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

[Tejun Heo <tj kernel org>]

Hello, Sasha.

On Thu, Aug 23, 2012 at 02:24:32AM +0200, Sasha Levin wrote:
> > I think the almost trivial nature of hlist hashtables makes this a bit
> > tricky and I'm not very sure but having this combinatory explosion is
> > a bit dazzling when the same functionality can be achieved by simply
> > combining operations which are already defined and named considering
> > hashtable.  I'm not feeling too strong about this tho.  What do others
> > think?
> 
> I'm thinking that this hashtable API will have 2 purposes: First, it would
> prevent the excessive duplication of hashtable implementations all around the code.
> 
> Second, it will allow more easily interchangeable hashtable implementations to
> find their way into the kernel. There are several maintainers who would be happy
> to see dynamically sized RCU hashtable, and I'm guessing that several more
> variants could be added based on needs in specific modules.
> 
> The second reason is why several things you've mentioned look the way they are:
> 
>  - No DEFINE_HASHTABLE(): I wanted to force the use of hash_init() since
> initialization for other hashtables may be more complicated than the static
> initialization for this implementation, which means that any place that used
> DEFINE_HASHTABLE() and didn't do hash_init() will be buggy.

I think this is problematic.  It looks exactly like other existing
DEFINE macros yet what its semantics is different.  I don't think
that's a good idea.

> I'm actually tempted in hiding hlist completely from hashtable users, probably
> by simply defining a hash_head/hash_node on top of the hlist_ counterparts.

I think that it would be best to keep this one simple & obvious, which
already has enough in-kernel users to justify its existence.  There
are significant benefits in being trivially understandable and
expectable.  If we want more advanced ones - say resizing, hybrid or
what not, let's make that a separate one.  No need to complicate the
common straight-forward case for that.

So, I think it would be best to keep this one as straight-forward and
trivial as possible.  Helper macros to help its users are fine but
let's please not go for full encapsulation.

Thanks.

-- 
tejun