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Date: Tue, 11 Mar 2014 15:07:33 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
Subject: Re: [PATCH v3 00/52] CPU hotplug: Fix issues with callback
 registration
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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?