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

Andrew Morton
2009-02-06T17:11:46Z

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?