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

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Date:	Wed, 19 Jun 2013 13:25:36 +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 v9] Cpufreq: Fix governor start/stop race condition

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 :)
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