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Re: [dm-devel] [PATCH v3 01/17] hashtable: introduce a small and naive hashtable



Hi Tejun,

On 08/22/2012 08:01 PM, Tejun Heo wrote:
> Hello, Sasha.
> 
> On Wed, Aug 22, 2012 at 04:26:56AM +0200, Sasha Levin wrote:
>> +#define DEFINE_HASHTABLE(name, bits)					\
>> +	struct hlist_head name[HASH_SIZE(bits)];
> 
> Shouldn't this be something like the following?
> 
> #define DEFINE_HASHTABLE(name, bits)					\
> 	struct hlist_head name[HASH_SIZE(bits)] =			\
> 		{ [0 ... HASH_SIZE(bits) - 1] = HLIST_HEAD_INIT };
> 
> Also, given that the declaration isn't non-trivial, you'll probably
> want a matching DECLARE_HASHTABLE() macro too.

I figured we might do a DEFINE_HASHTABLE() to prevent the need from using
hash_init() on hashtables defined this way, but I preferred not to since we may
end up wanting a more complex initialization (I'll explain why extensively below).

>> +/* Use hash_32 when possible to allow for fast 32bit hashing in 64bit kernels. */
>> +#define hash_min(val, bits) ((sizeof(val)==4) ? hash_32((val), (bits)) : hash_long((val), (bits)))
> 
> Why is the branching condition sizeof(val) == 4 instead of <= 4?

No reason, will fix.

> Also, no biggie but why isn't this macro in caps?

I had this plan in my mind to move it into linux/hash.h at some stage later, and
the API there uses low caps even for macros (hash_long()).

>> +/**
>> + * hash_add_rcu_size - add an object to a rcu enabled hashtable
>> + * @hashtable: hashtable to add to
>> + * @bits: bit count used for hashing
>> + * @node: the &struct hlist_node of the object to be added
>> + * @key: the key of the object to be added
>> + */
>> +#define hash_add_rcu_size(hashtable, bits, node, key)				\
>> +	hlist_add_head_rcu(node, &hashtable[hash_min(key, bits)]);
>> +
>> +/**
>> + * hash_add_rcu - add an object to a rcu enabled hashtable
>> + * @hashtable: hashtable to add to
>> + * @node: the &struct hlist_node of the object to be added
>> + * @key: the key of the object to be added
>> + */
>> +#define hash_add_rcu(hashtable, node, key)					\
>> +	hash_add_rcu_size(hashtable, HASH_BITS(hashtable), node, key)
> 
> Or maybe we're better off with hash_head_size() and hash_head()?  I'll
> expand on it later.  Please bear with me.
> 
>> +/**
>> + * hash_hashed - check whether an object is in any hashtable
>> + * @node: the &struct hlist_node of the object to be checked
>> + */
>> +#define hash_hashed(node) (!hlist_unhashed(node))
> 
> As the 'h' in hlist* stand for hash anyway and I think this type of
> thin wrappers tend to obfuscate more than anything else.
> 
>> +/**
>> + * hash_del - remove an object from a hashtable
>> + * @node: &struct hlist_node of the object to remove
>> + */
>> +static inline void hash_del(struct hlist_node *node)
>> +{
>> +	hlist_del_init(node);
>> +}
>> +
>> +/**
>> + * hash_del_rcu - remove an object from a rcu enabled hashtable
>> + * @node: &struct hlist_node of the object to remove
>> + */
>> +static inline void hash_del_rcu(struct hlist_node *node)
>> +{
>> +	hlist_del_init_rcu(node);
>> +}
> 
> If we do that, we can remove all these thin wrappers.
> 
>> +#define hash_for_each_size(name, bits, bkt, node, obj, member)			\
>> +	for (bkt = 0; bkt < HASH_SIZE(bits); bkt++)				\
>> +		hlist_for_each_entry(obj, node, &name[bkt], member)
> ..
>> +#define hash_for_each(name, bkt, node, obj, member)				\
>> +	hash_for_each_size(name, HASH_BITS(name), bkt, node, obj, member)
> ...
>> +#define hash_for_each_rcu_size(name, bits, bkt, node, obj, member)		\
>> +	for (bkt = 0; bkt < HASH_SIZE(bits); bkt++)				\
>> +		hlist_for_each_entry_rcu(obj, node, &name[bkt], member)
> ...
>> +#define hash_for_each_rcu(name, bkt, node, obj, member)				\
>> +	hash_for_each_rcu_size(name, HASH_BITS(name), bkt, node, obj, member)
> ...
>> +#define hash_for_each_safe_size(name, bits, bkt, node, tmp, obj, member)	\
>> +	for (bkt = 0; bkt < HASH_SIZE(bits); bkt++)                     	\
>> +		hlist_for_each_entry_safe(obj, node, tmp, &name[bkt], member)
> ...
>> +#define hash_for_each_safe(name, bkt, node, tmp, obj, member)			\
>> +	hash_for_each_safe_size(name, HASH_BITS(name), bkt, node,		\
>> +				tmp, obj, member)
> ...
>> +#define hash_for_each_possible_size(name, obj, bits, node, member, key)		\
>> +	hlist_for_each_entry(obj, node,	&name[hash_min(key, bits)], member)
> ...
>> +#define hash_for_each_possible(name, obj, node, member, key)			\
>> +	hash_for_each_possible_size(name, obj, HASH_BITS(name), node, member, key)
> ...
>> +#define hash_for_each_possible_rcu_size(name, obj, bits, node, member, key)	\
>> +	hlist_for_each_entry_rcu(obj, node, &name[hash_min(key, bits)], member)
> ...
>> +#define hash_for_each_possible_rcu(name, obj, node, member, key)		\
>> +	hash_for_each_possible_rcu_size(name, obj, HASH_BITS(name),		\
> ...
>> +#define hash_for_each_possible_safe_size(name, obj, bits, node, tmp, member, key)\
>> +	hlist_for_each_entry_safe(obj, node, tmp,				\
>> +		&name[hash_min(key, bits)], member)
> ...
>> +#define hash_for_each_possible_safe(name, obj, node, tmp, member, key)		\
>> +	hash_for_each_possible_safe_size(name, obj, HASH_BITS(name),		\
> 
> And also all these.  We'd only need hash_for_each_head() and
> hash_head().  hash_for_each_possible*() could be nice for convenience,
> I suppose.
> 
> I think the almost trivial nature of hlist hashtables makes this a bit
> tricky and I'm not very sure but having this combinatory explosion is
> a bit dazzling when the same functionality can be achieved by simply
> combining operations which are already defined and named considering
> hashtable.  I'm not feeling too strong about this tho.  What do others
> think?

