linux-kernel - RE: [PATCH 2.6.21-rc5] Flush MSI-X table writes (rev 3)

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Date:	Fri, 30 Mar 2007 13:49:20 -0700
From:	"Williams, Mitch A" <mitch.a.williams@...el.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Greg KH" <gregkh@...e.de>
Cc:	"Andrew Morton" <akpm@...ux-foundation.org>,
	<linux-pci@...ey.karlin.mff.cuni.cz>,
	<linux-kernel@...r.kernel.org>,
	"Kok, Auke-jan H" <auke-jan.h.kok@...el.com>
Subject: RE: [PATCH 2.6.21-rc5] Flush MSI-X table writes (rev 3)

Eric W. Biederman wrote:

>The bug report would be phrased as someone seeing "No irq for vector"
>on x86_64.  Unless they are a skilled developer they are unlikely to
>trace it down to not flushing posted writes to a MSI bar during irq
>migration.  It part it is a subtle hardware/software race.
>
>I have seen some unresolved bug reports described this way on machines
>that have MSI-X capable hardware.  The are against fedora core and I
>can't haven't been able to get the reporters to try 2.6.21-rcX and
>the other irq back ports don't work so well.
>
>I currently count 7 drivers in 2.6.21 that are susceptible to the race
>this bug closes.
>
>This version of the patch seems to meet all of the criteria for being
>obviously correct, and all it does is insert a stupid readl.  Not
>very dangerous.
>
>This fix removes a race between hardware and software that you likely
>need to migrate MSI-X irqs a lot to trigger.  Which can be the kind of
>thing that bytes you after you have a month of uptime.  This is a very
>nasty condition to track down from bug reports.  I'd rather not have a
>known culprit out there.

Agreed, this is a subtle bug, and was a real hairball to track down.
Even so, I'm surprised that nobody else has dug into this, since it
should affect anybody running MSI-X.  I originally thought I was seeing
a hardware bug, which is why I dug more deeply into the issue.

If Eric is seeing bug reports related to "no vector for IRQ" in the
wild, then I have to change my stance and agree that this should be
pushed to -stable.  Every one of those messages indicates that we
hit the race condition.

-Mitch
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