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

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Date:	Thu, 06 Feb 2014 21:35:51 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
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 02/06/2014 06:13 PM, Rafael J. Wysocki wrote:
> 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?
> 

Thank you! Same here, every patch depends only on the first patch in
the series. (Except the raid5 and the xen/balloon patches which don't
have any dependency).

But I'll be posting a v2 of this patchset soon with a rename of the
API..

Regards,
Srivatsa S. Bhat

>> ---
>>
>>  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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> 


-- 
Regards,
Srivatsa S. Bhat

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