lockdep: access percpu variable too early

lockdep: access percpu variable too early

[Ming Lei]

Hi,

The percpu variables of 'lockdep_stats' and 'cpu_lock_stat'
may be accessed before percpu area is brought up in case of
CONFIG_DEBUG_LOCKDEP/CONFIG_LOCK_STAT, so these variables
in non-boot CPUs will be initialized incorrectly.

As far as I think of, there are several solutions for the
problem:

	1, just enable 'debug_locks' until percpu area is built
	2, define the two percpu variables in a simple/stupid
	percpu way inside lockdep
	3, clear these two percpu variables of non-boot CPUs
	just after percpu area is built.
	4, ignore the problem

Looks each approache have its own disadvantage, so what
is your opinion on the problem? or better approach?

Thanks,
--
Ming Lei

Re: lockdep: access percpu variable too early

[Ingo Molnar]

* Ming Lei <ming.lei@canonical.com> wrote:

> Hi,
> 
> The percpu variables of 'lockdep_stats' and 'cpu_lock_stat' may be accessed before 
> percpu area is brought up in case of CONFIG_DEBUG_LOCKDEP/CONFIG_LOCK_STAT, so 
> these variables in non-boot CPUs will be initialized incorrectly.

What's the practical effect of it? Do we copy the value from the boot CPU and thus 
the secondary CPUs start with slightly increased stats?

Thanks,

	Ingo

Re: lockdep: access percpu variable too early

[Ming Lei]

On Wed, Mar 6, 2013 at 6:20 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> What's the practical effect of it? Do we copy the value from the boot CPU and thus
> the secondary CPUs start with slightly increased stats?

Yes, it is the major effect.

Also the arch dependent __my_cpu_offset might be initialized a bit late.

Thanks,
--
Ming Lei

Re: lockdep: access percpu variable too early

[Ingo Molnar]

* Ming Lei <ming.lei@canonical.com> wrote:

> On Wed, Mar 6, 2013 at 6:20 PM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > What's the practical effect of it? Do we copy the value from the boot CPU and 
> > thus the secondary CPUs start with slightly increased stats?
> 
> Yes, it is the major effect.
> 
> Also the arch dependent __my_cpu_offset might be initialized a bit late.

I suspect we could try to fix it - post-initializing it after percpu setup sounds 
like a viable option, if it does not get too ugly.

Thanks,

	Ingo

Re: lockdep: access percpu variable too early

[Ming Lei]

On Wed, Mar 6, 2013 at 6:34 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Ming Lei <ming.lei@canonical.com> wrote:
>
>> On Wed, Mar 6, 2013 at 6:20 PM, Ingo Molnar <mingo@kernel.org> wrote:
>> >
>> > What's the practical effect of it? Do we copy the value from the boot CPU and
>> > thus the secondary CPUs start with slightly increased stats?
>>
>> Yes, it is the major effect.
>>
>> Also the arch dependent __my_cpu_offset might be initialized a bit late.
>
> I suspect we could try to fix it - post-initializing it after percpu setup sounds
> like a viable option, if it does not get too ugly.

OK, I may post a patch for review, but the arch dependent __my_cpu_offset
still need to be initialized very early.

Thanks,
--
Ming Lei