From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754471AbcHXSFL (ORCPT ); Wed, 24 Aug 2016 14:05:11 -0400 Received: from mail-lf0-f66.google.com ([209.85.215.66]:35962 "EHLO mail-lf0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753057AbcHXSFJ (ORCPT ); Wed, 24 Aug 2016 14:05:09 -0400 Date: Wed, 24 Aug 2016 21:03:14 +0300 From: "Kirill A. Shutemov" To: "Paul E. McKenney" Cc: Geert Uytterhoeven , Steven Rostedt , Peter Zijlstra , Eric Dumazet , LKML , Ingo Molnar , jiangshanlai@gmail.com, dipankar@in.ibm.com, Andrew Morton , Mathieu Desnoyers , Josh Triplett , Thomas Gleixner , David Howells , Darren Hart , =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker , Oleg Nesterov , pranith kumar , Greg Ungerer Subject: Re: [PATCH tip/core/rcu 2/2] documentation: Record reason for rcu_head two-byte alignment Message-ID: <20160824180314.GA29871@node.shutemov.name> References: <20160822185609.GK10153@twins.programming.kicks-ass.net> <20160822151854.175dfea8@grimm.local.home> <20160822195445.GO3482@linux.vnet.ibm.com> <20160822211606.GU3482@linux.vnet.ibm.com> <20160823134314.GE3482@linux.vnet.ibm.com> <20160823142326.GF3482@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160823142326.GF3482@linux.vnet.ibm.com> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 23, 2016 at 07:23:26AM -0700, Paul E. McKenney wrote: > On Tue, Aug 23, 2016 at 03:45:51PM +0200, Geert Uytterhoeven wrote: > > Hi Paul, > > > > On Tue, Aug 23, 2016 at 3:43 PM, Paul E. McKenney > > wrote: > > > On Tue, Aug 23, 2016 at 08:39:18AM +0200, Geert Uytterhoeven wrote: > > >> On Mon, Aug 22, 2016 at 11:16 PM, Paul E. McKenney > > >> wrote: > > >> > On Mon, Aug 22, 2016 at 10:48:57PM +0200, Geert Uytterhoeven wrote: > > >> >> On Mon, Aug 22, 2016 at 9:54 PM, Paul E. McKenney > > >> >> wrote: > > >> >> > On Mon, Aug 22, 2016 at 03:18:54PM -0400, Steven Rostedt wrote: > > >> >> >> On Mon, 22 Aug 2016 20:56:09 +0200 > > >> >> >> Peter Zijlstra wrote: > > >> >> >> > > >> >> >> > > Don't we have __alignof__(void *) to avoid #ifdef CONFIG_M68K and > > >> >> >> > > other new macros ? > > >> >> > > > >> >> > Hmmm... Does __alignof__(void *) give two-byte alignment on m68k, > > >> >> > allowing something like this? Heh!!! It is already there. ;-) > > >> >> > > > >> >> > struct callback_head { > > >> >> > struct callback_head *next; > > >> >> > void (*func)(struct callback_head *head); > > >> >> > } __attribute__((aligned(sizeof(void *)))); > > >> >> > > >> >> No, it's aligning to sizeof(void *) (4 on m68k), not __alignof__(void *). > > >> > > > >> > Right you are. Commit 720abae3d68ae from Kirill A. Shutemov in November > > >> > 2015. > > >> > > > >> > Given that you haven't complained, I am guessing that this works for you. > > >> > If so, I can make the __call_rcu() WARN_ON() more strict. > > >> > Again, does the current state work for you? > > > > >> Yes it does. See also your commit 1146edcbef378922 ("rcu: Loosen __call_rcu()'s > > >> rcu_head alignment constraint"). > > > > > > Understood! > > > > > > But given that all architectures now provide at least four-byte alignment > > > for the rcu_head structure, isn't it now OK for me to tighten up __call_rcu()'s > > > check, for example, to this? > > > > > > WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1)); > > > > Yes, I agree with that. > > Very good, I have queued the following patch. > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 89d39c83d193733ed5fff1c480cd42c9de1da404 > Author: Paul E. McKenney > Date: Tue Aug 23 06:51:47 2016 -0700 > > rcu: Tighted up __call_rcu() rcu_head alignment check > > Commit 720abae3d68ae ("rcu: force alignment on struct > callback_head/rcu_head") forced the rcu_head (AKA callback_head) > structure's alignment to pointer size, that is, to 4-byte boundaries on > 32-bit systems and to 8-byte boundaries on 64-bit systems. This > commit therefore checks for this same alignment in __call_rcu(), > which used to check for two-byte alignment. > > Signed-off-by: Paul E. McKenney > Cc: Geert Uytterhoeven > Cc: Kirill A. Shutemov Looks good to me. Acked-by: Kirill A. Shutemov -- Kirill A. Shutemov