Re: [PATCH 0/3] x86: Fix panic vs. NMI issues

[Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>]

Hi,

Thanks for the feedback.

(2015/07/23 17:25), Michal Hocko wrote:
> Hi,
> 
> On Wed 22-07-15 11:14:21, Hidehiro Kawai wrote:
>> When an HA cluster software or administrator detects non-response
>> of a host, they issue an NMI to the host to completely stop current
>> works and take a crash dump.  If the kernel has already panicked
>> or is capturing a crash dump at that time, further NMI can cause
>> a crash dump failure.
>>
>> To solve this issue, this patch set does two things:
>>
>> - Don't panic on NMI if the kernel has already panicked
>> - Introduce "noextnmi" boot option which masks external NMI at the
>>   boot time (supported only for x86)
> 
> I am currently debugging the same issue for our customer. Curiously
> enough the issue happens on a Hitachi HW.

I found these issues by my white-box testing and source code
reading.  So, they haven't happened on our customers yet, but
possibly happen.

> I haven't posted my patch for an upstream review yet because I still
> do not have a feedback but I believe your solution is unnecessarily
> too complex. Unless I am missing something the following should be enough,
> no?

Your patch solves some cases, but I think it wouldn't cover
all cases where I want to solve.  How about the following cases?

1) panic -> acquire panic_lock -> unknown NMI on this CPU ->
   panic -> failed to acquire panic_lock -> infinite loop
==> no one processes kdump procedure. 

2) crash_kexec w/o entering panic -> acquire kexec_mutex ->
   unknown NMI on this CPU -> panic -> crash_kexec ->
   failed to acquire kexec_mutex -> return to panic -> smp_send_stop

Even if with your patch, case 2) causes infinite loop of
try_crash_kexec and no one processes kdump procedure.

Regards,

> ---
>>From ba6ef85d26113e720a630ea22b08efef5b70210f Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.cz>
> Date: Fri, 17 Jul 2015 15:17:08 +0200
> Subject: [PATCH] kexec: Never return from crash_kexec when kexex is in
>  progress
> 
> We had a report when kdump kernel hasn't booted after unknown NMI has
> been delivered and unknown_nmi_panic is enabled. The NMI is triggered
> by HW and it is delivered to all CPUs at the same time. The machine has
> hundreds of CPUs and the most plausible theory is that one CPU really
> manages to kick the kexec but it cannot shut down all the CPUs because
> they are processing NMI and so cannot process an IPI. Another CPU then
> manages to call smp_send_stop from a concurrent panic and this stops the
> kexec CPU which has managed to switch to the new kernel and doesn't run
> in the NMI mode anymore.
> 
> Fix this by making crash_kexec to never return if there is a kexec in
> progress. This can be done easily by relying on the fact that
> kexec_mutex will never be released for an ongoing kexec so we just have
> to loop over the try lock. The only tricky part is that
> kexec_crash_image might be not loaded when we have to return. The check
> has to be done under the lock. Extract the trylock and check into
> try_crash_kexec and make it return true only if crash kexec is disabled.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.cz>
> ---
>  kernel/kexec.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/kexec.c b/kernel/kexec.c
> index a785c1015e25..d61b1478167d 100644
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -1470,7 +1470,7 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
>  
>  #endif /* CONFIG_KEXEC_FILE */
>  
> -void crash_kexec(struct pt_regs *regs)
> +static bool try_crash_kexec(struct pt_regs *regs)
>  {
>  	/* Take the kexec_mutex here to prevent sys_kexec_load
>  	 * running on one cpu from replacing the crash kernel
> @@ -1490,7 +1490,20 @@ void crash_kexec(struct pt_regs *regs)
>  			machine_kexec(kexec_crash_image);
>  		}
>  		mutex_unlock(&kexec_mutex);
> +		return true;
>  	}
> +	return false;
> +}
> +
> +void crash_kexec(struct pt_regs *regs)
> +{
> +	/*
> +	 * Never return from this function if a kexec is in progress
> +	 * already because next steps might interfere with it.
> +	 * try_crash_kexec will never succeed in such a case.
> +	 */
> +	while (!try_crash_kexec(regs))
> +		cpu_relax();
>  }
>  
>  size_t crash_get_memory_size(void)
> 


-- 
Hidehiro Kawai
Hitachi, Ltd. Research & Development Group



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