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SubjectRe: [RFC PATCH 00/10] Cleanup ipi_call_lock[_irq]()/ipi_call_unlock[_irq]()
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On Tue, 2012-05-29 at 10:28 +0200, Peter Zijlstra wrote:
> On Tue, 2012-05-29 at 15:15 +0800, Yong Zhang wrote:
> > As discussed with Srivatsa [1], it seems there is no need to keep
> > ipi_call_[un]lock_irq() when cpu bring-up/down. Because:
> >
> > 1) call_function.lock used in smp_call_function_many() is just to protect
> > call_function.queue and &data->refs, cpu_online_mask is outside of the
> > lock. And it's not necessary to protect cpu_online_mask,
> > because data->cpumask is pre-calculate and even if a cpu is brougt up
> > when calling arch_send_call_function_ipi_mask(), it's harmless because
> > validation test in generic_smp_call_function_interrupt() will take care
> > of it.
> >
> > 2) For cpu down issue, stop_machine() will guarantee that no concurrent
> > smp_call_fuction() is processing.
>
> But that lock was only taken around setting a cpu online, so the offline
> case is pretty much irrelevant for these patches, right?

Ah, I see, some archs also did it on offline.

> That said, is there an alternative to stop_machine on the down side?
>
> I guess flipping the cpu offline and then doing synchronize_sched()
> should suffice.



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