Re: CPU Hotplug rework

From: Paul E. McKenney
Date: Wed Apr 11 2012 - 08:29:00 EST


On Wed, Apr 11, 2012 at 11:32:57AM +0530, Srivatsa S. Bhat wrote:
> On 04/11/2012 06:30 AM, Paul E. McKenney wrote:
>
> > On Tue, Apr 10, 2012 at 08:37:18PM -0400, Steven Rostedt wrote:
> >> On Tue, 2012-04-10 at 17:28 -0700, Paul E. McKenney wrote:
> >>
> >>>> Just to throw out the stupid silly approach.
> >>>>
> >>>> What about creating a "__register_cpu_notifier()" that just does:
> >>>>
> >>>> int __ref __register_cpu_notifier(struct notifier_block *nb)
> >>>> {
> >>>> return raw_notifier_chain_register(&cpu_chain, nb);
> >>>> }
> >>>>
> >>>> Also making cpu_maps_update_begin/done() global (and probably rename
> >>>> them).
> >>
> >> I just noticed that the cpu_maps_update_begin/done() are already global.
> >>
> >>>>
> >>>> and then in the above code do:
> >>>>
> >>>> cpu_maps_update_begin();
> >>>> __register_cpu_notifier(nb);
> >>>> do_setup();
> >>>> cpu_maps_update_done();
> >>>>
> >>>>
>
>
> Wow! Believe it or not, this is precisely the crux of the approach I was
> suggesting all along!! :-) Just that when put to code, it looked slightly
> different than this.. Sorry for not being clear.
>
> So here is what I proposed, in a simplified form:
>
> Modify the existing register_cpu_notifier() to this (by possibly giving
> it a different name):
>
> int __ref register_cpu_notifier(struct notifier_block *nb,
> int (*do_setup)(void))
> {
> int ret;
>
> cpu_maps_update_begin();
> ret = raw_notifier_chain_register(&cpu_chain, nb);
> do_setup();
> cpu_maps_update_done();
>
> return ret;
> }
>
> and then, in the caller, do:
>
> register_cpu_notifier(nb, do_setup);
>
> If the caller doesn't need any such extra setup, just do:
>
> register_cpu_notifier(nb, NULL);
>
>
> Of course, register_cpu_notifier() should handle NULL properly.
> (My patch [1] handles it, along with some other special cases.)
>
> That's it!
>
> Also, it is to be noted that cpu_maps_update_begin/done() are global, but
> not exported symbols - so modules can't use them. With the above approach,
> we need not make them exported symbols, since the caller need not care about
> these locks at all.
>
> >>>> Just saying,
> >>>
> >>> That does have some attractive properties, now that you mention it. ;-)
> >>
> >> Which property? Stupid or Silly ;-)
> >
> > As with any piece of software, no matter how small, both. ;-)
> >
> > Of course, __register_cpu_notifier() would need lockdep checking to make
> > sure that it wasn't called without the benefit of cpu_maps_update_begin().
>
>
> Not with my approach ;-) Its all automatically handled :-)

Good point, looks good!

Thanx, Paul

> > I might be missing something, but as long as that was in place, seems
> > like it is a lot simpler and easier to use than the alternatives that
> > Srivatsa and I were kicking around.
> >
>
>
> Hehe :-) Thanks for simplifying things, Steve!
>
>
> [1]. https://lkml.org/lkml/2012/3/1/39
>
> Regards,
> Srivatsa S. Bhat

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/