linux-kernel - Re: [this_cpu_xx V8 11/16] Generic support for this_cpu

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Message-ID: <alpine.DEB.2.00.0912220950030.12163@router.home>
Date:	Tue, 22 Dec 2009 09:54:26 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	Mel Gorman <mel@....ul.ie>,
	Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [this_cpu_xx V8 11/16] Generic support for this_cpu_cmpxchg

On Sat, 19 Dec 2009, Mathieu Desnoyers wrote:

> I am a bit concerned about the "generic" version of this_cpu_cmpxchg.
> Given that what LTTng needs is basically an atomic, nmi-safe version of
> the primitive (on all architectures that have something close to a NMI),
> this means that it could not switch over to your primitives until we add
> the equivalent support we currently have with local_t to all
> architectures. The transition would be faster if we create an
> atomic_cpu_*() variant which would map to local_t operations in the
> initial version.
>
> Or maybe have I missed something in your patchset that address this ?

NMI safeness is not covered by this_cpu operations.

We could add nmi_safe_.... ops?

The atomic_cpu reference make me think that you want full (LOCK)
semantics? Then use the regular atomic ops?
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