linux-kernel - Re: Perhaps a side effect regarding NMI returns

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Message-ID: <CAPM31RLSqD8uChMeQeKJWNDx2Ogb4MhN=gJFzdmCayh7ohU5MA@mail.gmail.com>
Date:	Mon, 28 Nov 2011 23:33:46 -0800
From:	Paul Turner <pjt@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: Perhaps a side effect regarding NMI returns

On Mon, Nov 28, 2011 at 8:53 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Mon, Nov 28, 2011 at 8:07 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>>
>> If enabling interrupts also enables NMIs, then there's no side effect.
>
> The only thing that enables NMI's again is 'iret', afaik. Of course,
> it can be *any* iret, so once you enable interrupts, you can get a
> timer interrupt, and the timer interrupt returning with iret will
> re-enable NMI's then too.
>

I double checked versus the manuals tonight after seeing this and I
believe this correct -- an iret is required; some of the verbage is
reasonably explicit to this fact.

> I would suggest that the actual NMI handler itself should probably
> never use that paranoid exit at all, and just always use a regular
> iret. Screw scheduling and TIF checks.
>
> Probably only the exceptions that can happen *during* NMI (eg debug,
> stack exception, double-fault etc) should use the paranoid versions
> that try to avoid using iret.
>
> Because I think you're right - we shouldn't call schedule() from
> within the NMI handler, even if we do enable interrupts and switch to
> the normal stack. Even if it probably does happen to work normally.
>
> But I have to admit to not necessarily thinking it through a lot.
>

At a high level I think we could apply the same logic from the first
paragraph; namely that we will see a timer tick within a jiffy (where
we will have an opportunity to process TIF flags) and it's better to
push schedule() out than the NMI handler.

That said, if TIF_RESCHED has been set by a higher priority RT task
this temporary inversion may not be appreciated.  It's probably worth
fixing properly.

- Paul
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