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Date:	Thu, 06 Feb 2014 13:43:08 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Len Brown <lenb@...nel.org>
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, linux-pm@...r.kernel.org
Subject: Re: [PATCH 38/51] intel-idle: Fix CPU hotplug callback registration

On Thursday, February 06, 2014 03:41:23 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 intel-idle code by using this latter form of callback registration.
> 
> Cc: Len Brown <lenb@...nel.org>
> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> Cc: linux-pm@...r.kernel.org
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>

This looks good to me.  Len, what do you think?

Srivatsa, how does it depend on the rest of your series?

> ---
> 
>  drivers/idle/intel_idle.c |   12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
> index 8e1939f..716ee5a 100644
> --- a/drivers/idle/intel_idle.c
> +++ b/drivers/idle/intel_idle.c
> @@ -681,14 +681,19 @@ static int __init intel_idle_init(void)
>  	if (intel_idle_cpuidle_devices == NULL)
>  		return -ENOMEM;
>  
> +	cpu_maps_update_begin();
> +
>  	for_each_online_cpu(i) {
>  		retval = intel_idle_cpu_init(i);
>  		if (retval) {
> +			cpu_maps_update_done();
>  			cpuidle_unregister_driver(&intel_idle_driver);
>  			return retval;
>  		}
>  	}
> -	register_cpu_notifier(&cpu_hotplug_notifier);
> +	__register_cpu_notifier(&cpu_hotplug_notifier);
> +
> +	cpu_maps_update_done();
>  
>  	return 0;
>  }
> @@ -698,10 +703,13 @@ static void __exit intel_idle_exit(void)
>  	intel_idle_cpuidle_devices_uninit();
>  	cpuidle_unregister_driver(&intel_idle_driver);
>  
> +	cpu_maps_update_begin();
>  
>  	if (lapic_timer_reliable_states != LAPIC_TIMER_ALWAYS_RELIABLE)
>  		on_each_cpu(__setup_broadcast_timer, (void *)false, 1);
> -	unregister_cpu_notifier(&cpu_hotplug_notifier);
> +	__unregister_cpu_notifier(&cpu_hotplug_notifier);
> +
> +	cpu_maps_update_done();
>  
>  	return;
>  }
> 
> --
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-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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