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

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Message-ID: <CAFgnuDdzjyUQnXZGMCnwP2mXr7eQ9n-ZxOxxG4KWdgPMKOjEfw@mail.gmail.com>
Date:	Mon, 24 Jun 2013 14:04:50 +0800
From:	Xiaoguang Chen <chenxg.marvell@...il.com>
To:	Xiaoguang Chen <chenxg@...vell.com>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	"rjw@...k.pl" <rjw@...k.pl>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ning Jiang <njiang1@...vell.com>,
	Zhoujie Wu <zjwu@...vell.com>, Yilu Mao <ylmao@...vell.com>
Subject: Re: [PATCH v9] Cpufreq: Fix governor start/stop race condition

Hi, Rafael
When can this patch be merged?

2013/6/19 Xiaoguang Chen <chenxg@...vell.com>:
> On 06/19/2013 03:55 PM, Viresh Kumar wrote:
>>
>> 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 :)
>
> Thank you for the professional instructions. It makes the code more
> beautiful. :)
>
> --
> Thanks
> Xiaoguang
>
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