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

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Message-ID: <501C407D.9080900@gmail.com>
Date:	Fri, 03 Aug 2012 23:19:57 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Tejun Heo <tj@...nel.org>
CC:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	paul.gortmaker@...driver.com, davem@...emloft.net,
	rostedt@...dmis.org, mingo@...e.hu, ebiederm@...ssion.com,
	aarcange@...hat.com, ericvh@...il.com, netdev@...r.kernel.org
Subject: Re: [RFC v2 1/7] hashtable: introduce a small and naive hashtable

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.
> 

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