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

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Message-Id: <20070202170500.57b6c3a3.akpm@linux-foundation.org>
Date:	Fri, 2 Feb 2007 17:05:00 -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 17:35:31 -0700
ebiederm@...ssion.com (Eric W. Biederman) wrote:

> When making the interrupt vectors per cpu I failed to handle a case
> during irq migration.  If the same interrupt comes in while we are
> servicing the irq but before we migrate it the pending bit in the
> local apic IRR register will be set for that irq.
> 
> After migrating the irq to another cpu and or vector the data
> structures will no longer be setup to handle this pending irq.  Then as
> soon as we return from servicing the irq we just migrated we will get
> a nasty: "No irq handler for vector" message. 
> 
> Since we do not disable irqs for edge triggered interrupts except
> in the smallest possible window during migration I cannot avoid
> migrating an irq in the unlikely event that it becomes pending.
> This is because by the time the irq could no longer become pending
> I have already updated all of my data structures.
> 
> Therefore this patch introduces an intermediate state that
> exists soley on the cpu where we are handling the irq during
> migration.  The irq number is changed to negative in the
> vector_irq data structure.
> 
> Once the migration operation is complete we know we will receive
> no more interrupts on this vector so the irq pending state for
> this irq will no longer be updated.  If the irq is not pending and
> we are in the intermediate state we immediately free the vector,
> otherwise in we free the vector in do_IRQ when the pending irq
> arrives.

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?
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