Re: [PATCH 2/2] x86/numa: instance all parsed numa node

[Andy Lutomirski <luto@amacapital.net>]

> On Jul 9, 2019, at 1:24 AM, Pingfan Liu <kernelfans@gmail.com> wrote:
> 
>> On Tue, Jul 9, 2019 at 2:12 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>> 
>>> On Tue, 9 Jul 2019, Pingfan Liu wrote:
>>>> On Mon, Jul 8, 2019 at 5:35 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>>>> It can and it does.
>>>> 
>>>> That's the whole point why we bring up all CPUs in the 'nosmt' case and
>>>> shut the siblings down again after setting CR4.MCE. Actually that's in fact
>>>> a 'let's hope no MCE hits before that happened' approach, but that's all we
>>>> can do.
>>>> 
>>>> If we don't do that then the MCE broadcast can hit a CPU which has some
>>>> firmware initialized state. The result can be a full system lockup, triple
>>>> fault etc.
>>>> 
>>>> So when the MCE hits a CPU which is still in the crashed kernel lala state,
>>>> then all hell breaks lose.
>>> Thank you for the comprehensive explain. With your guide, now, I have
>>> a full understanding of the issue.
>>> 
>>> But when I tried to add something to enable CR4.MCE in
>>> crash_nmi_callback(), I realized that it is undo-able in some case (if
>>> crashed, we will not ask an offline smt cpu to online), also it is
>>> needless. "kexec -l/-p" takes the advantage of the cpu state in the
>>> first kernel, where all logical cpu has CR4.MCE=1.
>>> 
>>> So kexec is exempt from this bug if the first kernel already do it.
>> 
>> No. If the MCE broadcast is handled by a CPU which is stuck in the old
>> kernel stop loop, then it will execute on the old kernel and eventually run
>> into the memory corruption which crashed the old one.
>> 
> Yes, you are right. Stuck cpu may execute the old do_machine_check()
> code. But I just found out that we have
> do_machine_check()->__mc_check_crashing_cpu() to against this case.
> 
> And I think the MCE issue with nr_cpus is not closely related with
> this series, can
> be a separated issue. I had question whether Andy will take it, if
> not, I am glad to do it.
> 
> 

Go for it. I’m not familiar enough with the SMP boot stuff that I would be able to do it any faster than you. I’ll gladly help review it.