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

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Message-ID: <20120803214414.GL15477@google.com>
Date:	Fri, 3 Aug 2012 14:44:14 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Sasha Levin <levinsasha928@...il.com>
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

Hello, Sasha.

On Fri, Aug 03, 2012 at 11:36:49PM +0200, Sasha Levin wrote:
> On 08/03/2012 11:30 PM, Tejun Heo wrote:
> The function definition itself is just a macro, for example:
> 
> 	#define MM_SLOTS_HASH_CMP(mm_slot, obj) ((mm_slot)->mm == (obj))

It seems like it would make things more difficult to follow and
error-prone.  I'd definitely prefer just using functions.

> 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.

It seems a bit too magical(tm) to me. ;)

> 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.

I can understand that but I think the benefit we're talking about is a
bit too miniscule to matter and to have two different interfaces.
What do others think?

Thanks.

-- 
tejun
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