Re: [V5 PATCH 1/4] panic/x86: Fix re-entrance problem due to panic on NMI

Michal Hocko
2015-11-24T20:27:19Z

On Tue 24-11-15 10:05:10, Steven Rostedt wrote:
> On Fri, Nov 20, 2015 at 06:36:44PM +0900, Hidehiro Kawai wrote:
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index 350dfb0..480a4fd 100644
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -445,6 +445,19 @@ extern int sysctl_panic_on_stackoverflow;
> >
> > extern bool crash_kexec_post_notifiers;
> >
> > +extern atomic_t panic_cpu;
> > +
> > +/*
> > + * A variant of panic() called from NMI context.
> > + * If we've already panicked on this cpu, return from here.
> > + */
> > +#define nmi_panic(fmt, ...) \
> > + do { \
> > + int this_cpu = raw_smp_processor_id(); \
> > + if (atomic_cmpxchg(&panic_cpu, -1, this_cpu) != this_cpu) \
> > + panic(fmt, ##__VA_ARGS__); \
>
> Hmm,
>
> What happens if:
>
> CPU 0: CPU 1:
> ------ ------
> nmi_panic();
>
> nmi_panic();
> <external nmi>
> nmi_panic();

I thought that nmi_panic is called only from the nmi context. If so how
can we get a nested NMI like that?
--
Michal Hocko
SUSE Labs