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

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Date:	Thu, 23 Aug 2012 09:54:15 +1000
From:	Ryan Mallon <rmallon@...il.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Sasha Levin <levinsasha928@...il.com>,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
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Subject: Re: [PATCH v3 01/17] hashtable: introduce a small and naive hashtable

On 23/08/12 04:01, 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.
> 
>> +/* 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?
> Also, no biggie but why isn't this macro in caps?

It should probably use gcc's statement expression extensions to prevent
side-effect issues with the arguments:

  #define hash_min ({		\
	sizeof(val) <= 4 ?	\
	hash_32(val, bits) :	\
	hash_long(val, bits));	\
  })

~Ryan

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