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Date: Tue, 10 Apr 2012 17:28:36 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Arjan van de Ven <arjan@infradead.org>,
        "rusty@rustcorp.com.au" <rusty@rustcorp.com.au>,
        "Rafael J. Wysocki" <rjw@sisk.pl>,
        Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>,
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        Tejun Heo <tj@kernel.org>, KOSAKI Motohiro <kosaki.motohiro@gmail.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Linux PM mailing list <linux-pm@vger.kernel.org>
Subject: Re: CPU Hotplug rework
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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