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

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Date:	Wed, 24 Nov 2010 10:45:21 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Pekka Enberg <penberg@...nel.org>
cc:	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, Pekka Enberg wrote:

> > The critical section begins with the retrieval of the tid and it ends with
> > the replacement of the tid with the newly generated one. This means that
> > all state data for the alloc and free operation needs to be retrieved in
> > that critical section. The change must be saved with the final
> > cmpxchg_double of the critical section.
>
> Right and we don't need a *memory barrier* here because we're
> accessing a per-CPU variable which means operations appear in-order.

The compiler is still free to rearrange the tid fetch. A possible
optimization that the compiler may do is to move the tid fetch into the
next if statement since that is the only block in which the tid variable
is actually used.

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