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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