linux-kernel - Re: [PATCH 2/2] x86_64 irq: Handle irqs pending in IRR during irq migration.

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Message-Id: <20070202180103.cf1e4217.akpm@linux-foundation.org>
Date:	Fri, 2 Feb 2007 18:01:03 -0800
From:	Andrew Morton <akpm@...l.org>
To:	ebiederm@...ssion.com (Eric W. Biederman)
Cc:	<linux-kernel@...r.kernel.org>, "Lu, Yinghai" <yinghai.lu@....com>,
	"Luigi Genoni" <luigi.genoni@...elli.com>,
	Ingo Molnar <mingo@...e.hu>,
	Natalie Protasevich <protasnb@...il.com>,
	Andi Kleen <ak@...e.de>
Subject: Re: [PATCH 2/2] x86_64 irq:  Handle irqs pending in IRR during irq
 migration.

On Fri, 02 Feb 2007 18:39:15 -0700
ebiederm@...ssion.com (Eric W. Biederman) wrote:

> Andrew Morton <akpm@...l.org> writes:
> 
> > So is this a for-2.6.20 thing?  The bug was present in 2.6.19, so
> > I assume it doesn't affect many people?
> 
> If it's not to late, and this patch isn't too scary.
> 
> It's a really rare set of circumstances that trigger it, but the
> possibility of being hit is pretty widespread, anything with
> more than one cpu, and more then one irq could see this.
> 
> The easiest way to trigger this is to have two level triggered irqs on
> two different cpus using the same vector.  In that case if one acks
> it's irq while the other irq is migrating to a different cpu 2.6.19
> get completely confused and stop handling interrupts properly.
> 
> With my previous bug fix (not to drop the ack when we are confused)
> the machine will stay up, and that is obviously correct and can't
> affect anything else so is probably a candidate for the stable tree.
> 
> With this fix everything just works.
> 
> I don't know how often a legitimate case of the exact same irq
> going off twice in a row is, but that is a possibility as well
> especially with edge triggered interrupts.
> 
> Setting up the test scenario was a pain, but by extremely limiting
> my choice of vectors I was able to confirm I survived several hundred
> of these events with in a couple of minutes no problem.
> 

OK, thanks.  Let's await Andi's feedback.
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