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

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Message-ID: <4E834FBA.1080709@goop.org>
Date:	Wed, 28 Sep 2011 09:47:54 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Jan Beulich <JBeulich@...e.com>,
	Stephan Diestelhorst <stephan.diestelhorst@....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, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH 00/10] [PATCH RFC V2] Paravirtualized ticketlocks

On 09/28/2011 09:10 AM, Linus Torvalds wrote:
> On Wed, Sep 28, 2011 at 8:55 AM, Jan Beulich <JBeulich@...e.com> wrote:
>>> just use "lock xaddw" there too.
>> I'm afraid that's not possible, as that might carry from the low 8 bits
>> into the upper 8 ones, which must be avoided.
> Oh damn, you're right. So I guess the "right" way to do things is with
> cmpxchg, but some nasty mfence setup could do it too.

Could do something like:

	if (ticket->head >= 254)
		prev = xadd(&ticket->head_tail, 0xff02);
	else
		prev = xadd(&ticket->head_tail, 0x0002);

to compensate for the overflow.

    J
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