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:

quoted

+#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?

quoted

+#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.

quoted

+#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? 

quoted

+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?

quoted

+	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.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>