linux-kernel - Re: [PATCH V4 6/6] cpufreq: governor: Create and traverse list of policy

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Date:	Wed, 10 Feb 2016 00:10:53 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Rafael Wysocki <rjw@...ysocki.net>,
	Juri Lelli <juri.lelli@....com>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Saravana Kannan <skannan@...eaurora.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Michael Turquette <mturquette@...libre.com>,
	Steve Muckle <steve.muckle@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	dietmar.eggemann@....com,
	Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V4 6/6] cpufreq: governor: Create and traverse list of
 policy_dbs to fix lockdep

On Tue, Feb 9, 2016 at 4:31 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> An instance of 'struct dbs_data' can support multiple 'struct
> policy_dbs_info' instances. To traverse all policy_dbs supported by a
> dbs_data, create a list of policy_dbs within dbs_data.
>
> We can traverse this list now, instead of traversing the loop for all
> online CPUs in update_sampling_rate(), to solve the circular dependency
> lockdep reported by Juri (and verified by Shilpa) earlier:

In addition to the previous comment, here's my changelog for this patch:

"The dbs_data_mutex lock is currently used in two places.  First,
cpufreq_governor_dbs() uses it to guarantee mutual exlusion between
invocations of governor operations from the core.  Second, it is used
by ondemand governor's update_sampling_rate() to ensure the stability
of data structures walked by it.

The second usage is quite problematic, because update_sampling_rate()
is called from a governor sysfs attribute's ->store callback and
that leads to a deadlock scenario involving cpufreq_governor_exit()
which runs under dbs_data_mutex.  Thus it is better to rework
the code so update_sampling_rate() doesn't need to acquire
dbs_data_mutex.

To that end, rework update_sampling_rate() to walk a list of
policy_dbs objects supported by the dbs_data one it has been called
for (instead of walking cpu_dbs_info object for all CPUs).  The list
manipulation is protected with dbs_data->mutex which also is held
around the execution of update_sampling_rate(), it is not necessary to
hold dbs_data_mutex in that function any more."

Thanks,
Rafael