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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: Sun, 19 Aug 2018 05:06:00 -0400
User-agent: Mutt/1.9.4 (2018-02-28)

On Fri, Aug 17, 2018 at 19:53:40 +0200, Paolo Bonzini wrote:
> On 15/08/2018 02:34, Emilio G. Cota wrote:
> > 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.
> Why do you need that?  Updates are protected by a mutex in RCU-protected
> lists, it is not necessary to make them atomic.  Also, feel free to
> implement a subset of the write-side macros, for example only

Yes I got confused, was thinking you wanted to support the
reverse traversal (simpleq doesn't even have the reverse pointers,
so I don't know how I reached that conclusion).

v2 incoming.



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