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Subject: Re: CPU Hotplug rework
From: Steven Rostedt <rostedt@goodmis.org>
To: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Cc: paulmck@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>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Milton Miller <miltonm@bga.com>, "mingo@elte.hu" <mingo@elte.hu>,
        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>
Date: Tue, 10 Apr 2012 20:09:59 -0400
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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,

-- Steve