Date:	Wed, 11 Apr 2012 05:28:43 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arjan van de Ven <arjan@...radead.org>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Milton Miller <miltonm@....com>,
	"mingo@...e.hu" <mingo@...e.hu>, Tejun Heo <tj@...nel.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linux PM mailing list <linux-pm@...r.kernel.org>,
	nikunj@...ux.vnet.ibm.com
Subject: Re: CPU Hotplug rework

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

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