Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933531Ab1CXAcc (ORCPT ); Wed, 23 Mar 2011 20:32:32 -0400 Received: from e39.co.us.ibm.com ([32.97.110.160]:51141 "EHLO e39.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933406Ab1CXAca (ORCPT ); Wed, 23 Mar 2011 20:32:30 -0400 Date: Wed, 23 Mar 2011 17:32:23 -0700 From: "Paul E. McKenney" To: James Bottomley Cc: Robert Love , Lai Jiangshan , Ingo Molnar , Jens Axboe , Neil Horman , "David S. Miller" , Alexey Kuznetsov , "Pekka Savola (ipv6)" , James Morris , Hideaki YOSHIFUJI , Patrick McHardy , Eric Dumazet , Stephen Hemminger , Tejun Heo , Jarek Poplawski , "linux-kernel@vger.kernel.org" , "devel@open-fcoe.org" , "linux-scsi@vger.kernel.org" , "netdev@vger.kernel.org" Subject: Re: [PATCH 02/36] scsi,rcu: convert call_rcu(fc_rport_free_rcu) to kfree_rcu() Message-ID: <20110324003223.GF2322@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <4D82D071.5020703@cn.fujitsu.com> <4D82D3FF.2080303@cn.fujitsu.com> <4D82D45A.30102@cn.fujitsu.com> <1300814913.19083.427.camel@fritz> <20110323065014.GU2322@linux.vnet.ibm.com> <1300889151.15899.4.camel@mulgrave.site> <20110323222456.GC2322@linux.vnet.ibm.com> <1300920332.15899.45.camel@mulgrave.site> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1300920332.15899.45.camel@mulgrave.site> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2377 Lines: 54 On Wed, Mar 23, 2011 at 05:45:32PM -0500, James Bottomley wrote: > On Wed, 2011-03-23 at 15:24 -0700, Paul E. McKenney wrote: > > On Wed, Mar 23, 2011 at 09:05:51AM -0500, James Bottomley wrote: > > > On Tue, 2011-03-22 at 23:50 -0700, Paul E. McKenney wrote: > > > > The kfree_rcu() definition is as > > > > follows: > > > > > > > > #define kfree_rcu(ptr, rcu_head) \ > > > > __kfree_rcu(&((ptr)->rcu_head), offsetof(typeof(*(ptr)), rcu_head)) > > > > > > Isn't this one of those cases where the obvious use of the interface is > > > definitely wrong? > > > > > > It's also another nasty pseudo C prototype. I know we do this sort of > > > thing for container_of et al, but I don't really think we want to extend > > > it. > > > > > > Why not make the interface take a pointer to the embedding structure and > > > one to the rcu_head ... that way all pointer mathematics can be > > > contained inside the RCU routines. > > > > Hello, James, > > > > If you pass in a pair of pointers, then it is difficult for RCU to detect > > bugs where the two pointers are unrelated. Yes, you can do some sanity > > checks, but these get cumbersome and have corner cases where they can > > be fooled. In contrast, Lai's interface allows the compiler to do the > > needed type checking -- unless the second argument is a field of type > > struct rcu_head in the structure pointed to by the first argument, the > > compiler will complain. > > > > Either way, the pointer mathematics are buried in the RCU API. > > > > Or am I missing something here? > > No ... I like the utility ... I just dislike the inelegance of having to > name a structure element in what looks like a C prototype. > > I can see this proliferating everywhere since most of our reference > counting release callbacks basically free the enclosing object ... Indeed! Improvements are welcome -- it is just that I am not convinced that the dual-pointer approach is really an improvement. The C preprocessor... It is ugly, inelegant, painful, annoying, and should have been strangled at birth -- but it is always there when you need it! Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/