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

[Emilio G. Cota]

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.

Thanks,

                Emilio