Re: [PATCH v2] x86/nmi: Add an emergency handler in nmi_desc & use it in nmi_shootdown_cpus()

[Waiman Long]

On 12/5/24 1:17 PM, Thomas Gleixner wrote:
> On Thu, Dec 05 2024 at 08:22, Waiman Long wrote:
> > On 12/5/24 8:12 AM, Thomas Gleixner wrote:
> > > > Actually, crash_nmi_callback() can return in the case of the crashing
> > > > CPUs, though all the other CPUs will not return once called. So I
> > > > believe the current form is correct. I will update the comment to
> > > > reflect that.
> > > Why would you continue servicing the NMI on a CPU which just crashed?
> > According to crash_nmi_callback(),
> >
> >           /*
> >            * Don't do anything if this handler is invoked on crashing cpu.
> >            * Otherwise, system will completely hang. Crashing cpu can get
> >            * an NMI if system was initially booted with nmi_watchdog
> > parameter.
> >            */
> >           if (cpu == crashing_cpu)
> >                   return NMI_HANDLED;
> >
> > The crashing CPU still has work to do after shutting down other CPUs. It
> > can't wait there forever without completing other crashing actions. The
> > only thing I can see we can do is to return immediately without
> > servicing other less important nmi handlers in the list.
> I understand that, but in case that the crashed CPU receives an NMI and
> sees that the emergency handler is set, shouldn't it stop the NMI
> processing instead of trying to go through perf and what not when the
> system is already in a fragile state. i.e.:
>
>         if (emergemcy_handler) {
>            emergency_handler();
>            return;
>         }

That is what I suggest in my last sentence. I will update the patch 
according.

Cheers,
Longman