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SubjectRe: [PATCH v3 00/52] CPU hotplug: Fix issues with callback registration
On Tue, 11 Mar 2014 02:03:52 +0530 "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com> wrote:

> Hi,
>
> Many subsystems and drivers have the need to register CPU hotplug callbacks
> from their init routines and also perform initialization for the CPUs that are
> already online. But unfortunately there is no race-free way to achieve this
> today.
>
> For example, consider this piece of code:
>
> get_online_cpus();
>
> for_each_online_cpu(cpu)
> init_cpu(cpu);
>
> register_cpu_notifier(&foobar_cpu_notifier);
>
> put_online_cpus();
>
> This is not safe because there is a possibility of an ABBA deadlock involving
> the cpu_add_remove_lock and the cpu_hotplug.lock.
>
> CPU 0 CPU 1
> ----- -----
>
> Acquire cpu_hotplug.lock
> [via get_online_cpus()]
>
> CPU online/offline operation
> takes cpu_add_remove_lock
> [via cpu_maps_update_begin()]
>
> Try to acquire
> cpu_add_remove_lock
> [via register_cpu_notifier()]
>
> CPU online/offline operation
> tries to acquire cpu_hotplug.lock
> [via cpu_hotplug_begin()]

Can't we fix this by using a different (ie: new) lock to protect
cpu_chain?



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