linux-kernel - Re: [RFC PATCH] x86 NMI-safe INT3 and Page Fault (v5)

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Message-Id: <20080417142614.23c3849f.akpm@linux-foundation.org>
Date:	Thu, 17 Apr 2008 14:26:14 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
Cc:	mingo@...e.hu, hpa@...or.com, jeremy@...p.org, rostedt@...dmis.org,
	fche@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] x86 NMI-safe INT3 and Page Fault (v5)

On Thu, 17 Apr 2008 17:16:25 -0400
Mathieu Desnoyers <compudj@...stal.dyndns.org> wrote:

> > Should we or should we not make in_interrupt() return true in NMI? 
> > "should", I expect.
> > 
> > If not, we'd need to do something else to communicate the current
> > processing state down to the BUG-handling code.
> > 
> 
> You bring an interesting question. In practice, since this BUG_ON could
> only happen if we have an NMI nested over another NMI or an nmi which
> fails to decrement its HARDNMI_MASK. Given that the HARDIRQ_MASK is
> incremented right after the HARDNMI_MASK increment (the reverse is also
> true), really bad things (TM) must have happened for the BUG_ON to be
> triggered outside of the __irq_enter()/__irq_exit() scope of the NMI
> below the buggy one.
> 
> But since this code is there to extract as much information as possible
> when things go wrong, I would say it's safer to, at least, add
> HARDNMI_MASK to irq_count().
> 
> Instead, though, I think we could add :
> 
> if (in_nmi())
>    panic("Fatal exception in non-maskable interrupt");
> 
> to die().

But that's just one site.  There might be (now, or in the future) other
code under BUG() which tests in_interrupt().

And most of the places where we test for in_interrupt() and in_irq()
probably want that to return true is we're in NMI too.  After all, it's an
interrupt.  

> That would be clearer. I just added it to x86_32, but can't
> find where x86_64 reports the "fatal exception in interrupt" and friends
> message. Any idea ?

Dunno - maybe it just doesn't have it.  Maybe it was never the right thing
to do.

> By dealing with this case specifically, I think we don't really have to
> add HARDNMI_MASK to irq_count(), considering it's normally an HARDIRQ
> too.


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