linux-kernel - Re: [PATCH V2] cpufreq: Disallow ->resolve_freq() for drivers providing ->target

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Message-ID: <CAJZ5v0j1vX+TwjcDxhbfd2ZruxWz7hFfiMUD6NL_+ZHyDqcU5g@mail.gmail.com>
Date:	Fri, 22 Jul 2016 01:53:13 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Linux PM <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH V2] cpufreq: Disallow ->resolve_freq() for drivers
 providing ->target_index()

On Fri, Jul 22, 2016 at 1:45 AM, Steve Muckle <steve.muckle@...aro.org> wrote:
> On Fri, Jul 22, 2016 at 01:32:00AM +0200, Rafael J. Wysocki wrote:
>> On Fri, Jul 22, 2016 at 1:22 AM, Steve Muckle <steve.muckle@...aro.org> wrote:
>> > On Fri, Jul 22, 2016 at 01:22:22AM +0200, Rafael J. Wysocki wrote:
>> >> OK, applied.
>> >
>> > FWIW I do have a concern on this patch, I think it adds unnecessary
>> > overhead.
>>
>> It isn't unnecessary.  It prevents an otherwise possible kernel crash
>> from happening.
>
> The logic may not be unecessary, but the overhead is. The crash could be
> prevented in a way that doesn't require repeatedly checking a pointer
> that doesn't change.

Well, you had the ->resolve_freq check in your patch, didn't you?

Viresh simply added a ->target_index check to it.

Now, you can argue that this is one check too many, but as long as
drivers are allowed to implement ->target without implementing
->resolve_freq, the *number* of checks in this routine cannot be
reduced.

There are three possible cases and two checks are required to
determine which case really takes place.