Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH net-next 1/2] random32: add prandom_u32_lt_N and convert "misuses" of reciprocal_divide | Date | Thu, 16 Jan 2014 09:28:33 +0000 |
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From: Eric Dumazet > 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.
Maybe rename the current reciprocal_divide() as well?
In the above code it isn't immediately obvious that the result of a division is as useful as that of a modulus! OTOH the high 32 bits of a 32x32 multiply are (slightly) more obviously in the required range.
David
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