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

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Message-ID: <501C4471.4090706@gmail.com>
Date:	Fri, 03 Aug 2012 23:36:49 +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 11:30 PM, Tejun Heo wrote:
>> 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.
> I don't know.  What's the difference?  In terms of LOC, it might even
> not save any thanks to the extra function definition, right?  I don't
> think it's saving enough complexity to justify a separate rather
> unusual interface.

The function definition itself is just a macro, for example:

	#define MM_SLOTS_HASH_CMP(mm_slot, obj) ((mm_slot)->mm == (obj))

As an alternative, what do you think about simplifying that to be just a 'cond' instead of a function? Something like:

	hash_get(&mm_slots_hash, mm, struct mm_slot, hash, mm);

In that case, the last param ("mm") will get unrolled to a condition like this:

	if ((obj)->mm == key)

Which will be simple and easy for the user.


The only reason I want to keep this interface is that most cases I've stumbled so far were easy short comparisons of a struct member with the key, and I don't want to make them more complex than they need to be. I probably will switch hash_get() to use hash_for_each_possible() as well, which will cut down on how hash_get() is a separate case.
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