[Sheepdog] [PATCH 1/2] sheep: sheep: handle node change event first

[MORITA Kazutaka]

At Sun, 01 Apr 2012 15:15:16 +0800,
Liu Yuan wrote:
> 
> On 04/01/2012 02:46 PM, MORITA Kazutaka wrote:
> 
> > I don't think the approach can guarantee that all nodes process the
> > membership events in the same order because some nodes may have
> > outstanding I/Os but the others may not.
> > 
> 
> 
> I think we need to clarify that there are two kind of outstanding IO
> requests
> 	a) queued in the cpg queue, not issued yet
> 	b) already issued, being processed on the fly
> 
> I don't think the a) cause any problem, because,
> 
> when we get a confchg event, considering there are outstanding IO
> reuqests in the cpg queue, if we flush them before confchg, most of them
> will risk to be failed or wasteful operation because
> 	1) in case some nodes are down and they don't get a fresh knowledge of
> that, so the operation targeted on those down nodes are doomed to be failed
> 	2) in case new nodes join, the targeted object location might be
> migrated already
> 
> and only b) might do.

Yes, I meant b).

> 
> Currently, with my path, 2) will lead the confchg to be queued instead
> of being processed immediately. The queuing of more than one confchg
> will cause problem.
> 
> So this situation would be healed by awaiting infly IOs completion(spin
> lock or aync notification), then insert confchg at the head of cpg
> queue, that guaranteed to be processed.

We cannot wait in start_cpg_event_work() because it is in the main
thread.

Thanks,

Kazutaka