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

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Date:	Tue, 5 Jan 2010 12:13:05 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Andi Kleen <andi@...stfloor.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"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:

> So a spinlock is as cheap as an atomic when there is no contention (which
> is the common single-thread case - the real cost of both lock and atomic
> is simply the fact that CPU serialization is expensive), but when there is
> actual lock contention, I bet the atomic xadd is going to be shown to be
> superior.
>
> Remember: we commonly claim that 'spin_unlock' is basically free on x86 -
> and that's true, but it is _only_ true for the uncontended state.

Its also free if the MESI algorithm has been tuned in such a way that the
exclusive cacheline that was just acquired is not immediately released
after a single access.

If the critical section protected by the spinlock is small then the
delay will keep the cacheline exclusive until we hit the unlock. This
is the case here as far as I can tell.


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