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

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Date:	Wed, 24 Nov 2010 13:53:47 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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 Wed, 24 Nov 2010, Jeremy Fitzhardinge wrote:

> 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()?

ACCESS_ONCE does not prevent reordering if used once it seems when one
reads the comments. ACCESS_ONCE() uses volatile? Uggh.


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