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Re: [dm-devel] [PATCH v3 01/17] hashtable: introduce a small and naive hashtable

On 08/24/2012 10:33 PM, Tejun Heo wrote:
> Hello, Sasha.
> On Fri, Aug 24, 2012 at 10:11:55PM +0200, Sasha Levin wrote:
>>> If this implementation is about the common trivial case, why not just
>>> have the usual DECLARE/DEFINE_HASHTABLE() combination?
>> When we add the dynamic non-resizable support, how would DEFINE_HASHTABLE() look?
> Hmmm?  DECLARE/DEFINE are usually for static ones.

Yup, but we could be using the same API for dynamic non-resizable and static if
we go with the DECLARE/hash_init. We could switch between them (and other
implementations) without having to change the code.

>>> I don't know.  If we stick to the static (or even !resize dymaic)
>>> straight-forward hash - and we need something like that - I don't see
>>> what the full encapsulation buys us other than a lot of trivial
>>> wrappers.
>> Which macros do you consider as trivial within the current API?
>> Basically this entire thing could be reduced to DEFINE/DECLARE_HASHTABLE and
>> get_bucket(), but it would make the life of anyone who wants a slightly
>> different hashtable a hell.
> Wouldn't the following be enough to get most of the benefits?
> * hash_head()
> * hash_for_each_head()
> * hash_add*()
> * hash_for_each_possible*()
 * hash_for_each*() ?

Why do we need hash_head/hash_for_each_head()? I haven't stumbled on a place yet
that needed direct access to the bucket itself.

Consider the following list:

 - hash_init
 - hash_add
 - hash_del
 - hash_hashed
 - hash_for_each_[rcu, safe]
 - hash_for_each_possible[rcu, safe]

This basically means 11 macros/functions that would let us have full
encapsulation and will make it very easy for future implementations to work with
this API instead of making up a new one. It's also not significantly (+~2-3)
more than the ones you listed.

>> I think that right now the only real trivial wrapper is hash_hashed(), and I
>> think it's a price worth paying to have a single hashtable API instead of
>> fragmenting it when more implementations come along.
> I'm not objecting strongly against full encapsulation but having this
> many thin wrappers makes me scratch my head.
> Thanks.

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