linux-kernel - Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking

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Message-ID: <af88b0d6c81fbf480eee4cef120ff710.squirrel@www.firstfloor.org>
Date:	Sat, 3 Sep 2011 01:14:55 +0200
From:	"Andi Kleen" <andi@...stfloor.org>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"Peter Zijlstra" <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Nick Piggin" <npiggin@...nel.dk>, "Avi Kivity" <avi@...hat.com>,
	"Marcelo Tosatti" <mtosatti@...hat.com>,
	"KVM" <kvm@...r.kernel.org>, "Andi Kleen" <andi@...stfloor.org>,
	"Xen Devel" <xen-devel@...ts.xensource.com>,
	"Jeremy Fitzhardinge" <jeremy.fitzhardinge@...rix.com>,
	"Stefano Stabellini" <stefano.stabellini@...citrix.com>
Subject: Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while
 blocking


> But, erm, does that even make sense?  I'm assuming the NMI reason port
> tells the CPU why it got an NMI.  If multiple CPUs can get NMIs and
> there's only a single reason port, then doesn't that mean that either 1)
> they all got the NMI for the same reason, or 2) having a single port is
> inherently racy?  How does the locking actually work there?

All the code to determine NMI reasons is inherently racy,
and each new NMI user makes it worse.

-Andi

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