linux-kernel - Re: Spinlocks: Factor our GENERIC_LOCKBREAK in order to avoid spin with irqs disable

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Message-ID: <48738EAC.6090707@goop.org>
Date:	Tue, 08 Jul 2008 08:58:36 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Lameter <clameter@....com>,
	Petr Tesarik <ptesarik@...e.cz>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: Spinlocks: Factor our GENERIC_LOCKBREAK in order to avoid spin
 with irqs disable

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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