linux-kernel - Re: [PATCH 0/2] printk vs rq->lock and xtime lock

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Date:	Mon, 24 Mar 2008 15:31:45 +0100
From:	Marcin Slusarz <marcin.slusarz@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] printk vs rq->lock and xtime lock

On Mon, Mar 24, 2008 at 01:24:24PM +0100, Peter Zijlstra wrote:
> Hi Linus,
> 
> As to the regression reported by Marcin; what happens is that we invoke
> printk() while holding the xtime lock for writing. printk() will call
> wake_up_klogd() which tries to enqueue klogd on some rq.
> 
> The known deadlock here is calling printk() while holding rq->lock, which
> would then try to recusively lock the rq again when trying to wake klogd.
> 
> The new deadlock is due to task enqueues setting an hrtimer, which requires
> reading the time, which will result in a live-lock when the printk() call-
> site is holding the xtime lock for writing.
> 
> Thomas would like to preserve the printk() information if possible, hence my
> proposal of printk_nowakeup(). It will do everything printk() normally does,
> except ommit to wake up of klogd. The call is explicitly not EXPORTed so that
> its use is confined to core kernel code.
> 
> Marcin, could you please test these two patches to confirm they do indeed
> solve your issue as well?

I've successfully tested these patches. Thanks.
Tested-by: Marcin Slusarz <marcin.slusarz@...il.com>

Marcin
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