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

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Date:	Thu, 13 Jun 2013 14:46:31 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Xiaoguang Chen <chenxg@...vell.com>
Cc:	rjw@...k.pl, cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, njiang1@...vell.com,
	zjwu@...vell.com, ylmao@...vell.com, chenxg.marvell@...il.com
Subject: Re: [PATCH v5] cpufreq: fix governor start/stop race condition

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>
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