linux-kernel - Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks

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Message-ID: <4E83703C.2010907@goop.org>
Date:	Wed, 28 Sep 2011 12:06:36 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Stephan Diestelhorst <stephan.diestelhorst@....com>,
	"H. Peter Anvin" <hpa@...or.com>, Jan Beulich <JBeulich@...e.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nick Piggin <npiggin@...nel.dk>,
	the arch/x86 maintainers <x86@...nel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks

On 09/28/2011 11:49 AM, Linus Torvalds wrote:
> But I don't care all *that* deeply. I do agree that the xaddw trick is
> pretty tricky. I just happen to think that it's actually *less* tricky
> than "read the upper bits separately and depend on subtle ordering
> issues with another writer that happens at the same time on another
> CPU".
>
> So I can live with either form - as long as it works. I think it might
> be easier to argue that the xaddw is guaranteed to work, because all
> values at all points are unarguably atomic (yeah, we read the lower
> bits nonatomically, but as the owner of the lock we know that nobody
> else can write them).

Exactly.  I just did a locked add variant, and while the code looks a
little simpler, it definitely has more actual complexity to analyze.

    J
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