Re: [PATCH 01/51] CPU hotplug: Provide lockless versions of callback registration functions

[Toshi Kani]

On Tue, 2014-02-11 at 22:48 +0530, Gautham R Shenoy wrote:
> On Tue, Feb 11, 2014 at 09:33:56AM -0700, Toshi Kani wrote:
> > 
> > I agree that introducing a reader-writer semaphore allows concurrent
> > executions.  Adding yet another hotplug lock is a bit unfortunate,
> > though.
> > 
> 
> I agree with this last part. We already have enough locks for
> cpu-hotplug. Another one sounds one too many!!
> 
> 
> > This may be a dumb question, but can't we simply do this way?
> > 
> >         get_online_cpus();
> > 
> >         for_each_online_cpu(cpu)
> >                 init_cpu(cpu);
> > 
> >         put_online_cpus();
> >
>       -------- Someone chooses to hotplug a cpu here ------
>       -------- But this subsystem might miss out on knowing
>                about it since it hasn't registered its
>                notifier yet! 
> 
> >         register_cpu_notifier(&foobar_cpu_notifier);


How about this?  foo_cpu_notifier returns NOP when foo_notifier_ready is
false.

        register_cpu_notifier(&foobar_cpu_notifier);

        get_online_cpus();

        for_each_online_cpu(cpu)
                init_cpu(cpu);

	foo_notifier_ready = true;

        put_online_cpus();

Thanks,
-Toshi

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