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

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Message-ID: <CAKohpoky1-hj7cwnKc58svh8Z1Yas1=q4uJEK8mWcb56uqAcng@mail.gmail.com>
Date:	Wed, 6 Aug 2014 13:40:30 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Saravana Kannan <skannan@...eaurora.org>
Cc:	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>
Subject: Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem to
 be held for duration of changing governors [v2]

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?

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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