Re: [PATCH v3] x86/mce: Don't participate in rendezvous process once nmi_shootdown_cpus() was made

From: Luck, Tony
Date: Wed Feb 22 2017 - 13:50:51 EST

On Wed, Feb 22, 2017 at 12:11:14PM +0800, Xunlei Pang wrote:
> + /*
> + * Cases to bail out to avoid rendezvous process timeout:
> + * 1)If this CPU is offline.
> + * 2)If crashing_cpu was set, e.g. entering kdump,
> + * we need to skip cpus remaining in 1st kernel.
> + */
> + if (cpu_is_offline(cpu) ||
> + (crashing_cpu != -1 && crashing_cpu != cpu)) {
> u64 mcgstatus;
> mcgstatus = mce_rdmsrl(MSR_IA32_MCG_STATUS);

I think we should document the remaining race conditions. I don't
think there is any good way to eliminate them, and they are already
pretty small windows.

I think the sequence of events looks like:

1 Panic occurs
2 nmi_shootdown_cpus() sets crashing_cpu
3 send NMI to everyone else
4 wait up to a second for other CPUs to take NMI
5 go to kexec code
6 start new kernel
7 new kernel establishes #MC handler

If one of the other cpus triggers a machine check while
getting to, or in, the NMI handler ... then that cpu will
skip processing (if RIPV is set).

Between '2' and '5' if crashing_cpu gets a machine check it
will execute in the old kernel handler, and do the right thing.

There's a fuzzy area between '6' and '7' where a machine check
might not end up in the right code.

>From '7' onwards the kexec kernel will handle and machine
checks caused by kdump.