Re: [RFC] how to perform a safe NMI stack trace on all CPUs on x86?

[long.wanglong]

On 2015/5/13 22:26, Jiri Kosina wrote:
> On Wed, 13 May 2015, 王龙 wrote:
> 
>> Hi all,
>>
>> In kernel before 3.19, when trigger_all_cpu_backtrace() is called on x86, 
>> it will trigger an NMI on each CPU and call show_regs(). But this can lead
>> to a hard lock up if the NMI comes in on another printk().
>>
>> The commit a9edc88093287183ac934be44f295f183b2c62dd (x86/nmi: Perform a safe 
>> NMI stack trace on all CPUs) fix this problem on kernel mainline. when the NMI 
>> triggers, it switches the printk routine for that CPU to call a NMI safe printk 
>> function that records the printk in a per_cpu seq_buf descriptor. After all 
>> NMIs have finished recording its data, the seq_bufs are printed in a safe 
>> context. But how do we fix this problem in older version of kernel(eg, 3.10 stable)? 
>> The 3.10 stable has no "switch printk routine" and "seq_buf" infrastructures.
>>
>> Could anyone give me some ideas?
> 
> Either you backport seq_buf-based aproach to the older kernel, or, if you 
> are working on 3.4 kernel or earlier (basically any kernel preceeding the 
> printk() revamp that happened in 7ff9554bb57 and after), you can use 
> slightly simpler aproach.
> 
> It's an aproach we used initially when finding out the issue for the first 
> time, and it is proven to work as well (but it's not applicable after Kay 
> added all the complexity to printk()).
> 
> You can see it in our SLE11 kernel tree, available on
> 	
> 	http://kernel.suse.com/cgit/kernel/commit/?h=SLE11-SP4&id=8d62ae68ff61d77ae3c4899f05dbd9c9742b14c9
> 
> for example.
> 
> It's up to you to judget which is the least painful way :)
> 

Hi Jiri Kosina,

For 3.10 stable, the only way to solve this problem is backport seq_buf-based aproach.

I will backport necessary patches to 3.10 stable. Welcome you to review my backport patches.

Best Regards
Wang Long




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