linux-kernel - Re: [PATCH net-next 2/2] reciprocal_divide: correction/update of the algorithm

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Message-ID: <1389831470.11912.40.camel@bwh-desktop.uk.level5networks.com>
Date:	Thu, 16 Jan 2014 00:17:50 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Daniel Borkmann <dborkman@...hat.com>
CC:	<davem@...emloft.net>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"Austin S Hemmelgarn" <ahferroin7@...il.com>,
	Jesse Gross <jesse@...ira.com>,
	"Jamal Hadi Salim" <jhs@...atatu.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Matt Mackall <mpm@...enic.com>,
	Pekka Enberg <penberg@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	"Andy Gospodarek" <andy@...yhouse.net>,
	Veaceslav Falico <vfalico@...hat.com>,
	"Jay Vosburgh" <fubar@...ibm.com>,
	Jakub Zawadzki <darkjames-ws@...kjames.pl>
Subject: Re: [PATCH net-next 2/2] reciprocal_divide: correction/update of
 the algorithm

On Thu, 2014-01-16 at 00:23 +0100, Daniel Borkmann wrote:
[...]
> --- a/include/linux/reciprocal_div.h
> +++ b/include/linux/reciprocal_div.h
[...]
> + * RECIPROCAL_VALUE_TO_ZERO can be used to express an element, which
> + * used as the argument to reciprocal_divide always yields zero.
>   */
[...]
> +#define RECIPROCAL_VALUE_RESULT_TO_ZERO ((struct reciprocal_value){.sh1 = 32})
[...]
> +static inline u32 reciprocal_divide(u32 a, struct reciprocal_value R)
>  {
> -	return (u32)(((u64)A * R) >> 32);
> +	u32 t = (u32)(((u64)a * R.m) >> 32);
> +	return (t + ((a - t) >> R.sh1)) >> R.sh2;
[...]

(a - t) has type u32.
So (a - t) >> 32 has undefined behaviour.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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