Re: [PATCH v2 1/2] xen: fix a (latent) cpupool-related race during domain destroy

[Juergen Gross <jgross@suse.com>]

On 15/07/16 13:52, Dario Faggioli wrote:
> On Fri, 2016-07-15 at 12:36 +0200, Juergen Gross wrote:
>> On 15/07/16 12:14, Dario Faggioli wrote:
>>> In particular, I'm probably not fully understanding, from that
>>> commit
>>> changelog, what is the set of operations/command that I should run
>>> to
>>> check whether or not I reintroduced the issue back.
>> You need to create a domain in a cpupool and destroy it again while
>> some dom0 process still is holding a reference to it (resulting in a
>> zombie domain). Then try to destroy the cpupool.
>>
> Ah, I see. I wasn't get the fact that it needed to be a zombie domain
> from anywhere.
> 
>>> What am I missing?
>> The domain being a zombie domain might change the picture. Moving it
>> to
>> cpupool0 was failing before my patch and it might do so again with
>> your
>> patch applied.
>>
> Mmmm... I don't immediately see the reason why moving a zombie domain
> fails either, but I guess I'll have to try.

Searching through the history I found commit
934e7baa6c12d19cfaf24e8f8e27d6c6a8b8c5e4 which might has removed the
problematic condition (cpupool->n_dom being non-zero while d->cpupool
was NULL already).

> But then, correct me if I'm wrong, the situation is like this:
>  - right now there's a (potential) race between domain's scheduling 
>    data destruction and domain removal from a cpupool;
>  - with my patch, the race goes away, but we risk not being able to 
>    destroy a cpupool with a zombie domain in it.

This one has been observed.

I do remember the following critical cases:

- removing a cpupool with a zombie domain
- shutting down the system with a domain in a cpupool
- not sure about the combination of both cases (shutting down with
  zombie domain in a cpupool): is this even possible without another
  bug in the hypervisor or dom0?

> Therefore, I still think this patch is correct, but I'm up for
> investigating further and finding a way to solve the "zombie in
> cpupool" issue as well.

I'm not saying your patch is wrong. I just wanted to give you a hint
about the history of the stuff you are changing. :-)

If it is working I'd really prefer it over the current situation.


Juergen

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