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

[Emilio G. Cota]

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
> INSERT_{HEAD,TAIL,AFTER} and REMOVE_HEAD.

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.

Thanks,

                E.