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

From: Paolo Bonzini
Subject: Re: [Qemu-ppc] [PATCH 3/3] qom: implement CPU list with an RCU QLIST
Date: Fri, 17 Aug 2018 19:53:40 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

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

Anyway, the only difference between non-RCU and RCU-protected update
macros should be the write barriers (atomic_rcu_set).

>> , 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.

You're right, I forgot about CPU_NEXT.


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