Re: [RFC PATCH 00/10] Cleanup ipi_call_lock[_irq]()/ipi_call_unlock[_irq]()

[Peter Zijlstra <peterz@infradead.org>]

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?

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.