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Date: Tue, 5 Jan 2010 12:46:12 -0800 (PST)
From: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Christoph Lameter <cl@linux-foundation.org>, Andi Kleen <andi@firstfloor.org>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Minchan Kim <minchan.kim@gmail.com>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "hugh.dickins" <hugh.dickins@tiscali.co.uk>, Nick Piggin <nickpiggin@yahoo.com.au>, Ingo Molnar <mingo@elte.hu>
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On Tue, 5 Jan 2010, Peter Zijlstra wrote:
>
> On Tue, 2010-01-05 at 10:25 -0800, Linus Torvalds wrote:
> > The readers are all hitting the 
> > lock (and you can try to solve the O(n*2) issue with back-off, but quite 
> > frankly, anybody who does that has basically already lost 
> 
> /me sneaks in a reference to local spinning spinlocks just to have them
> mentioned.

Were you in on the discussion with (I think) Andy Glew about that? I don't 
think he ever came up with an actual workable solution, but he tried. He 
worked for Intel at the time (maybe still does), and was looking at 
architectural improvements. I think you need a "compare-and-exchange-2- 
separate-words" instruction to make it work (not "cmpxchg8/16b" - 
literally two _different_ words).

I think m68k can do it. Even then it's a lot more expensive than a regular 
lock for any reasonable common case.

		Linus

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