Messages in this thread Patch in this message | | | Date | Thu, 12 May 2022 14:14:20 +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, 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".
Thanks!
diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c index 4e3a839ae146..808d3e75fb2d 100644 --- a/arch/x86/kernel/reboot.c +++ b/arch/x86/kernel/reboot.c @@ -896,7 +896,11 @@ void nmi_shootdown_cpus(nmi_shootdown_cb callback) msecs--; } - /* Leave the nmi callback set */ + /* + * Leave the nmi callback set, shootdown is a one-time thing. Clearing + * the callback could result in a NULL pointer dereference if a CPU + * (finally) responds after the timeout expires. + */ } static inline void nmi_shootdown_cpus_on_restart(void)
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