Date: Wed, 8 Jun 2011 21:17:58 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Peter Zijlstra <peterz@infradead.org>
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
Subject: Re: [debug patch] printk: Add a printk killswitch to robustify NMI
 watchdog messages
Message-ID: <20110608191758.GA12457@elte.hu>
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 <20110606161749.GA22157@elte.hu>
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* Peter Zijlstra <peterz@infradead.org> wrote:

> I came up with the below hackery, seems to actually boot and such 
> on a lockdep enabled kernel (although Ingo did report lockups with 
> a partial version of the patch, still need to look at that).
> 
> The idea is to use the console_sem.lock instead of the semaphore 
> itself, we flush the console when console_sem.count > 0, which 
> means its uncontended. Its more or less equivalent to 
> down_trylock() + up(), except it never releases the sem internal 
> lock, and optimizes the count fiddling away.
> 
> It doesn't require a wakeup because any real semaphore contention 
> will still be spinning on the spinlock instead of enqueueing itself 
> on the waitlist.
> 
> Its rather ugly, exposes semaphore internals in places it 
> shouldn't, although we could of course expose some primitives for 
> this, but then people might thing it'd be okay to use them etc..
> 
> /me puts on the asbestos underwear

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 ...

Thanks,

	Ingo
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