RE: [PATCH 3/7] netfilter: xt_HMARK: modulus is expensive for hash calculation

[Eric Dumazet <eric.dumazet@xxxxxxxxx>]

On Thu, 2012-05-17 at 09:16 +0100, David Laight wrote:
> > From: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
> > 
> > Use:
> > 
> > ((u64)(HASH_VAL * HASH_SIZE)) >> 32
> > 
> > as suggested by David S. Miller.
> 
> That (u64) cast is very unlikely to have any effect.
> If you want a 64 bit result from the product of two
> 32 bit values, you have to cast one of the 32 bit values
> prior to the multiply - as in the patch below.

Hey, Changelog is a bit wrong (for several reasons) but code is correct.

return (((u64)hash * info->hmodulus) >> 32) + info->hoffset;



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