Messages in this thread | | | Date | Thu, 12 May 2022 14:35:18 +0000 | From | Sean Christopherson <> | Subject | Re: [PATCH 1/2] x86/crash: Disable virt in core NMI crash handler to avoid double list_add |
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On Thu, May 12, 2022, Sean Christopherson wrote: > On Thu, May 12, 2022, Thomas Gleixner wrote: > > Sean, > > > > On Wed, May 11 2022 at 23:43, Sean Christopherson wrote: > > > @@ -840,6 +858,20 @@ void nmi_shootdown_cpus(nmi_shootdown_cb callback) > > > unsigned long msecs; > > > local_irq_disable(); > > > > > > + /* > > > + * Invoking multiple callbacks is not currently supported, registering > > > + * the NMI handler twice will cause a list_add() double add BUG(). > > > + * The exception is the "nop" handler in the emergency reboot path, > > > + * which can run after e.g. kdump's shootdown. Do nothing if the crash > > > + * handler has already run, i.e. has already prepared other CPUs, the > > > + * reboot path doesn't have any work of its to do, it just needs to > > > + * ensure all CPUs have prepared for reboot. > > > > This is confusing at best. The double list add is just one part of the > > problem, which would be trivial enough to fix. > > > > The real point is that after the first shoot down all other CPUs are > > stuck in crash_nmi_callback() and won't respond to the second NMI. > > Well that's embarrasingly obvious in hindsight. > > > > > So trying to run this twice is completely pointless and guaranteed to > > run into the timeout. > > > > > + */ > > > + if (shootdown_callback) { > > > + WARN_ON_ONCE(callback != nmi_shootdown_nop); > > > + return; > > > > Instead of playing games with the callback pointer, I prefer to make > > this all explicit. Delta patch below. > > Much better. If you're planning on doing fixup, can you also include a comment > tweak about why the callback is left set? If not, I'll do it in v2. A bit > overkill, but it's another thing that for me was obvious only once I realized "why".
Actually, I'll send a v2, there's some more cleanup that can be done if patch 2 uses cpu_crash_disable_virtualization().
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