linux-kernel - Re: [PATCH v5] cpufreq: fix governor start/stop race condition

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Date:	Thu, 13 Jun 2013 18:03:12 +0800
From:	Xiaoguang Chen <chenxg.marvell@...il.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Xiaoguang Chen <chenxg@...vell.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, cpufreq@...r.kernel.org,
	linux-pm@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	njiang1@...vell.com, zjwu@...vell.com, ylmao@...vell.com
Subject: Re: [PATCH v5] cpufreq: fix governor start/stop race condition

2013/6/13 Viresh Kumar <viresh.kumar@...aro.org>:
> On 13 June 2013 14:31, Xiaoguang Chen <chenxg@...vell.com> wrote:
>> cpufreq governor stop and start should be kept in sequence.
>> If not, there will be unexpected behavior, for example:
>>
>> we have 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, it will
>> call cpufreq_add_policy_cpu. on which it first stops userspace
>> governor, and then starts userspace governor.
>>
>> Now if the sequence of above two cases interleaves, it becames
>> below sequence:
>>
>> 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 one policy's governor multi-times
>> Governor stop should only do once for one policy, after it is stopped,
>> no other governor stop should be executed. also add one mutext to
>> protect __cpufreq_governor so governor operation can be kept in sequence.
>>
>> Signed-off-by: Xiaoguang Chen <chenxg@...vell.com>
>> ---
>>  drivers/cpufreq/cpufreq.c | 24 ++++++++++++++++++++++++
>>  include/linux/cpufreq.h   |  1 +
>>  2 files changed, 25 insertions(+)
>
> I forgot to tell you earlier but please update changelog everytime you
> send a new version.
>
> Acked-by: Viresh Kumar <viresh.kumar@...aro.org>

Thanks
I'll pay attention next time :)

Xiaoguang
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