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Date: Fri, 6 Feb 2009 09:10:39 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
       Peter Zijlstra <peterz@infradead.org>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       Steven Rostedt <srostedt@redhat.com>
Subject: Re: [PATCH 2/4] nmi: add generic nmi tracking state
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On Fri, 6 Feb 2009 08:25:52 -0500 (EST) Steven Rostedt <rostedt@goodmis.org> wrote:

> 
> On Thu, 5 Feb 2009, Andrew Morton wrote:
> > >  
> > > +/*
> > > + * Are we in NMI context?
> > > + */
> > > +#define in_nmi()	(preempt_count() & NMI_OFFSET)
> > > +
> > >  #if defined(CONFIG_PREEMPT)
> > >  # define PREEMPT_INATOMIC_BASE kernel_locked()
> > >  # define PREEMPT_CHECK_OFFSET 1
> > > @@ -167,6 +178,8 @@ extern void irq_exit(void);
> > >  #define nmi_enter()				\
> > >  	do {					\
> > >  		ftrace_nmi_enter();		\
> > > +		BUG_ON(in_nmi());		\
> > > +		add_preempt_count(NMI_OFFSET);	\
> > >  		lockdep_off();			\
> > >  		rcu_nmi_enter();		\
> > >  		__irq_enter();			\
> > > @@ -177,6 +190,8 @@ extern void irq_exit(void);
> > >  		__irq_exit();			\
> > >  		rcu_nmi_exit();			\
> > >  		lockdep_on();			\
> > > +		BUG_ON(!in_nmi());		\
> > > +		sub_preempt_count(NMI_OFFSET);	\
> > >  		ftrace_nmi_exit();		\
> > >  	} while (0)
> > >  
> > 
> > Well that was tidy.
> > 
> > We're sure that no present or future architecture will for some weird
> > reason nest NMIs?
> 
> That would be fun to implement. Not the in_nmi code, but the handling of 
> nested NMIs. How would you be able to save the state when the NMI occurred 
> without being preempted by another NMI?

Like with normal interrupts?

As long as the number of sources is finite, nested NMIs could work OK.

> I think the arch that has nested NMIs will have many more issues to solve 
> in the kernel than this one.

I have a vague memory that x86 can do this.

<googles a bit>

What's all this about?
https://www.x86-64.org/pipermail/discuss/2005-October/007010.html
http://kerneltrap.org/index.php?q=mailarchive/linux-kernel/2008/2/12/830704/thread

I expect that even if it is possible, we can live without it.

And if I'm wrong, it'll be easy to accommodate by adding a new counter
into the task_struct or thread_struct.

Does your above implementation make in_interrupt() return true if
in_nmi()?  I think it doesn't, but should?