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Date:   Tue, 15 May 2018 14:20:42 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        syzkaller <syzkaller@googlegroups.com>,
        Fengguang Wu <fengguang.wu@intel.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: printk feature for syzbot?
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Hello,

On (05/11/18 09:37), Steven Rostedt 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.

Good to know!
I thought that NMI can nest in some weird cases, like a breakpoint from
NMI. This must be super tricky, given that nested NMI will corrupt the
stack of the previous NMI, etc. Anyway.

> > 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
> >
[..]
> 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.

Thanks. So you are also checking the preempt_count().

	-ss