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

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Date:	Wed, 08 Jun 2011 23:49:18 +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 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.

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