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

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Date:	Wed, 28 Jan 2009 13:34:13 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Arjan van de Ven <arjan@...radead.org>
cc:	Peter Zijlstra <peterz@...radead.org>,
	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>,
	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, Arjan van de Ven wrote:

> On Wed, 28 Jan 2009 12:24:48 -0500 (EST)
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > > ---
> > I'll have skip the NMI test and see if it locks up any place else.
> 
> if you get an NMI during MTRR changing you are more or less screwed.
> Really. (the cpu is uncachable etc)

Heh, the NMI is just another place that has the IPI problem. A different
issue to the MTRR. Well, same bug, different place.

> 
> the MTRR code should disable NMIs as much as possible.
> 
> and for sure in -RT, the MTRR change section should not be preemptable
> at all.

Yeah, that is why I found this. The current code calls kfree and such from 
the IPI if it is allocated (not to mention the kmalloc in 
smp_call_function. Both of which can sleep in RT. I was forcing the 
data=NULL case in RT to keep it from entering the allocation code paths. 
But this is where I found the deadlock with MTRR (and NMI).

-- Steve

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