Message-ID: <48738EAC.6090707@goop.org>
Date: Tue, 08 Jul 2008 08:58:36 -0700
From: Jeremy Fitzhardinge <jeremy@goop.org>
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To: Nick Piggin <nickpiggin@yahoo.com.au>
CC: Rik van Riel <riel@redhat.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Christoph Lameter <clameter@sgi.com>, Petr Tesarik <ptesarik@suse.cz>,
       Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org
Subject: Re: Spinlocks: Factor our GENERIC_LOCKBREAK in order to avoid spin
 with irqs disable
References: <Pine.LNX.4.64.0805061220570.30604@schroedinger.engr.sgi.com> <200807081207.34453.nickpiggin@yahoo.com.au> <487301B0.8050801@goop.org> <200807081841.50422.nickpiggin@yahoo.com.au>
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Nick Piggin wrote:
> Sure. Btw. I have no problems with your patchset, but if SMP guests
> are seriously used on x86, just remember the fairness issue. At some
> point you might find you'll need an even more advanced lock for Xen.
>   

Perhaps.  The spin lock has a bounded number of iterations before it 
goes into the vcpu-blocking path.  At that point, it's no longer subject 
to the whims of the cache coherency protocol, and is explicitly woken.  
But it may still have issues when competing with a rapid lock-unlocker.

A ticket-lock variant with some kind of directed yield might be the way 
to go if it turns out to be a problem.

    J
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