linux-kernel - Re: [PATCH 43/51] hwmon, via-cputemp: Fix CPU hotplug callback registration

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Message-ID: <20140206004431.GB1308@roeck-us.net>
Date:	Wed, 5 Feb 2014 16:44:31 -0800
From:	Guenter Roeck <linux@...ck-us.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, Jean Delvare <jdelvare@...e.de>,
	lm-sensors@...sensors.org
Subject: Re: [PATCH 43/51] hwmon, via-cputemp: Fix CPU hotplug callback
 registration

On Thu, Feb 06, 2014 at 03:42:19AM +0530, 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 hwmon via-cputemp code by using this latter form of callback
> registration.
> 
> Cc: Jean Delvare <jdelvare@...e.de>
> Cc: Guenter Roeck <linux@...ck-us.net>
> Cc: lm-sensors@...sensors.org
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>

Applied.

Thanks,
Guenter
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