Re: [git pull] scheduler fixes

Alexey Zaytsev
2009-01-31T17:49:21Z

On Sat, Jan 31, 2009 at 20:23, Peter Zijlstra <peterz@infradead.org> wrote:
> On Sat, 2009-01-31 at 18:11 +0100, Peter Zijlstra wrote:
>> > diff --git a/kernel/sched.c b/kernel/sched.c
>> > index 52bbf1c..5686bb5 100644
>> > --- a/kernel/sched.c
>> > +++ b/kernel/sched.c
>> > @@ -4440,7 +4450,7 @@ void __kprobes sub_preempt_count(int val)
>> > /*
>> > * Underflow?
>> > */
>> > - if (DEBUG_LOCKS_WARN_ON(val > preempt_count()))
>> > + if (DEBUG_LOCKS_WARN_ON(val > preempt_count() - (!!kernel_locked())))
>> > return;
>> > /*
>> > * Is the spinlock portion underflowing?
>
> Since the commit msg of 01e3eb8 says:
>
> kernel_locked() is not a valid test in IRQ context (we update the
> BKL's ->lock_depth and the preempt count separately and non-atomicalyy),
> so we cannot put it into the generic preempt debugging checks which
> can run in IRQ contexts too.
>

Is the comment actually valid? From arch/x86/kernel/irq_32.c:
do_softirq() actually does
curctx = current_thread_info();
irqctx = softirq_ctx[smp_processor_id()];
irqctx->tinfo.task = curctx->task;

and so does execute_on_irq_stack().
So kernel_locked() should be valid. It corresponds to the thread
that is being interrupted.

And answering an earlier question, this happens only on i386 and only
with 4K stacks because x86_64 dosn't have a separate softirq stack,
so the preempt count diring the soft irq is at least IRQ_EXIT_OFFSET.

(If I understood the things correctly)

> Another possibility would be writing it like:
>
> if (DEBUG_LOCKS_WARN_ON(val > preempt_count() -
> (in_interrupt() ? 0 : !!kernel_locked())))
>
> Which might just work because we're in sub_preempt_count, before we
> actually do the subtraction, so in_interrupt() will still be true.
>
>
>
>