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Re: [Xen-devel] [PATCH 3/5] xen: RCU/x86/ARM: discount CPUs that were idle when grace period started.

On Mon, 2017-08-14 at 14:54 +0100, Tim Deegan wrote:
> At 15:24 +0200 on 14 Aug (1502724279), Dario Faggioli wrote:
> > About the former... I'm not sure which check of rcp->cur you're
> > referring to. I think it's the one in rcu_check_quiescent_state(),
> > but
> > then, I'm not sure where to actually put the barrier...
> I mean whatever one causes the CPU to DTRT about the new grace
> period.
> AFAICT that's the one in __rcu_pending().  The important thing is
> that
> that read mustn't be allowed to happen before the write to the
> idle_cpumask.
Interestingly enough, someone seems to have had a very similar
discussion before:


>   I'd be inclined to put the barrier right after the
> cpumask_set_cpu() in rcu_idle_enter().
And with conclusions similar to these: :-)


I've no idea why they then didn't put an actual barrier in place. This
thread mention s390's ordering, as at the time, this mechanism was only
in used there, but they've not added one when making things general.

IAC, I'm fine putting down one. :-)

Thanks and Regards,
<<This happens because I choose it to happen!>> (Raistlin Majere)
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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