linux-kernel - Re: [debug patch] printk: Add a printk killswitch to robustify NMI watchdog messages

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Date:	Mon, 06 Jun 2011 18:00:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Arne Jansen <lists@...-jansens.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	efault@....de, npiggin@...nel.dk, akpm@...ux-foundation.org,
	frank.rowand@...sony.com, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [debug patch] printk: Add a printk killswitch to robustify NMI
 watchdog messages

On Mon, 2011-06-06 at 17:52 +0200, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > Needs more staring at, preferably by someone who actually 
> > understands that horrid mess :/ Also, this all still doesn't make 
> > printk() work reliably while holding rq->lock.
> 
> So, what about my suggestion to just *remove* the wakeup from there 
> and use the deferred wakeup mechanism that klogd uses.
> 
> That would make printk() *visibly* more robust in practice.

That's currently done from the jiffy tick, do you want to effectively
delay releasing the console_sem for the better part of a jiffy?

> [ It would also open up the way to possibly make printk() NMI entry 
>   safe - currently we lock up if we printk in an NMI or #MC context 
>   that happens to nest inside a printk(). ]

Well, for that to happen you also need to deal with logbuf_lock nesting.
Personally I think using printk() from NMI context is quite beyond sane.
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