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

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Message-ID: <501C4E92.1070801@gmail.com>
Date:	Sat, 04 Aug 2012 00:20:02 +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 11:48 PM, Tejun Heo wrote:
> Hello,
> 
> On Fri, Aug 03, 2012 at 11:41:34PM +0200, Sasha Levin wrote:
>> I forgot to comment on that one, sorry.
>>
>> If we put hash entries after struct hash_table we don't take the
>> bits field size into account, or did I miss something?
> 
> So, if you do the following,
> 
> 	struct {
> 		struct {
> 			int i;
> 			long ar[];
> 		} B;
> 		long __ar_storage[32];
> 	} A;

struct A should have been an union, right?

> It should always be safe to dereference A.B.ar[31].  I'm not sure
> whether this is something guaranteed by C tho.  Maybe compilers are
> allowed to put members in reverse order but I think we already depend
> on the above.

why is accessing A.B.ar[31] safe?

__ar_storage is only 32*sizeof(long) bytes long, while struct B would need to be 32*sizeof(long) + sizeof(int) bytes long so that A.B.ar[31] access would be safe.


> Thanks.
> 

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