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

Steven Rostedt
2009-02-06T13:26:02Z

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