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Date: Sat, 3 Sep 2011 01:14:55 +0200
Subject: Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while
 blocking
From: "Andi Kleen" <andi@firstfloor.org>
To: "Jeremy Fitzhardinge" <jeremy@goop.org>
Cc: "Peter Zijlstra" <peterz@infradead.org>, "H. Peter Anvin" <hpa@zytor.com>,
        "Linus Torvalds" <torvalds@linux-foundation.org>,
        "Ingo Molnar" <mingo@elte.hu>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
        "Nick Piggin" <npiggin@kernel.dk>, "Avi Kivity" <avi@redhat.com>,
        "Marcelo Tosatti" <mtosatti@redhat.com>, "KVM" <kvm@vger.kernel.org>,
        "Andi Kleen" <andi@firstfloor.org>,
        "Xen Devel" <xen-devel@lists.xensource.com>,
        "Jeremy Fitzhardinge" <jeremy.fitzhardinge@citrix.com>,
        "Stefano Stabellini" <stefano.stabellini@eu.citrix.com>
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> 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