From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750887Ab1CPEIR (ORCPT ); Wed, 16 Mar 2011 00:08:17 -0400 Received: from e37.co.us.ibm.com ([32.97.110.158]:60519 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750765Ab1CPEIN (ORCPT ); Wed, 16 Mar 2011 00:08:13 -0400 Date: Tue, 15 Mar 2011 21:02:03 -0700 From: "Paul E. McKenney" To: Arnd Bergmann Cc: Lai Jiangshan , Ingo Molnar , LKML , Manfred Spraul Subject: Re: [PATCH V4 1/1] rcu: introduce kfree_rcu() Message-ID: <20110316040203.GB2273@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <4D7F356C.8020903@cn.fujitsu.com> <201103151302.09381.arnd@arndb.de> <20110315121941.GD2167@linux.vnet.ibm.com> <201103151407.24800.arnd@arndb.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <201103151407.24800.arnd@arndb.de> 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 On Tue, Mar 15, 2011 at 02:07:24PM +0100, Arnd Bergmann wrote: > On Tuesday 15 March 2011, Paul E. McKenney wrote: > > And it makes use of statically allocated structures a bit clunky. > > How do statically allocated structures relate to this? I would > expect that you never call kfree_rcu on them, so it shouldn't > matter. > > > Yet another approach is to use the low-order bit of the rcu_head pointer, > > given that the rcu_head structure does have to be aligned. If this bit > > is set, then the function pointer could be interpreted as an offset. > > This approach might also allow a slab_free_rcu() to be constructed, given > > that the full 32 bits of the function pointer would be available. > > For example, if the upper 16 bits are zero, the low-order 16 bits are > > the offset. If the upper 16 bits are 0x1, then the low-order 16 bits > > might be an index that selects the desired slab cache. > > This solution sounds like a clear improvement over the patch that Lai > Jiangshan posted, without any downsides. Except that I was forgetting that we don't really have any way to stop people from handing us misaligned rcu_head structures -- that topic came up last time as well. Or were the people mentioning that possibility being overly paranoid? Thanx, Paul