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Date: Mon, 29 Nov 2010 14:09:35 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
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To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
cc: akpm@linux-foundation.org, Pekka Enberg <penberg@cs.helsinki.fi>,
        linux-kernel@vger.kernel.org, Eric Dumazet <eric.dumazet@gmail.com>,
        Tejun Heo <tj@kernel.org>
Subject: Re: [thisops uV2 04/10] x86: Support for
 this_cpu_add,sub,dec,inc_return
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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.