I'm thinking that this hashtable API will have 2 purposes: First, it would
prevent the excessive duplication of hashtable implementations all around the code.

Second, it will allow more easily interchangeable hashtable implementations to
find their way into the kernel. There are several maintainers who would be happy
to see dynamically sized RCU hashtable, and I'm guessing that several more
variants could be added based on needs in specific modules.

The second reason is why several things you've mentioned look the way they are:

 - No DEFINE_HASHTABLE(): I wanted to force the use of hash_init() since
initialization for other hashtables may be more complicated than the static
initialization for this implementation, which means that any place that used
DEFINE_HASHTABLE() and didn't do hash_init() will be buggy.

 - hash_hashed(): Different hashtable implementations may not use hlist, so
letting them assume that doing hlist operations on hashtable objects is wrong.

 - Lack of hash_head(): I didn't want to expose the internal structure of the
hashtable to the user. This might make it hard to implement hashtables that
resize themselves if we let the user hold a bucket in his hands.

I'm actually tempted in hiding hlist completely from hashtable users, probably
by simply defining a hash_head/hash_node on top of the hlist_ counterparts.

Now, I know that I mention a lot of these hashtables that might or might not be
added later on in the kernel, this is mostly due to the interest in dynamic
resizable hashtables which was shown during the RFC, and the fact that there is
a solid implementation of a dynamic RCU hashtable in urcu which might be a great
benefit in the kernel as well (I'm also looking at it while working on this
hashtable to verify that both could share an API in the future).

Regarding the amount of new macros, we can address this by splitting off any
dynamic elements into a new hashtable type. This will let us reduce quite a lot
of macros (removing all the _size() ones at least).

> Also, can you please audit the comments on top of each macro?  They
> have wrong names and don't differentiate the different variants very
> well.

Sure, sorry about that.

Thanks,
Sasha

> Thanks.
> 


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