netdev - Re: [v1 PATCH 7/14] netfilter: Use rhashtable_lookup instead of lookup

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Date:	Mon, 16 Mar 2015 09:28:30 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>, kaber@...sh.net
Subject: Re: [v1 PATCH 7/14] netfilter: Use rhashtable_lookup instead of
 lookup_compare

On 03/16/15 at 08:14pm, Herbert Xu wrote:
> On Mon, Mar 16, 2015 at 08:28:42AM +0000, Thomas Graf wrote:
> > On 03/15/15 at 09:44pm, Herbert Xu wrote:
> > > The use of rhashtable_lookup_compare in nft_hash is gratuitous
> > > since the comparison function is just doing memcmp.  Furthermore,
> > > there is cruft embedded in the comparson function that should
> > > instead be moved into the caller of the lookup.
> > > 
> > > This patch moves that cruft over and replacces the call to
> > > rhashtable_lookup_compare with rhashtable_lookup.
> > > 
> > > Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>
> > 
> > The reason for the indirection was to not bypass the abstraction
> > nft_data_cmp() provides.
> > 
> > No objection to the change but maybe leave a comment in
> > nft_data_cmp() that if one changes nft_data_cmp() one needs to
> > look at nft_hash and see if the direct use of rhashtable_lookup()
> > is still valid.
> 
> Well we could preserve nft_data_cmp if necessary.  It'll just
> mean an extra indirect call to do the compare when needed.

I like Dave's idea of implementing the lookup as a macro to avoid
the redirect for both rhashtable and API users requiring their own
compare function to inline the compare and hash function:

#define rhashtable_lookup_compare(ht, obj, compare_fn, obj_hash_fn, arg)

Leaving indirect hash calls to the slow path only. This was also
the initial design of the compare lookup variantion until it
"evolved" away from that due to lack of users of that API ;-)
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