> From: Michal Hocko [mailto:mhocko@xxxxxxxxxx] > On Wed 29-07-15 05:48:47, 河合英宏 / KAWAI，HIDEHIRO wrote: > > Hi, > > > > > From: linux-kernel-owner@xxxxxxxxxxxxxxx [mailto:linux-kernel-owner@xxxxxxxxxxxxxxx] 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. > > Yes this is true but it is no different from the current state, isn't > it? So if you want to handle that then it deserves a separate patch. > It is certainly not harmful wrt. panic behavior. > > > So, we should still use atomic_cmpxchg() in nmi_panic() to > > prevent other cpus from running panic routines. > > Not sure what you mean by that. I mean that we should use the same logic as my V2 patch like this: #define nmi_panic(fmt, ...) \ do { \ if (atomic_cmpxchg(&panic_cpu, -1, raw_smp_processor_id()) \ == -1) \ panic(fmt, ##__VA_ARGS__); \ } while (0) By using atomic_cmpxchg here, we can ensure that only this cpu runs panic routines. It is important to prevent a NMI-context cpu from calling panic_smp_self_stop(). void panic(const char *fmt, ...) { ... * `old_cpu == -1' means we are the first comer. * `old_cpu == this_cpu' means we came here due to panic on NMI. */ this_cpu = raw_smp_processor_id(); old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu); if (old_cpu != -1 && old_cpu != this_cpu) panic_smp_self_stop(); Please assume that CPU 0 calls nmi_panic() in NMI context and CPU 1 calls panic() in normal context at tha same time. If CPU 1 set panic_cpu before CPU 0 does, CPU 1 runs panic routines and CPU 0 return from the nmi handler. Eventually CPU 0 is stopped by nmi_shootdown_cpus(). If CPU 0 set panic_cpu before CPU 1 does, CPU 0 runs panic routines. CPU 1 calls panic_smp_self_stop(), and wait for NMI by nmi_shootdown_cpus(). Anyway, I tested my approach and it worked fine. Regards, Kawai ��.n��������+%������w��{.n�����{����* jg��� �����ݢj����G���� ���j:+v���w�m������w�������h�����٥
