Re: [PATCH v2 38/52] drivers/base/topology.c: Fix CPU hotplug callback registration

[Greg Kroah-Hartman]

On Fri, Feb 14, 2014 at 01:27:51PM +0530, 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_notifier_register_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_notifier_register_done();
> 
> 
> Fix the topology code by using this latter form of callback registration.
> 
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>

Acked-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
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