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

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Date:	Thu, 16 Jan 2014 01:46:42 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	Daniel Borkmann <dborkman@...hat.com>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.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, Jan 16, 2014 at 12:17:50AM +0000, Ben Hutchings wrote:
> 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.

Good catch, totally overlooked that.

Thanks,

  Hannes

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