Re: [PATCH 2/2] mm: page allocator: Do not drain per-cpu lists via IPI from page allocator context

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Thu, 2012-01-12 at 16:51 +0200, Gilad Ben-Yossef wrote:
> What I can't figure out is why we don't need  get/put_online_cpus()
> pair around each and every call
> to on_each_cpu everywhere? and if we do, perhaps making it a part of
> on_each_cpu is the way to go?
> 
> Something like:
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index f66a1b2..cfa3882 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -691,11 +691,15 @@ void on_each_cpu(void (*func) (void *info), void
> *info, int wait)
>  {
>         unsigned long flags;
> 
> +       BUG_ON(in_atomic());
> +
> +       get_online_cpus();
>         preempt_disable();

Your preempt_disable() here serializes against hotplug..

>         smp_call_function(func, info, wait);
>         local_irq_save(flags);
>         func(info);
>         local_irq_restore(flags);
>         preempt_enable();
> +       put_online_cpus();
>  }
>  EXPORT_SYMBOL(on_each_cpu); 

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