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

Michal Hocko
2015-07-29T08:23:38Z

On Wed 29-07-15 05:48:47, 河合英宏 / KAWAI，HIDEHIRO wrote:
> Hi,
>
> > From: linux-kernel-owner@vger.kernel.org [mailto:linux-kernel-owner@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.

Yes this is true but it is no different from the current state, isn't
it? So if you want to handle that then it deserves a separate patch.
It is certainly not harmful wrt. panic behavior.

> So, we should still use atomic_cmpxchg() in nmi_panic() to
> prevent other cpus from running panic routines.

Not sure what you mean by that.

--
Michal Hocko
SUSE Labs