[Ksummit-discuss] [TECH TOPIC] Printk softlockups

[Jiri Kosina]

On Thu, 15 May 2014, Jan Kara wrote:

[ ... snip ... ]
> So I would really like as much involved people as possible to sit down in
> one room and think over what guarantees do we want from printk, which
> complexity is acceptable, and hopefully we can agree on a way accepted by
> all parties to resolve the issue.
> 
> People involved in the discussion:
> Jan Kara <jack at suse.cz>
> Andrew Morton <akpm at linux-foundation.org>
> Steven Rosted <rostedt at goodmis.org>
> Alan Cox <gnomes at lxorguk.ukuu.org.uk>

Yes, this story is indeed frustrating.

What is worse, printk() needs even more surgery so that it really doesn't 
lockup the machines 'super-hard' when called from NMI context.

We've spent a non-trivial amount of time in fixing this [1]. It might be a 
natural followup to the discussion you are proposing, as we are basically 
making printk() even more compilcated with that patchset ... but for a 
good reason as well.

Currently, pritnk() is able (and we've seen in happening) to just 
completely lock up the machine with all processess stuck in NMI context, 
which is rather undebuggable, so we better have it fixed. But yes, 
admittedly, it makes printk() code yet more complex, which might cause 
headache to people who are already afraid of your patchset.

[1] https://lkml.org/lkml/2014/5/9/118

-- 
Jiri Kosina
SUSE Labs