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

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Message-ID: <501AAF47.3090708@gmail.com>
Date:	Thu, 02 Aug 2012 18:48:07 +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 06:15 PM, Josh Triplett wrote:
> On Thu, Aug 02, 2012 at 03:04:19PM +0200, Sasha Levin wrote:
>> On 08/02/2012 01:23 PM, Sasha Levin wrote:
>>>> #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'.
>>>
>>
>> What if we just use two possible decelerations? One of static structs and one for regular ones.
>>
>> struct hash_table {
>>         size_t bits;
>>         struct hlist_head buckets[];
>> };
>>
>> #define DEFINE_HASHTABLE(name, bits)                                    \
>>         union {                                                         \
>>                 struct hash_table name;                                 \
>>                 struct {                                                \
>>                         size_t bits;                                    \
> 
> This shouldn't use "bits", since it'll get expanded to the macro
> argument.
> 
>>                         struct hlist_head buckets[1 << bits];           \
>>                 } __name;                                               \
> 
> __##name
> 
>>         }
>>
>> #define DEFINE_STATIC_HASHTABLE(name, bit)                              \
>>         static struct hash_table name = { .bits = bit,                  \
>>                 .buckets = { [0 ... (bit - 1)] = HLIST_HEAD_INIT } }
> 
> You probably wanted to change that to [0 ... ((1 << bit) - 1)] , to
> match DEFINE_HASHTABLE.

I wrote it by hand and didn't compile test, will fix all of those.

> Since your definition of DEFINE_HASHTABLE would also work fine when used
> statically, why not just always use that?
> 
> #define DEFINE_STATIC_HASHTABLE(name, bits) static DEFINE_HASHTABLE(name, bits) = { .name.bits = bits }

It will get defined fine, but it will be awkward to use. We'd need to pass anonymous union to all the functions that handle this hashtable, which isn't pretty.

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