Re: [PATCH v2] x86/mce: Don't participate in rendezvous process once nmi_shootdown_cpus() was made

From: Xunlei Pang <xpang@redhat.com>
To: Borislav Petkov <bp@alien8.de>, Xunlei Pang <xlpang@redhat.com>
Cc: Prarit Bhargava <prarit@redhat.com>,
	Kiyoshi Ueda <k-ueda@ct.jp.nec.com>,
	Tony Luck <tony.luck@intel.com>,
	x86@kernel.org, kexec@lists.infradead.org,
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
	Junichi Nomura <j-nomura@ce.jp.nec.com>,
	Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
	Dave Young <dyoung@redhat.com>
Subject: Re: [PATCH v2] x86/mce: Don't participate in rendezvous process once nmi_shootdown_cpus() was made
Date: Mon, 20 Feb 2017 21:29:24 +0800	[thread overview]
Message-ID: <58AAEF34.9000303@redhat.com> (raw)
In-Reply-To: <20170220110941.vwcm3je3e4kkei6o@pd.tnic>

On 02/20/2017 at 07:09 PM, Borislav Petkov wrote:
> On Mon, Feb 20, 2017 at 02:10:37PM +0800, Xunlei Pang wrote:
>> @@ -1128,8 +1129,9 @@ void do_machine_check(struct pt_regs *regs, long error_code)
>>  	 */
>>  	int lmce = 1;
>>  
>> -	/* If this CPU is offline, just bail out. */
>> -	if (cpu_is_offline(smp_processor_id())) {
>> +	/* If nmi shootdown happened or this CPU is offline, just bail out. */
>> +	if (cpus_shotdown() ||
> I don't like "cpus_shotdown" - it doesn't hint at all that this is
> special-handling crash/kdump.
>
> And more importantly, I want it to be obvious that we do let the
> crashing CPU into the MCE handler.

Ok, I will export crashing_cpu and use it directly in mce handler.

>
> Why?
>
> If we didn't, you will not handle *any* MCE, even a fatal one, during
> dumping memory so if that dump is corrupted from the MCE, you won't
> know. And I don't want to be the one staring at the corrupted dump and
> wondering why I'm seeing what I'm seeing.
>
> IOW, if we get a fatal MCE during dumping then we should go and die.
> This is much better than silently corrupting the dump and not even
> saying anything about it.
>

My thought is that it doesn't matter after kdump boots as new mce handler
will be installed. If we get a fatal MCE during kdumping, the new handler will
handle the cpus running kdump kernel correctly.

There is a small window between crash and kdump kernel boot, so if a SRAO comes
within this window it will also cause the mce synchronization problem on the crashing
cpu if we don't bail out the crashing cpu.

Regards,
Xunlei

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