linux-kernel - Re: [PATCH net-next 1/2] random32: add prandom_u32_lt_N and convert "misuses" of reciprocal

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Date:	Wed, 15 Jan 2014 19:14:10 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jakub Zawadzki <darkjames-ws@...kjames.pl>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: Re: [PATCH net-next 1/2] random32: add prandom_u32_lt_N and convert
 "misuses" of reciprocal_divide

On Thu, 2014-01-16 at 00:23 +0100, Daniel Borkmann wrote:

> @@ -1220,7 +1219,7 @@ static unsigned int fanout_demux_hash(struct packet_fanout *f,
>  				      struct sk_buff *skb,
>  				      unsigned int num)
>  {
> -	return reciprocal_divide(skb->rxhash, num);
> +	return (u32)(((u64) skb->rxhash * num) >> 32);
>  }
>  

This is unfortunate.

(This reverts one of your patch : f55d112e529386 )

Please add a helper to explain what's going on here, and on many other
spots we do this computation (as in get_rps_cpu()).
Few people really understand this.

Or keep reciprocal_divide() as is, and introduce a new set of functions
for people really wanting the precise divides.



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