linux-kernel - Re: [PATCH 2/5] cpufreq: governor: Create separate sysfs-ops

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Date:	Wed, 3 Feb 2016 12:03:42 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Juri Lelli <juri.lelli@....com>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	Rafael Wysocki <rjw@...ysocki.net>,
	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,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] cpufreq: governor: Create separate sysfs-ops

On 02-02-16, 17:01, Juri Lelli wrote:
> Hi Rafael,
> 
> On 02/02/16 17:35, Rafael J. Wysocki wrote:
> > On Tue, Feb 2, 2016 at 4:47 PM, Juri Lelli <juri.lelli@....com> wrote:

> > > This patch cleans things up a lot, that's good.
> > >
> > > One thing I'm still concerned about, though: don't we need some locking
> > > in place for some of the store operations on governors attributes? Are
> > > store_{ignore_nice_load, sampling_down_fact, etc} safe without locking?
> > 
> > That would require some investigation I suppose.

Yeah, that protection is required. Sorry about that.

> > > It seems that we can call them from different cpus concurrently.
> > 
> > Yes, we can.
> > 
> > One quick-and-dirty way of dealing with that might be to introduce a
> > "sysfs lock" into struct dbs_data and hold that around the invocation
> > of gattr->store() in the sysfs_ops's ->store callback.

s/dirty/sane ? :)

> Can't we actually try to use the policy->rwsem (or one of the core
> locks) + wait_for_completion approach as we do in cpufreq core?

policy->rwsem will defeat the purpose of this change as it will
reintroduce the ABBA issue.

-- 
viresh