linux-kernel - Re: [PATCH 1/12] cpufreq: governor: Close dbs

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Date:	Fri, 19 Feb 2016 03:34:09 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/12] cpufreq: governor: Close dbs_data update race condition

On Fri, Feb 19, 2016 at 3:27 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 18-02-16, 17:20, Rafael J. Wysocki wrote:
>> On Thu, Feb 18, 2016 at 6:24 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>> > On 18-02-16, 02:19, Rafael J. Wysocki wrote:
>
>> >> @@ -112,7 +112,7 @@ static ssize_t governor_store(struct kob
>> >>
>> >>       mutex_lock(&dbs_data->mutex);
>> >>
>> >> -     if (gattr->store)
>> >> +     if (dbs_data->usage_count && gattr->store)
>> >
>> > That's not gonna be enough. The above lock doesn't guarantee
>> > protection with any such races.
>
> Oops, I completely misread it. Really sorry about that.
>
> But now that I have read the code again, I wonder why we need this protection at
> all. The first thing we do after decrementing the usage_count counter, is we put
> the kobject. Which will ensure that the sysfs files are all gone. So, what is
> the race we are trying to fix then?

The ->store() callbacks for different attributes may do silly stuff
like walking all CPUs in the system and updating per-CPU data for
them.  If the dbs_data the callback has been called for is going away,
this is pointless at best and may be actually harmful depending on
what the callback is really doing.

Thanks,
Rafael