linux-kernel - Re: [thisops uV2 04/10] x86: Support for this_cpu_add,sub,dec,inc

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Message-ID: <alpine.DEB.2.00.1011291408050.4836@router.home>
Date:	Mon, 29 Nov 2010 14:09:35 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	akpm@...ux-foundation.org, Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [thisops uV2 04/10] x86: Support for
 this_cpu_add,sub,dec,inc_return

On Mon, 29 Nov 2010, Mathieu Desnoyers wrote:

> Ah, ok, so you don't mind if var and val have different types. You just want
> the compiler to be able to cast one into the other without complaining.
>
> Isn't it already checked by "typeof(var) pfo_ret__ = val;" ?

Right. I will drop that strange fragment.

> But at the end, you return :
>
>   pfo_ret__ + (val);
>
> So if pfo_ret has a type smaller than val (e.g. unsigned short vs unsigned int),
> unless I'm mistakened, the type returned by percpu_add_return_op will be an
> unsigned int (the larger of the two), but the atomic operation is performed on
> an unsigned short. This might cause confusion for the caller. Casting
>
>   pfo_ret__ + (typeof(var)) (val);
>
> might be appropriate.

The return needs to be constrained to pfo_ret__. If the value wraps around
then we do not want a value larger than the variable returned.

	pfo_ret__ += val;
	pfo_ret;

will do I think.

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