Re: [RFC v2 1/7] hashtable: introduce a small and naive hashtable

Sasha Levin
2012-08-03T21:19:57Z

On 08/03/2012 07:15 PM, Tejun Heo wrote:
> Hello, Sasha.
>
> On Fri, Aug 03, 2012 at 04:23:02PM +0200, Sasha Levin wrote:
>> +#define DEFINE_STATIC_HASHTABLE(n, b) \
>> + static struct hash_table n = { .bits = (b), \
>> + .buckets = { [0 ... ((1 << (b)) - 1)] = HLIST_HEAD_INIT } }
>
> What does this "static" mean?
>
>> +#define DEFINE_HASHTABLE(n, b) \
>> + union { \
>> + struct hash_table n; \
>> + struct { \
>> + size_t bits; \
>> + struct hlist_head buckets[1 << (b)]; \
>> + } __##n ; \
>> + };
>
> Is this supposed to be embedded in struct definition? If so, the name
> is rather misleading as DEFINE_* is supposed to define and initialize
> stand-alone constructs. Also, for struct members, simply putting hash
> entries after struct hash_table should work.

It would work, but I didn't want to just put them in the union since I feel it's safer to keep them in a separate struct so they won't be used by mistake,

> Wouldn't using DEFINE_HASHTABLE() for the first macro and
> DEFINE_HASHTABLE_MEMBER() for the latter be better?

Indeed that sounds better, will fix.

>> +#define HASH_BITS(name) ((name)->bits)
>> +#define HASH_SIZE(name) (1 << (HASH_BITS(name)))
>> +
>> +__attribute__ ((unused))
>
> Are we using __attribute__((unused)) for functions defined in headers
> instead of static inline now? If so, why?
>
>> +static void hash_init(struct hash_table *ht, size_t bits)
>> +{
>> + size_t i;
>
> I would prefer int here but no biggie.

Just wondering, is there a particular reason behind it?

>> + ht->bits = bits;
>> + for (i = 0; i < (1 << bits); i++)
>> + INIT_HLIST_HEAD(&ht->buckets[i]);
>> +}
>> +
>> +static void hash_add(struct hash_table *ht, struct hlist_node *node, long key)
>> +{
>> + hlist_add_head(node,
>> + &ht->buckets[hash_long((unsigned long)key, HASH_BITS(ht))]);
>> +}
>> +
>> +
>> +#define hash_get(name, key, type, member, cmp_fn) \
>> +({ \
>> + struct hlist_node *__node; \
>> + typeof(key) __key = key; \
>> + type *__obj = NULL; \
>> + hlist_for_each_entry(__obj, __node, &(name)->buckets[ \
>> + hash_long((unsigned long) __key, \
>> + HASH_BITS(name))], member) \
>> + if (cmp_fn(__obj, __key)) \
>> + break; \
>> + __obj; \
>> +})
>
> As opposed to using hash_for_each_possible(), how much difference does
> this make? Is it really worthwhile?

Most of the places I've switched to using this hashtable so far (4 out of 6) are using hash_get(). I think that the code looks cleaner when you an just provide a comparison function instead of implementing the iteration itself.

I think hash_for_for_each_possible() is useful if the comparison condition is more complex than a simple comparison of one of the object members with the key - there's no need to force it on all the users.

>
> Thanks.