Hi, > From: linux-kernel-owner at vger.kernel.org [mailto:linux-kernel-owner at 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. So, we should still use atomic_cmpxchg() in nmi_panic() to prevent other cpus from running panic routines. > > +void nmi_panic(const char *fmt, ...) > > +{ > > + /* > > + * We have to back off if the NMI has preempted an ongoing panic and > > + * allow it to finish > > + */ > > + if (atomic_read(&panic_cpu) == raw_smp_processor_id()) > > + return; > > + > > + panic(); > > +} > > +EXPORT_SYMBOL(nmi_panic);
