linux-kernel - Re: CPU Hotplug rework

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Message-ID: <1334102999.23924.232.camel@gandalf.stny.rr.com>
Date:	Tue, 10 Apr 2012 20:09:59 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	paulmck@...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 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


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