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SubjectRe: [PATCH 08/51] arm, hw-breakpoint: Fix CPU hotplug callback registration
Hi Srivatsa,

On Wed, Feb 05, 2014 at 10:06:04PM +0000, 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).

Hmm, the code in question (for this patch) runs from an arch_initcall. How
can you generate CPU hotplug operations at that stage?

> 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 hw-breakpoint code in arm by using this latter form of callback
> registration.

I guess you introduce __register_cpu_notifier somewhere earlier in the
series, so it's best if you take this all via your tree.


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