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

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Message-ID: <501A633B.3010509@gmail.com>
Date:	Thu, 02 Aug 2012 13:23:39 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Josh Triplett <josh@...htriplett.org>
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 08/02/2012 12:32 PM, Josh Triplett wrote:
> 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 } }

The limitation of this approach is that the struct hash_table variable must be 'static', which is a bit limiting - see for example the use of hashtable in 'struct user_namespace'.

> 
> - Josh Triplett
> 

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