linux-kernel - Re: CPU Hotplug rework

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Message-ID: <20120411002836.GM2428@linux.vnet.ibm.com>
Date:	Tue, 10 Apr 2012 17:28:36 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	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>
Subject: Re: CPU Hotplug rework

On Tue, Apr 10, 2012 at 08:09:59PM -0400, Steven Rostedt wrote:
> On Sat, 2012-04-07 at 01:22 +0530, Srivatsa S. Bhat wrote:
> 
> > Wrapping only do_setup() within get/put_online_cpus() wouldn't serve our
> > purpose, since the race with CPU Hotplug would still exist, just like
> > before. So, let's consider what happens when we wrap both the functions
> > within get/put_online_cpus():
> > 
> > 	get_online_cpus();
> > 	register_cpu_notifier(nb, mask);
> > 	do_setup(mask);
> > 	put_online_cpus();
> > 
> > Unfortunately this leads to an ABBA deadlock (see below).
> >  
> 
> 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).
> 
> and then in the above code do:
> 
> 	cpu_maps_update_begin();
> 	__register_cpu_notifier(nb);
> 	do_setup();
> 	cpu_maps_update_done();
> 
> 
> Just saying,

That does have some attractive properties, now that you mention it.  ;-)

							Thanx, Paul

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