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

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Date:	Wed, 3 Feb 2016 12:27:45 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Saravana Kannan <skannan@...eaurora.org>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	Juri Lelli <juri.lelli@....com>,
	Rafael Wysocki <rjw@...ysocki.net>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.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, 20:03, Saravana Kannan wrote:
> This is the hotplug case I mentioned. The sysfs file removals will happen
> only for the last CPU in that policy (we thankfully optimized that part last
> year). We also know that multiple CPUs can't be hotplugged at the same time.
> So, in the start of cpufreq_offline_prepare, we just need to check if this
> is the last CPU in the policy and if that's the case, do the gov sysfs
> remove and then grab the policy lock and do all our crap. If a read is going
> on, that's going to finish before the sysfs attr remove can go ahead and it
> can grab the policy lock if it needs to and that still won't cause a
> deadlock because we haven't yet grabbed the policy lock in
> cpufreq_offline_prepare(). If the read comes after the sysfs remove, then
> the read is obviously going to fail (we can depend on the sysfs framework on
> doing its job there).

IMHO, these are all dirty hacks we should stay away from. Adding such
hunks in code is considered a band-aid kind of solution and hurts
readability badly. The new solution (new governor show/store)
implement this in a very clean and proper way I feel..

Others are free to disagree though :)

-- 
viresh