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

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Date:	Wed, 8 Jun 2011 22:52:53 +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 Wed, 2011-06-08 at 21:17 +0200, Ingo Molnar wrote:
> > Hm, the no-wakeup aspect seems rather useful.
> > 
> > Could we perhaps remove console_sem and replace it with a mutex and 
> > do something like this with a mutex and its ->wait_lock?
> > 
> > We'd have two happy side effects:
> > 
> >  - we'd thus remove one of the last core kernel semaphore users
> >  - we'd gain lockdep coverage for console locking as a bonus ... 
> 
> The mutex thing is more complex due to the mutex fast path, the 
> advantage of the semaphore is its simple implementation that always 
> takes the internal lock.
> 
> I guess I can make it happen, but its a tad more tricky.

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.

If that's notpossible then i think you and Linus are right that it's 
not worth creating all the per arch fastpath special cases for 
something like this.

The non-removal of the console_sem is sad though. Sniff.

Thanks,

	Ingo
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