linux-kernel - Re: [PATCH] cpufreq: Set policy to non-NULL only after all hotplug online work is done

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Date:	Tue, 25 Feb 2014 14:04:16 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Saravana Kannan <skannan@...eaurora.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arm-msm@...r.kernel.org,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] cpufreq: Set policy to non-NULL only after all hotplug online work is done

On Tuesday, February 25, 2014 02:20:57 PM Viresh Kumar wrote:
> On 25 February 2014 01:53, Saravana Kannan <skannan@...eaurora.org> wrote:
> > I was simplifying the scenario that causes it. We change the min/max using
> > ADJUST notifiers for multiple reasons -- thermal being one of them.
> >
> > thermal/cpu_cooling is one example of it.
> 
> Just to understand the clear picture, you are actually hitting this bug? Or
> is this only a theoretical bug?
> 
> > So, cpufreq_update_policy() can be called on any CPU. If that races with
> > someone offlining a CPU and onlining it, you'll get this crash.
> 
> Then shouldn't that be fixed by locks? I think yes. That makes me agree with
> Srivatsa more here.
> 
> Though I would say that your argument was also valid that 'policy' shouldn't be
> up for sale unless it is prepared to. And for that reason only I
> floated that question
> earlier: What exactly we need to make sure is initialized in policy? Because
> policy might keep changing in future as well and that needs locks to protect
> that stuff. Like min/max/governor/ etc..

Well, that depends on what the current users expect it to look like initially.
It should be initialized to the point in which all of them can handle it
correctly.

> So, probably a solution here might be a mix of both. Initialize policy to this
> minimum level and then make sure locking is used correctly..

Yes.

> > The idea would exist, but we can just call cpufreq_generic_get() and pass it
> > policy->clk if it is not NULL. Does that work for you?
> 
> No. Not all drivers implement clk interface. And so clk doesn't look to be the
> right parameter. I thought maybe 'policy' can be the right parameter and
> then people can get use policy->cpu to get cpu id out of it.
> 
> But even that doesn't look to be a great idea. X86 drivers may share policy
> structure for CPUs that don't actually share a clock line. And so they do need
> right CPU number as parameter instead of policy. As they might be doing
> some tricky stuff there. Also, we need to make sure that ->get() returns
> the frequency at which CPU x is running.

That's not going to work in at least some cases anyway, because for some types
of HW we simply can't retrieve the current frequency in a non-racy way.

Thanks!

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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