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

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Message-ID: <alpine.LFD.2.00.1001051251090.3630@localhost.localdomain>
Date:	Tue, 5 Jan 2010 13:00:18 -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, Linus Torvalds wrote:
>
> I think you need a "compare-and-exchange-2-separate-words" instruction 
> to make it work (not "cmpxchg8/16b" - literally two _different_ words).

Btw, I might be misremembering - Andy was looking at various lockless 
algorithms too. Maybe the problem was the non-local space requirement. 
There were several spin-lock variants that would be improved if we could 
pass a cookie from the 'lock' to the 'unlock'.

In fact, even the ticket locks would be improved by that, since we could 
then possibly do the unlock as a plain 'store' rather than an 'add', and 
keep the nex-owner cookie in a register over the lock rather than unlock 
by just incrementing it in the nasty lock cacheline.

		Linus
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