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

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Date:	Wed, 8 Jun 2011 23:57:01 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Ingo Molnar <mingo@...e.hu>, 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, linux-tip-commits@...r.kernel.org
Subject: Re: [debug patch] printk: Add a printk killswitch to robustify NMI
 watchdog messages

On Wed, 8 Jun 2011, Peter Zijlstra wrote:

> On Wed, 2011-06-08 at 22:52 +0200, Ingo Molnar wrote:
> > Hm, i thought it would be possible to only express it via the 
> > slowpath: if mutex_trylock() succeeds then *all* execution goes into 
> > the slowpath so we don't have to take all the fastpaths into account.
> 
> Right, but you first have to take wait_lock, then do the trylock, but
> that's complicated for asm/mutex-null.h because trylock will then also
> try to obtain the wait_lock.
> 
> You can do it by creating ___mutex_trylock_slowpath() which contains the
> meat of __mutex_trylock_slowpath() and then implement
> atomic_mutex_trylock{_irq,_irqsave,} using that, not releasing wait_lock
> on success.
> 
> Shouldn't be too bad, but it ain't too pretty either.
> 
> Furthermore, like I said in my initial patch, I share Thomas' worry
> about 'creative' usage of these primitives.

We are way better off with the semaphore abuse confined to printk.c.

A mutex would give us lockdep coverage, but due to the strict owner
semantics - which we have already proven in -rt by converting it to a
mutex - we can annotate console_sem lockdep wise and still keep the
nifty semaphore abuse.

Further I don't have any worries about -rt either as a RT task using
printf is doomed anyway and we should not encourage that by making it
somehow more deterministic.

Thanks,

	tglx
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