On Tue, Jul 9, 2019 at 2:12 PM Thomas Gleixner <t...@linutronix.de> wrote: > > On Tue, 9 Jul 2019, Pingfan Liu wrote: > > On Mon, Jul 8, 2019 at 5:35 PM Thomas Gleixner <t...@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. Thanks and regards, Pingfan
