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

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Date:	Thu, 2 Aug 2012 09:15:56 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Sasha Levin <levinsasha928@...il.com>
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 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.

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 }

One downside: you can't use this to define a global non-static hash
table, because you can't have a global non-static anonymous union.
Using the non-union form would actually allow a global non-static hash
table:

#define DEFINE_HASHTABLE_INIT(name, bits) struct hash_table name = { .bits = bits, .buckets = { [0 ... ((1 << bits) - 1)] = HLIST_HEAD_INIT } }

/* elsewhere */
extern struct hash_table name;

I don't know if that seems like a good idea or not.

- Josh Triplett
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