linux-kernel - Re: [RFC 1/4] hashtable: introduce a small and naive hashtable

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Message-ID: <50197E4A.7020408@gmail.com>
Date:	Wed, 01 Aug 2012 21:06:50 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Tejun Heo <tj@...nel.org>
CC:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	paul.gortmaker@...driver.com
Subject: Re: [RFC 1/4] hashtable: introduce a small and naive hashtable

On 08/01/2012 08:27 PM, Tejun Heo wrote:
> On Wed, Aug 01, 2012 at 08:24:32PM +0200, Sasha Levin wrote:
>> On 08/01/2012 08:21 PM, Tejun Heo wrote:
>>> On Wed, Aug 01, 2012 at 08:19:52PM +0200, Sasha Levin wrote:
>>>> If we switch to using functions, we could no longer hide it anywhere
>>>> (we'd need to either turn the buckets into a struct, or have the
>>>> user pass it around to all functions).
>>>
>>> Create an outer struct hash_table which remembers the size?
>>
>> Possible. I just wanted to avoid creating new structs where they're not really required.
>>
>> Do you think it's worth it for eliminating those two macros?
> 
> What if someone wants to allocate hashtable dynamically which isn't
> too unlikely?  I think it's best to stay away from macro tricks as
> much as possible although I gotta admit I fall into the macro trap
> more often than I would like.

Using a struct makes the dynamic case much easier, but it complicates the static case.

Previously we could create the buckets statically.

Consider this struct:

struct hash_table {
	u32 bits;
	struct hlist_head buckets[];
};

We can't make any code that wraps this to make it work properly statically allocated nice enough to be acceptable.


What if when creating the buckets, we actually allocate bits+1 buckets, and use the last bucket not as a bucket but as the bitcount? It looks like a hack but I think it's much nicer than the previous.
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