linux-kernel - Re: [linux-pm] Bug in PCI core

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Date:	Fri, 13 Oct 2006 10:49:33 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Adam Belay <abelay@....EDU>,
	Arjan van de Ven <arjan@...radead.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Greg KH <greg@...ah.com>, linux-pci@...ey.karlin.mff.cuni.cz,
	Linux-pm mailing list <linux-pm@...ts.osdl.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [linux-pm] Bug in PCI core

On Fri, Oct 13, 2006 at 06:09:09PM +0100, Alan Cox wrote:
> Ar Gwe, 2006-10-13 am 12:34 -0400, ysgrifennodd Adam Belay:
> > I agree this needs to be fixed.  However, as I previously mentioned,
> > this isn't the right place to attack the problem.  Remember, this wasn't
> > originally a kernel regression.  Rather it's a workaround for a known
> 
> It's a kernel regression. It used to be reliable to read X resource
> addresses at any time.

No it didn't.  It's undefined behaviour to perform *any* PCI config
access to the device while it's doing a D-state transition.  It may have
happened to work with the chips you tried it with, but more likely you
never hit that window because X simply didn't try to do that.

> > Finally, it's worth noting that this issue is really a corner-case, and
> > in most systems it's extremely rare that even incorrect userspace apps
> > would have any issue.
> 
> Except just occasionally and randomly in the field, probably almost
> undebuggable and irreproducable - the very worst conceivable kind of
> bug.

Indeed.  Only now we have software producing it, rather than hardware
producing it.  That's actually an improvement I think, since it forces
awareness of the issue.
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