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Re: [Xen-devel] [GIT PULL] Fix lost interrupt race in Xen event channels

To: "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [GIT PULL] Fix lost interrupt race in Xen event channels
From: "Jan Beulich" <JBeulich@xxxxxxxxxx>
Date: Mon, 30 Aug 2010 10:22:24 +0100
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "Xen-devel@xxxxxxxxxxxxxxxxxxx" <Xen-devel@xxxxxxxxxxxxxxxxxxx>, Tom Kopec <tek@xxxxxxx>, Daniel Stodden <Daniel.Stodden@xxxxxxxxxx>
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>>> On 30.08.10 at 11:15, Keir Fraser <keir.fraser@xxxxxxxxxxxxx> wrote:
> On 30/08/2010 10:06, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:
>> However, do you also think that pirq_unmask_and_notify() is safe
>> to be called twice? I would think the double EOI potentially sent to
>> Xen could lead to an interrupt getting ack-ed that didn't even get
>> started to be serviced yet.
> Erm, well if this is a race that happens only occasionally, does it matter?
> Worst case you get another interrupt straight away. Only a problem if it
> happens often enough to cause a performance issue or even livelock interrupt
> storm.

I wasn't worried about the level case, but rather the edge one. I
forgot, however, that edge ones are ACKTYPE_NONE in Xen, so
indeed there shouldn't be a problem unless IRQs get moved
around at a very high rate (which clearly they shouldn't).


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