linux-kernel - Re: [PATCH 2/4] nmi: add generic nmi tracking state

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Date:	Fri, 6 Feb 2009 08:25:52 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 2/4] nmi: add generic nmi tracking state


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?

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

-- Steve

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