linux-kernel - Re: Regression on cpufreq in v3.12-rc1

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Message-ID: <523C66C5.9040008@linux.vnet.ibm.com>
Date:	Fri, 20 Sep 2013 20:46:21 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	Linus Walleij <linus.walleij@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Regression on cpufreq in v3.12-rc1

On 09/20/2013 03:05 PM, Viresh Kumar wrote:
> On 20 September 2013 14:19, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>> Hmm... Remember I told you last time that I have another way of fixing
>> it up, probably we need that now..
>>
>> I wanted to add another variable to reflect if a cpufreq_driver is registered
>> or not, and if not then return early from these routines..
>>
>> I will get that in now, please see if you can give it a try..
>>
>> But I am still surprised how are we reaching this place before your cpufreq
>> driver gets registered..
> 
> Once we know what's going on in your system, please test attached patch
> (Will send it separately once you have tested it):
> 
> commit 389fbc3c8ad7c339cd2d9572d73c355b7b967823
> Author: Viresh Kumar <viresh.kumar@...aro.org>
> Date:   Fri Sep 20 14:55:31 2013 +0530
> 
>     cpufreq: check cpufreq driver is valid and cpufreq isn't disabled
> in cpufreq_get()
> 
>     cpufreq_get() can be called from external drivers which might not
> be aware if
>     cpufreq driver is registered or not. And so we should actually
> check if cpufreq
>     driver is registered or not and also if cpufreq is active or
> disabled, at the
>     beginning of cpufreq_get().
> 
>     Otherwise call to lock_policy_rwsem_read() might hit BUG_ON(!policy).
> 
>     Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>

The patch looks reasonable to me. Earlier cpufreq_cpu_get() used to do the
necessary checks, but commit 6eed9404 removed the call to that function in
cpufreq_get(). So adding those checks back seem like the right fix to me.
Also, looking at commit 6eed9404, I think show() and store() also suffer
from a similar fate. So do you think we need to add these checks there as well?
I'm not sure, since I can't think of a situation in which show() or store()
can be invoked before the cpufreq-driver is registered.. or, is such a
situation possible with cpufreq_disabled()?

Regards,
Srivatsa S. Bhat

> ---
>  drivers/cpufreq/cpufreq.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 82ecbe3..db004a8 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -1460,6 +1460,9 @@ unsigned int cpufreq_get(unsigned int cpu)
>  {
>         unsigned int ret_freq = 0;
> 
> +       if (cpufreq_disabled() || !cpufreq_driver)
> +               return -ENOENT;
> +
>         if (!down_read_trylock(&cpufreq_rwsem))
>                 return 0;
> 

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