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:	Wed, 06 Aug 2014 06:09:41 -0400
From:	Prarit Bhargava <prarit@...hat.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	Saravana Kannan <skannan@...eaurora.org>,
	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>
Subject: Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem
 to be held for duration of changing governors [v2]



On 08/06/2014 04:10 AM, Viresh Kumar wrote:
> On 6 August 2014 03:36, Saravana Kannan <skannan@...eaurora.org> wrote:
>> Stephen and I looked into this. This is not a sysfs framework difference.
>> The reason we don't have this issue when we use global tunables is because
>> we add the attribute group to the cpufreq_global_kobject and that kobject
>> doesn't have a kobj_type ops similar to the per policy kobject. So,
>> read/write to those attributes do NOT go through the generic show/store ops
>> that wrap every other cpufreq framework attribute read/writes.
>>
>> So, none of those read/write do any kind of locking. They don't race with
>> POLICY_EXIT (because we remove the sysfs group first thing in POLICY_EXIT)
>> but might still race with START/STOPs (not sure, haven't looked closely
>> yet).
>>
>> For example, writing to sampling_rate of ondemand governor might cause a
>> race in update_sampling_rate(). It could race and happen between a STOP and
>> POLICY_EXIT (triggered by hotplug, gov change, etc).
> 
> This sounds good but I couldn't prove it. Doing this on my dual core exynos
> doesn't give me that crash report and it should?

Are you sure you're not seeing another lockdep warning?  That was my problem --
there was an xfs related lockdep warning which then resulted in lockdep being
disabled from that point on.

P.

> 
> diff --git a/drivers/cpufreq/exynos-cpufreq.c b/drivers/cpufreq/exynos-cpufreq.c
> index 1e0ec57..027b6f7 100644
> --- a/drivers/cpufreq/exynos-cpufreq.c
> +++ b/drivers/cpufreq/exynos-cpufreq.c
> @@ -139,7 +139,7 @@ static int exynos_cpufreq_cpu_init(struct
> cpufreq_policy *policy)
>  }
> 
>  static struct cpufreq_driver exynos_driver = {
> -       .flags          = CPUFREQ_STICKY | CPUFREQ_NEED_INITIAL_FREQ_CHECK,
> +       .flags          = CPUFREQ_STICKY |
> CPUFREQ_NEED_INITIAL_FREQ_CHECK | CPUFREQ_HAVE_GOVERNOR_PER_POLICY,
>         .verify         = cpufreq_generic_frequency_table_verify,
>         .target_index   = exynos_target,
>         .get            = cpufreq_generic_get,
> 
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