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SubjectRe: printk feature for syzbot?
On Fri, 11 May 2018 18:50:04 +0900
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> wrote:

> On (05/11/18 11:17), Dmitry Vyukov wrote:
> >
> > From what I see, it seems that interrupts can be nested:
>
> Hm, I thought that in general IRQ handlers run with local IRQs
> disabled on CPU. So, generally, IRQs don't nest. Was I wrong?
> NMIs can nest, that's true; but I thought that at least IRQs
> don't.

We normally don't run nested interrupts, although as the comment in
preempt.h says:

* The hardirq count could in theory be the same as the number of
* interrupts in the system, but we run all interrupt handlers with
* interrupts disabled, so we cannot have nesting interrupts. Though
* there are a few palaeontologic drivers which reenable interrupts in
* the handler, so we need more than one bit here.

And no, NMI handlers do not nest. Yes, we deal with nested NMIs, but in
those cases, we just set a bit as a latch, and return, and when the
first NMI is complete, it checks that bit and if it is set, it executes
another NMI handler.

>
> > https://syzkaller.appspot.com/bug?id=72eddef9cedcf81486adb9dd3e789f0d77505ba5
> > https://syzkaller.appspot.com/bug?id=66fcf61c65f8aa50bbb862eb2fde27c08909a4ff
> >
> > Will this in_nmi()/in_irq()/in_serving_softirq()/else be enough to
> > untangle output printed by such nested interrupts?
>
> Well, hm. __irq_enter() does preempt_count_add(HARDIRQ_OFFSET) and
> __irq_exit() does preempt_count_sub(HARDIRQ_OFFSET). So, technically,
> you can store
>
> preempt_count() & HARDIRQ_MASK
> preempt_count() & SOFTIRQ_MASK
> preempt_count() & NMI_MASK
>
> in that extended context tracking. The numbers will not tell you
> the IRQ line number, for instance, but at least you'll be able to
> distinguish different hard/soft IRQs, NMIs. Just an idea, I didn't
> check it, may be it won't work at all.
>
> Ideally, the serial log should be like this
>
> i:1 ... foo()
> i:1 ... bar()
> i:2 ... foo() // __irq_enter()
> i:2 ... bar()
> i:2 ... buz() // __irq_exit()
> i:1 ... buz()
>
> but I may be completely wrong.
>
> Petr and Steven probably will have better ideas.

I handle nesting of different contexts in the ftrace ring buffer using
the preempt count. See trace_recursive_lock/unlock() in
kernel/trace/ring_buffer.c.

-- Steve

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