Re: [PATCH v3 4/4] printk/nmi: Increase the size of NMI buffer and make it configurable

[Daniel Thompson <daniel.thompson@linaro.org>]

On 18/12/15 17:00, Daniel Thompson wrote:
>>> The MCE handlers should only call printk() when they decide to panic
>>> and *after* busting the spinlocks. At this point deferring printk()
>>> until it is safe is not very helpful.
>>>
>>> When we bust the spinlocks we should probably restore the normal
>>> printk() function to give best chance of the failure messages making
>>> it out.
>>
>> The problem is that we do not know what locks need to be busted. There
>> are too many consoles and too many locks involved. Also busting locks
>> open another can of worms.
>
> Yes, I agree that busting the spinlocks doesn't avoid all risk of deadlock.
>
> Probably I've been placing too much weight on the importance of getting
> messages out when dying. You're right that surviving far enough through
> a panic to trigger kdump or reset is equally (or more) important in many
> scenarios than getting a failure message out.
>
> However on a system with nothing but "while(1) {}" hooked up to panic()
> then its worth risking a lock up. In this case restoring normal printk()
> behavior and dumping the NMI buffers would be worthwhile.

An a (much) later thread[1] Andrew Morton described this comment as 
non-committal. Sorry for that.

I don't object to the overall approach taken by Petr, merely that I 
think there are cases where the current patchset doesn't put in quite 
enough effort to issue messages.

Panic triggered during NMI is the only case I can think of and that, I 
think, could be addressed by adding an extra call to printk_nmi_flush() 
during panic(). It should probably only cover cases where we *don't* 
call kdump and the panic handler doesn't restart the machine... so just 
after the pr_emerg("...end kernel panic") would be OK for me.


Daniel.


[1]: http://thread.gmane.org/gmane.linux.ports.arm.kernel/482845