with trace
enabled [0] in order to try to figure out what's happening?

I assume this only happens when running nested in KVM?

Thanks, Roger.

[0] 
http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=tools/hotplug/Linux/launch-xenstore.in

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