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

From: Xiaoguang Chen
Date: Wed Jun 19 2013 - 04:31:52 EST

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. :)


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