linux-kernel - Re: [rfc][patch] queued spinlocks (i386)

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Message-Id: <20070323114047.b60f2acc.dada1@cosmosbay.com>
Date:	Fri, 23 Mar 2007 11:40:47 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ravikiran G Thirumalai <kiran@...lex86.org>
Subject: Re: [rfc][patch] queued spinlocks (i386)

On Fri, 23 Mar 2007 11:32:44 +0100
Nick Piggin <npiggin@...e.de> wrote:

> On Fri, Mar 23, 2007 at 11:04:18AM +0100, Ingo Molnar wrote:
> > 
> > * Nick Piggin <npiggin@...e.de> wrote:
> > 
> > > Implement queued spinlocks for i386. [...]
> > 
> > isnt this patented by MS? (which might not worry you SuSE/Novell guys, 
> > but it might be a worry for the rest of the world ;-)
> 
> Hmm, it looks like they have implemented a system where the spinning
> cpu sleeps on a per-CPU variable rather than the lock itself, and
> the releasing cpu writes to that variable to wake it.  They do this
> so that spinners don't continually perform exclusive->shared
> transitions on the lock cacheline. They call these things queued
> spinlocks.  They don't seem to be very patent worthy either, but
> maybe it is what you're thinking of?
> 
> I'm not as concerned about the contended performance of spinlocks
> for Linux as MS seems to be for windows (they seem to be very proud
> of this lock). Because if it is a big problem then IMO it is a bug.
> 
> This was just something I had in mind when the hardware lock
> starvation issue came up, so I thought I should quickly code it up
> and RFC...  actually it makes contended performance worse, but I'm
> not too worried about that because I'm happy I was able to implement
> it without increasing data size or number of locked operations.

Sure, but please note that you should rename your patch to :

"Implement queued spinlocks for i486"

:)
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