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

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Date:	Thu, 2 Aug 2012 03:32:44 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	Tejun Heo <tj@...nel.org>, 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 Thu, Aug 02, 2012 at 12:00:33PM +0200, Sasha Levin wrote:
> 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.

What about using a C99 flexible array member?  Kernel style prohibits
variable-length arrays, but I don't think the same rationale applies to
flexible array members.

struct hash_table {
    size_t count;
    struct hlist_head buckets[];
};

#define DEFINE_HASH_TABLE(name, length) struct hash_table name = { .count = length, .buckets = { [0 ... (length - 1)] = HLIST_HEAD_INIT } }

- Josh Triplett
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