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

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Date:	Thu, 06 Feb 2014 13:43:59 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	paulus@...ba.org, oleg@...hat.com, rusty@...tcorp.com.au,
	peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, mingo@...nel.org,
	paulmck@...ux.vnet.ibm.com, tj@...nel.org, walken@...gle.com,
	ego@...ux.vnet.ibm.com, linux@....linux.org.uk,
	linux-kernel@...r.kernel.org,
	Viresh Kumar <viresh.kumar@...aro.org>,
	cpufreq@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH 35/51] acpi-cpufreq: Fix CPU hotplug callback registration

On Thursday, February 06, 2014 03:40:53 AM 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_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>

Looks OK to me.  How does it depend on the rest of your series?

> ---
> 
>  drivers/cpufreq/acpi-cpufreq.c |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/cpufreq/acpi-cpufreq.c b/drivers/cpufreq/acpi-cpufreq.c
> index 18448a7..e2eb471 100644
> --- a/drivers/cpufreq/acpi-cpufreq.c
> +++ b/drivers/cpufreq/acpi-cpufreq.c
> @@ -907,15 +907,16 @@ static void __init acpi_cpufreq_boost_init(void)
>  
>  		acpi_cpufreq_driver.boost_supported = true;
>  		acpi_cpufreq_driver.boost_enabled = boost_state(0);
> -		get_online_cpus();
> +
> +		cpu_maps_update_begin();
>  
>  		/* Force all MSRs to the same value */
>  		boost_set_msrs(acpi_cpufreq_driver.boost_enabled,
>  			       cpu_online_mask);
>  
> -		register_cpu_notifier(&boost_nb);
> +		__register_cpu_notifier(&boost_nb);
>  
> -		put_online_cpus();
> +		cpu_maps_update_done();
>  	}
>  }
>  
> 
> --
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-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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