From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Andi Kleen" <andi@firstfloor.org>
Subject: Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while
 blocking
Date: Sat, 3 Sep 2011 01:14:55 +0200
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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