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

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Message-ID: <alpine.DEB.1.10.0901281334200.25359@gandalf.stny.rr.com>
Date:	Wed, 28 Jan 2009 13:52:47 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.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, 28 Jan 2009, Peter Zijlstra wrote:

> 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.

Is there any reason to force the user to have to deal with a wait? We can 
add this code to handle the failed alloc case, and I can even make it a 
bit better to only do the copy *d = data; when the RELEASE flag is set.

-- Steve

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