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

[Xunlei Pang]

On 02/23/2017 at 02:50 AM, Luck, Tony wrote:
> 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.
>

Agree, will update the comment.

Regards,
Xunlei