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

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Message-ID: <501A4FC1.8040907@gmail.com>
Date:	Thu, 02 Aug 2012 12:00:33 +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, josh@...htriplett.org
Subject: Re: [RFC 1/4] hashtable: introduce a small and naive hashtable

On 08/02/2012 12:45 AM, Tejun Heo wrote:
> On Thu, Aug 02, 2012 at 12:41:56AM +0200, Sasha Levin wrote:
>> How would your DEFINE_HASHTABLE look like if we got for the simple
>> 'struct hash_table' approach?
> 
> I think defining a different enclosing anonymous struct which the
> requested number of array entries and then aliasing the actual
> hash_table to that symbol should work.  It's rather horrible and I'm
> not sure it's worth the trouble.

I agree that this is probably not worth the trouble.

At the moment I see two alternatives:

1. Dynamically allocate the hash buckets.

2. Use the first bucket to store size. Something like the follows:

	#define HASH_TABLE(name, bits)	\
        	struct hlist_head name[1 << bits + 1];

	#define HASH_TABLE_INIT (bits) ({name[0].next = bits});

And then have hash_{add,get} just skip the first bucket.


While it's not a pretty hack, I don't see a nice way to avoid having to dynamically allocate buckets for all cases.
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