linux-kernel - Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem to be held for duration of changing governors [v2]

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Date:	Tue, 5 Aug 2014 06:29:12 -0000
From:	skannan@...eaurora.org
To:	"Viresh Kumar" <viresh.kumar@...aro.org>
Cc:	"Saravana Kannan" <skannan@...eaurora.org>,
	"Prarit Bhargava" <prarit@...hat.com>,
	"Stephen Boyd" <sboyd@...eaurora.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Lenny Szubowicz" <lszubowi@...hat.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"Robert SchÃ¶ne" 
	<robert.schoene@...dresden.de>
Subject: Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem
 to be held for duration of changing governors [v2]


Viresh Kumar wrote:
> On 5 August 2014 01:46, Saravana Kannan <skannan@...eaurora.org> wrote:
>> The problem is when between one thread trying to cat a governor's file
>> (say,
>> sampling_rate) vs the governor getting a POLICY_EXIT.
>
> I don't see two threads racing against each other here. Simply changing
> the governor from conservative->ondemand creates this.
>
> Or is it that the kernel is detecting two different orders of taking lock?
>
> But during governor change, isn't the sysfs lock taken first as we are
> storing a value to "scaling_governor"? So, isn't this a sysfs lock first
> in all cases?
>
> In short, I am still unclear about the *exact* problem here.
>
>> Could you please look at my policy free/remove patches? If you can do
>> that,
>> I can work on a fix for this. It might also be simpler to fix after my
>> patch
>> series (not sure, might be).
>
> I had an overall look of those on the day you posted them, but haven't
> commented yet as was going away..
>
> There is no way those can land in 3.17-rc1 atleast and so we still have
> some time to get them pushed..
>
> Anyway, they are my number two priority and the number one is this bug,
> which we have to fix in stable kernels as well. So, a dependency on your
> series wouldn't work..

Sigh... ok. I too will try to fix this one. I already have something in
mind for this.

Looks like Srivatsa has gone off the grid too. I'm hoping at least one of
you can do a review of my series. Come on guys, not everyone has to work
on the same patch/issue. :-(

-Saravana

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