From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753967Ab2HCWTj (ORCPT ); Fri, 3 Aug 2012 18:19:39 -0400 Received: from mail-bk0-f46.google.com ([209.85.214.46]:42230 "EHLO mail-bk0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753319Ab2HCWTg (ORCPT ); Fri, 3 Aug 2012 18:19:36 -0400 Message-ID: <501C4E92.1070801@gmail.com> Date: Sat, 04 Aug 2012 00:20:02 +0200 From: Sasha Levin User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:14.0) Gecko/20120730 Thunderbird/14.0 MIME-Version: 1.0 To: Tejun Heo CC: torvalds@linux-foundation.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, paul.gortmaker@windriver.com, davem@davemloft.net, rostedt@goodmis.org, mingo@elte.hu, ebiederm@xmission.com, aarcange@redhat.com, ericvh@gmail.com, netdev@vger.kernel.org Subject: Re: [RFC v2 1/7] hashtable: introduce a small and naive hashtable References: <1344003788-1417-1-git-send-email-levinsasha928@gmail.com> <1344003788-1417-2-git-send-email-levinsasha928@gmail.com> <20120803171515.GH15477@google.com> <501C407D.9080900@gmail.com> <20120803213017.GK15477@google.com> <501C458E.7050000@gmail.com> <20120803214806.GM15477@google.com> In-Reply-To: <20120803214806.GM15477@google.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. >