[Xen-devel] RE: [PATCH][ioemu] Ignore the first watch fire in xenstore_process_dm_command_event()

["Cui, Dexuan" <dexuan.cui@xxxxxxxxx>]

Ian Jackson wrote:
> Cui, Dexuan writes ("[PATCH][ioemu] Ignore the first watch fire in
> xenstore_process_dm_command_event()"): 
>> After changeset 19679: ec2bc4b9fa32 (xend: hot-plug PCI devices at
>> boot-time) and the related ioemu commits, I notice there is a race
>> condition that could break the device assignment of hvm guest. ...
>> A straightforward thought is: we can ignore the first watch
>> fire. Please see the below patch.
> 
> I don't think this can possibly be right.  xenstore watches are cues
> to read the path at which the watch has fired, but number of watch
> firings is not conserved.  So the actual watch processing code should
> be idempotent.
> 
> When we start processing a command we should either (i) delete it from
> xenstore immediately, so that future watch triggerings either don't
> see the command or actually see genuine new invocations, 
Thanks for your suggestion.
I prefer this method. I'll post a new patch.

> or (ii) make
> a note that we're processing the command and arrange not to reprocess
> it until something (us or xend, probably) has deleted it.
> 
> I'm not sure exactly how the protocol works here ...
> 

Thanks,
-- Dexuan


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