linux-kernel - Re: printk feature for syzbot?

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Date:   Fri, 11 May 2018 18:50:04 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        syzkaller <syzkaller@...glegroups.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Fengguang Wu <fengguang.wu@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: printk feature for syzbot?

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.

> 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.

	-ss