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

From: long.wanglong
Date: Thu May 14 2015 - 08:13:10 EST

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
> 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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