linux-kernel - Re: [PATCH 2/11] cpufreq: governor: Use common mutex for dbs

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Date:	Thu, 4 Feb 2016 17:46:07 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Juri Lelli <juri.lelli@....com>,
	Steve Muckle <steve.muckle@...aro.org>,
	Saravana Kannan <skannan@...eaurora.org>
Subject: Re: [PATCH 2/11] cpufreq: governor: Use common mutex for dbs_data protection

On Thu, Feb 4, 2016 at 6:09 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 04-02-16, 00:16, Rafael J. Wysocki wrote:
>> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>>
>> Every governor relying on the common code in cpufreq_governor.c
>> has to provide its own mutex in struct common_dbs_data.  However,
>> those mutexes are never used at the same time
>
> Why do you think so? I thought they can always be used in parallel.
>
> Consider 2 or more policies, one can have ondemand as the governor,
> whereas other one can have conservative.
>
> If CPUs go online/offline or if governors are switching in parallel,
> then cpufreq_governor_dbs() can very much run in parallel for ondemand
> and conservative.
>
> Or am I missing something here ?

Well, so perhaps the changelog is inaccurate.

However, what's wrong with using a single mutex then?

Thanks,
Rafael