> On Jul 9, 2019, at 1:24 AM, Pingfan Liu <kernelfans@xxxxxxxxx> wrote: > >> On Tue, Jul 9, 2019 at 2:12 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: >> >>> On Tue, 9 Jul 2019, Pingfan Liu wrote: >>>> On Mon, Jul 8, 2019 at 5:35 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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.
