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Date:	Thu, 06 Feb 2014 21:34:00 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
CC:	Len Brown <lenb@...nel.org>, paulus@...ba.org, oleg@...hat.com,
	rusty@...tcorp.com.au, peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, mingo@...nel.org,
	paulmck@...ux.vnet.ibm.com, tj@...nel.org, walken@...gle.com,
	ego@...ux.vnet.ibm.com, linux@....linux.org.uk,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH 38/51] intel-idle: Fix CPU hotplug callback registration

On 02/06/2014 06:13 PM, Rafael J. Wysocki wrote:
> On Thursday, February 06, 2014 03:41:23 AM Srivatsa S. Bhat wrote:
>> Subsystems that want to register CPU hotplug callbacks, as well as perform
>> initialization for the CPUs that are already online, often do it as shown
>> below:
>>
>> 	get_online_cpus();
>>
>> 	for_each_online_cpu(cpu)
>> 		init_cpu(cpu);
>>
>> 	register_cpu_notifier(&foobar_cpu_notifier);
>>
>> 	put_online_cpus();
>>
>> This is wrong, since it is prone to ABBA deadlocks involving the
>> cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently
>> with CPU hotplug operations).
>>
>> Instead, the correct and race-free way of performing the callback
>> registration is:
>>
>> 	cpu_maps_update_begin();
>>
>> 	for_each_online_cpu(cpu)
>> 		init_cpu(cpu);
>>
>> 	/* Note the use of the double underscored version of the API */
>> 	__register_cpu_notifier(&foobar_cpu_notifier);
>>
>> 	cpu_maps_update_done();
>>
>>
>> Fix the intel-idle code by using this latter form of callback registration.
>>
>> Cc: Len Brown <lenb@...nel.org>
>> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
>> Cc: linux-pm@...r.kernel.org
>> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
> 
> This looks good to me.  Len, what do you think?
> 

Thanks a lot Rafael!

> Srivatsa, how does it depend on the rest of your series?
> 

It depends only on the first patch in the series:
http://article.gmane.org/gmane.linux.kernel/1641640

But don't take this patch yet, we are discussing a possible rename
of the function cpu_maps_update_begin()/done(). So I'll post a v2
after the name is finalized.

Thank you!

Regards,
Srivatsa S. Bhat





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