linux-kernel - Re: [git pull] scheduler fixes

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Date:	Sat, 31 Jan 2009 20:49:10 +0300
From:	Alexey Zaytsev <alexey.zaytsev@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [git pull] scheduler fixes

On Sat, Jan 31, 2009 at 20:23, Peter Zijlstra <peterz@...radead.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.
>
>
>
>
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