linux-kernel - Re: [PATCH v9] Cpufreq: Fix governor start/stop race condition

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Message-ID: <51C16B97.3040304@marvell.com>
Date:	Wed, 19 Jun 2013 16:28:07 +0800
From:	Xiaoguang Chen <chenxg@...vell.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	"rjw@...k.pl" <rjw@...k.pl>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ning Jiang <njiang1@...vell.com>,
	Zhoujie Wu <zjwu@...vell.com>, Yilu Mao <ylmao@...vell.com>,
	"chenxg.marvell@...il.com" <chenxg.marvell@...il.com>
Subject: Re: [PATCH v9] Cpufreq: Fix governor start/stop race condition

On 06/19/2013 03:55 PM, Viresh Kumar wrote:
> On 19 June 2013 12:30, Xiaoguang Chen <chenxg@...vell.com> wrote:
>> Cpufreq governor's stop and start operation should be kept in sequence.
>> If not, there will be unexpected behavior, for example:
>>
>> There are 4 CPUs and policy->cpu=CPU0, CPU1/2/3 are linked to CPU0.
>> The normal sequence is as below:
>>
>> 1) Current governor is userspace, One application tries to set
>> governor to ondemand. It will call __cpufreq_set_policy in which it
>> will stop userspace governor and then start ondemand governor.
>>
>> 2) Current governor is userspace, Now CPU0 hotplugs in CPU3 (put CPU3 online),
>> It will call cpufreq_add_policy_cpu in which it first stops userspace
>> governor, and then starts userspace governor.
>>
>> Now if the sequence of above two cases interleaves, It becomes like below:
>>
>> 1) Application stops userspace governor
>> 2)                                      Hotplug stops userspace governor
>> 3) Application starts ondemand governor
>> 4)                                      Hotplug starts a governor
>>
>> In step 4, Hotplug is supposed to start userspace governor, But now
>> the governor has been changed by application to ondemand, So hotplug
>> starts ondemand governor again !!!!
>>
>> The solution is: Do not allow stop policy's governor multi-times.
>> Governor should only be stopped once for one policy, After it is stopped,
>> No other governor stop operation should be executed. also add one mutex to
>> protect __cpufreq_governor so governor operation can be kept in sequence.
>>
>> Signed-off-by: Xiaoguang Chen <chenxg@...vell.com>
>> Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
>> ---
>>   drivers/cpufreq/cpufreq.c | 24 ++++++++++++++++++++++++
>>   include/linux/cpufreq.h   |  1 +
>>   2 files changed, 25 insertions(+)
> All good now. Sorry for the noise :)
Thank you for the professional instructions. It makes the code more 
beautiful. :)

-- 
Thanks
Xiaoguang

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