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Re: [Qemu-ppc] [PATCH 3/3] qom: implement CPU list with an RCU QLIST

From: Emilio G. Cota
Subject: Re: [Qemu-ppc] [PATCH 3/3] qom: implement CPU list with an RCU QLIST
Date: Tue, 14 Aug 2018 20:34:53 -0400
User-agent: Mutt/1.9.4 (2018-02-28)

On Tue, Aug 14, 2018 at 08:26:54 +0200, Paolo Bonzini wrote:
> On 13/08/2018 18:38, Emilio G. Cota wrote:
> > Fix it by implementing the CPU list as an RCU QLIST. This requires
> > a little bit of extra work to insert CPUs at the tail of
> > the list and to iterate over the list in reverse order (see previous patch).
> > 
> > One might be tempted to just insert new CPUs at the head of the list.
> > However, I think this might lead to hard-to-debug issues, since it is
> > possible that callers are assuming that CPUs are inserted at the tail
> > (just like spapr code did in the previous patch). So instead of auditing
> > all callers, this patch simply keeps the old behaviour.
> Why not add an RCU_QSIMPLEQ

Because we can't atomically update both head.last and item.next.

> , or even use an array since the quadratic
> behavior should not be an issue?  The advantage of the array is that
> reverse iteration becomes trivial.

I just gave this a shot. IMO implementing CPU_NEXT based on the
array is too ugly to live.

I think the poor man's tail insert + the CPU_FOREACH_REVERSE
are a better compromise.



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