linux-kernel - Re: [thiscpuops upgrade 10/10] Lockless (and preemptless) fastpaths for slub

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Message-ID: <4CED695E.5050502@goop.org>
Date:	Wed, 24 Nov 2010 11:37:02 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Christoph Lameter <cl@...ux.com>
CC:	Pekka Enberg <penberg@...nel.org>, akpm@...ux-foundation.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [thiscpuops upgrade 10/10] Lockless (and preemptless) fastpaths
 for slub

On 11/24/2010 08:17 AM, Christoph Lameter wrote:
> On Wed, 24 Nov 2010, Pekka Enberg wrote:
>
>>> +       /*
>>> +        * The transaction ids are globally unique per cpu and per operation on
>>> +        * a per cpu queue. Thus they can be guarantee that the cmpxchg_double
>>> +        * occurs on the right processor and that there was no operation on the
>>> +        * linked list in between.
>>> +        */
>>> +       tid = c->tid;
>>> +       barrier();
>> You're using a compiler barrier after every load from c->tid. Why?
> To make sure that the compiler does not do something like loading the tid
> later. The tid must be obtained before the rest of the information from
> the per cpu slab data is retrieved in order to ensure that we have a
> consistent set of data to operate on.

Isn't that best expressed with ACCESS_ONCE()?

    J
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