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

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Message-ID: <501ABEE2.10007@gmail.com>
Date:	Thu, 02 Aug 2012 19:54:42 +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 07:44 PM, Josh Triplett wrote:
> On Thu, Aug 02, 2012 at 06:48:07PM +0200, Sasha Levin wrote:
>> 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.
> 
> No, it'll still use the anonymous union, so you'll still have a thing of
> type "struct hash_table" with the given name, and you can use that name
> with the hash-table functions.

We can use 'struct hash_table' directly, but then the call will look awkward :)

Consider this case (I've placed arbitrary values into size and name:

/* I've "preprocessed" the DEFINE macro below */
union {
	struct hash_table table;
	struct {
		size_t bits;
		struct hlist_head buckets[32];
	}
} my_hashtable;

void foo(struct hash_table *table)
{
/* Do something */
}

int main(void)
{
	foo(my_hashtable); /* This is what the user expects to work, and won't work in this case */

	foo(&my_hashtable.table); /* This is what he has to do, which means the user has to know about the internal structure of the union */
}
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