Re: [v2 PATCH 3/10] rhashtable: Allow hashfn to be unset

On 03/23/15 at 08:12am, Herbert Xu wrote:

On Sun, Mar 22, 2015 at 12:32:57PM +0000, Thomas Graf wrote:

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Sure but why not just store key_len/4 in ht->p.key_len then if you
opt in to jhash2() in rhashtable_init()?

Because that breaks rhashtable_compare/memcmp.

Thanks. Didn't see that.

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+	if (!__builtin_constant_p(params.key_len))
+		hash = ht->p.hashfn(key, ht->key_len, tbl->hash_rnd);

I don't understand this. It looks like you only consider
params->key_len if it's constant.

If params->key_len is not constant, then params == ht->p.

I must be missing something obvious. Who guarantees that? I can see
that's true for the current callers but what prevents anybody from
using rhashtable_lookup_fast() with a key length not known at compile
time and pass it as rhashtable_params?

They shouldn't be doing that.  The whole point of this function
is to have it inlined so all external callers of it should be
supplying a constant parameter.  We could add a __ variant that
is only called by rhashtable if you like so we can enforce this
in rhashtable_lookup_fast.

If you add such constraints it must be clearly documented. There
is no way of figuring this out right now without reading the entire
rhashtable code (and talking to you).

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I still don't get this. Why do we fall back to jhash2() if
params.key_len is set but not if only ht->p.key_len is set?

Because if params.key_len is not set then we have no idea whether
we should use jhash or jhash2 because ht->p.key_len cannot be
known at compile time.  This is only used by netfilter currently
as it has a key-length set at run-time.

Sorry, still not getting this ;-)

nft_hash sets key_len to set->klen and passes it to rhashtable_init().
rhashtable_init() should then fall back to jhash() or jhash2() if no
hashfn is provided. Why is the logic in rht_key_hashfn() different?
Actually, in which case is ht->p.hashfn not set in rht_key_hashfn()?