linux-kernel - Re: [RFC][PATCH 6/8] mm: handle_speculative

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Message-ID: <alpine.LFD.2.00.1001051241190.3630@localhost.localdomain>
Date:	Tue, 5 Jan 2010 12:46:12 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Minchan Kim <minchan.kim@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()



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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