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Date: Mon, 6 Jun 2011 18:08:10 +0200
From: Ingo Molnar <mingo@...e.hu>
To: Peter Zijlstra <peterz@...radead.org>
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
* Peter Zijlstra <peterz@...radead.org> wrote:
> 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?
Yes, and we already do it in some other circumstances. Can you see
any problem with that? klogd is an utter slowpath anyway.
> > [ 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. [...]
That we could do as a robustness patch: detect when the current CPU
already holds it and do not lock up on that. This would also allow
printk() to work within a crashing printk(). (assuming the second
printk() does not crash - in which case it's game over anyway)
> Personally I think using printk() from NMI context is quite beyond
> sane.
Yeah, quite so, but it *can* happen so if we can make it work as a
free side-effect of a printk()-robustness increasing patch, why not?
Thanks,
Ingo
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