[libvirt] [PATCH v2 4/5] qemu_hotplug: Fix a rare race condition when detaching a device twice

[Peter Krempa]

On Thu, Mar 14, 2019 at 14:56:48 +0100, Michal Privoznik wrote:
> On 3/14/19 2:18 PM, Peter Krempa wrote:
> > On Thu, Mar 14, 2019 at 13:22:38 +0100, Michal Privoznik wrote:

[...]

> > 
> > How can this be considered success? Also this introduces a possible
> > regression. The DEVICE_DELETED event should be fired only after the
> > device was entirely unplugged. Claiming success before seeing the event
> > can lead to another race when qemu deleted the device from the internal
> > list so that 'device_del' does not see it any more but did not finish
> > cleanup fully.
> > 
> > We need to start the '*Remove' handler only after the DEVICE_DELETED
> > event was received.
> 
> I beg to differ. If we were to report error here users would see the API
> failing with error "Device not found". So they'd run 'virsh dumpxml' only to
> find the device there. I don't find such behaviour sane. If one API tells me
> a devie is not there then another one shall not tell otherwise.

Well. The user semantics can be confusing here. What we can't allow
though is that some of the steps done in the qemuDomainRemove*Device
will fail because qemu will still have some internal reference to some
backend object. What I'd find more of a problem is that I'd try to
attach a similar device only to be told that it already exists.
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