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

[Xiaoguang Chen]

On 06/19/2013 03:55 PM, Viresh Kumar wrote:
> On 19 June 2013 12:30, Xiaoguang Chen <chenxg@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> > Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> > ---
> >   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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