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From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <19faab37-6728-b649-488e-c6dc2e5be052@redhat.com>
Date: Tue, 14 Aug 2018 08:26:54 +0200
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Subject: Re: [Qemu-devel] [PATCH 3/3] qom: implement CPU list with an RCU
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To: "Emilio G. Cota" <cota@braap.org>, qemu-devel@nongnu.org
Cc: Peter Crosthwaite <crosthwaite.peter@gmail.com>, Richard Henderson <rth@twiddle.net>, David Gibson <david@gibson.dropbear.id.au>, Alexander Graf <agraf@suse.de>, Riku Voipio <riku.voipio@iki.fi>, Laurent Vivier <laurent@vivier.eu>, qemu-ppc@nongnu.org

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, 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.

Thanks,

Paolo