linux-kernel - Re: [PATCH 35/51] acpi-cpufreq: Fix CPU hotplug callback registration

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Date:	Fri, 7 Feb 2014 09:39:36 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Paul Mackerras <paulus@...ba.org>, oleg@...hat.com,
	rusty@...tcorp.com.au, Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>, walken@...gle.com,
	ego@...ux.vnet.ibm.com,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH 35/51] acpi-cpufreq: Fix CPU hotplug callback registration

On 6 February 2014 03:40, Srivatsa S. Bhat
<srivatsa.bhat@...ux.vnet.ibm.com> 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 acpi-cpufreq code by using this latter form of callback registration.
>
> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> Cc: cpufreq@...r.kernel.org
> Cc: linux-pm@...r.kernel.org
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>

Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
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