Subject: Re: [debug patch] printk: Add a printk killswitch to robustify NMI
 watchdog messages
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: Arne Jansen <lists@die-jansens.de>,
        Linus Torvalds <torvalds@linux-foundation.org>, mingo@redhat.com,
        hpa@zytor.com, linux-kernel@vger.kernel.org, efault@gmx.de,
        npiggin@kernel.dk, akpm@linux-foundation.org, frank.rowand@am.sony.com,
        tglx@linutronix.de, linux-tip-commits@vger.kernel.org
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Date: Wed, 08 Jun 2011 23:49:18 +0200
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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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