[V2 PATCH 1/3] x86/panic: Fix re-entrance problem due to panic on NMI

[河合英宏 / KAWAI，HIDEHIRO]

Hi,

> From: linux-kernel-owner at vger.kernel.org [mailto:linux-kernel-owner at vger.kernel.org] On Behalf Of Hidehiro Kawai
> (2015/07/27 23:34), Michal Hocko wrote:
> > On Mon 27-07-15 10:58:50, Hidehiro Kawai wrote:
[...]
> > The check could be also relaxed a bit and nmi_panic would
> > return only if the ongoing panic is the current cpu when we really have
> > to return and allow the preempted panic to finish.
> 
> It's reasonable.  I'll do that in the next version.

I noticed atomic_read() is insufficient.  Please consider the following
scenario.

CPU 1: call panic() in the normal context
CPU 0: call nmi_panic(), check the value of panic_cpu, then call panic()
CPU 1: set 1 to panic_cpu
CPU 0: fail to set 0 to panic_cpu, then do an infinite loop
CPU 1: call crash_kexec(), then call kdump_nmi_shootdown_cpus()

At this point, since CPU 0 loops in NMI context, it never executes
the NMI handler registered by kdump_nmi_shootdown_cpus().  This means
that no register states are saved and no cleanups for VMX/SVM are
performed.  So, we should still use atomic_cmpxchg() in nmi_panic() to
prevent other cpus from running panic routines.

> > +void nmi_panic(const char *fmt, ...)
> > +{
> > +	/*
> > +	 * We have to back off if the NMI has preempted an ongoing panic and
> > +	 * allow it to finish
> > +	 */
> > +	if (atomic_read(&panic_cpu) == raw_smp_processor_id())
> > +		return;
> > +
> > +	panic();
> > +}
> > +EXPORT_SYMBOL(nmi_panic);