Re: next-20090107: WARNING: at kernel/sched.c:4435 sub_preempt_count

[Alexey Zaytsev]

On Wed, Jan 14, 2009 at 05:00, Nick Piggin <npiggin@xxxxxxx> wrote:
> On Sun, Jan 11, 2009 at 03:49:45AM +0100, Ingo Molnar wrote:
>>
>> * Alexey Zaytsev <alexey.zaytsev@xxxxxxxxx> wrote:
>>
>> > One more instance of http://marc.info/?l=linux-kernel&m=123134586202636&w=2
>> > Added Ingo Molnar to CC.
>>
>> added Nick on Cc:. Nick, it's about:
>>
>> > commit 7317d7b87edb41a9135e30be1ec3f7ef817c53dd
>> > Author: Nick Piggin <nickpiggin@xxxxxxxxxxxx>
>> > Date:   Tue Sep 30 20:50:27 2008 +1000
>> >
>> >    sched: improve preempt debugging
>>
>> causing a seemingly spurious warning.
>
> I don't know how it is spurious... Presumably the sequence _would_ have
> caused preempt count to go negative if the bkl were not held...
>
> __do_softirq does a __local_bh_disable on entry, and it seems like the
> _local_bh_enable on exit is what causes this warning. So something is
> unbalanced somehow. Or is it some weird thing we do in early boot that
> I am missing?
>
> Can you put in some printks around these functions in early boot to
> get an idea of what preempt_count is doing?
>

Hi again.

Finally got to debug this. The preempt count on the first __do_softirq entry
ever is 0, as it is set in irq_ctx_init(). The interrupted swapper
thread happens
to be in the kernel_locked() state at the moment, so the warning.

I don't understand why the softirq preempt count is initialized to 0. Should not
it be SOFTIRQ_OFFSET instead?
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