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

[Saravana Kannan]

On 02/25/2014 05:04 AM, Rafael J. Wysocki wrote:
> On Tuesday, February 25, 2014 02:20:57 PM Viresh Kumar wrote:
> > On 25 February 2014 01:53, Saravana Kannan <skannan@xxxxxxxxxxxxxx> 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?
This is a real bug. But the actual caller of cpufreq_update_policy() is 
a driver that's local to our tree. I'm just giving examples of upstream 
code that act in a similar way.

> > > 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.

Yes, so let's not make it available until all of it is initialized. I 
don't like the piece meal check. 6 months down the lane someone making 
changes might not remember this. The problem also applies for drivers 
that might not be upstreamed, etc.

> > 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.

Rafael, It's not clear what you mean by the yes. Do you want to 
initialize it partly and make it available. I think that's always wrong.

> > > 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.

I think there's been a misunderstanding of what I'm proposing. The 
reference to policy->clk is only to get rid of the dependency that 
cpufreq_generic_get() has on the per cpu policy variable being filled. 
You can do that by just removing calls to get _IF_ clk is filled in.

Viresh,

I'll look at the patches later today and respond. Although, I would have 
been nice you had helped me fix any issues with my patch than coming up 
with new ones. Kinda removes to motivation for submitting patches upstream.

Regards,
Saravana

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