linux-kernel - Re: Buggy IPI and MTRR code on low memory

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Date:	Wed, 28 Jan 2009 19:20:34 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>, jens.axboe@...cle.com
Subject: Re: Buggy IPI and MTRR code on low memory

On Wed, 2009-01-28 at 12:24 -0500, Steven Rostedt wrote:
> On Wed, 28 Jan 2009, Peter Zijlstra wrote:
> > ---
> > Subject: x86: fix potential deadlock in set_mtrr()
> > From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Date: Wed Jan 28 17:17:32 CET 2009
> > 
> > smp_call_function() can fall-back to waiting on completion in case of
> > low memory (GFP_ATOMIC). set_mtrr() relies on the async behaviour of !wait.
> > 
> > This would deadlock.
> > 
> > Fix this by providing per-cpu csd's and using __smp_call_function_single().
> 
> I applied the patch but it still locked up in the testing NMI 
> watchdog code. Not saying your patch is at fault, because we never made it 
> to your code. This is probably not an issue, since if we have low memory 
> at bootup, we have much bigger problems to deal with.
> 
> I'll have skip the NMI test and see if it locks up any place else.

Right, the NMI code does exactly the same, in that case your patch might
well be the best way forward.

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