Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752087AbaFFSqM (ORCPT ); Fri, 6 Jun 2014 14:46:12 -0400 Received: from e38.co.us.ibm.com ([32.97.110.159]:58164 "EHLO e38.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751437AbaFFSqJ (ORCPT ); Fri, 6 Jun 2014 14:46:09 -0400 Date: Fri, 6 Jun 2014 11:46:05 -0700 From: "Paul E. McKenney" To: Greg Ungerer Cc: One Thousand Gnomes , Linux Kernel Development , Linux/m68k Subject: Re: rcu alignment warning tripping on m68k Message-ID: <20140606184605.GT4581@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <538696A0.5090601@uclinux.org> <20140529141151.21ed6f95@alan.etchedpixels.co.uk> <5387DF05.1020103@uclinux.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5387DF05.1020103@uclinux.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14060618-1344-0000-0000-0000020E9726 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 30, 2014 at 11:29:41AM +1000, Greg Ungerer wrote: > On 29/05/14 23:11, One Thousand Gnomes wrote: > > On Thu, 29 May 2014 12:08:32 +1000 > > Greg Ungerer wrote: > > > >> Hi All, > >> > >> Inside kernel/rcy/tree.c in __call_rcu() it does an alignment check on > >> the head pointer passed in. This trips on m68k systems, because they only > >> need alignment of 32bit quantities to 16bit boundaries. > > > > __alignof perhaps ? > > That might do. Change then becomes something like: > > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -2467,7 +2467,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_ > unsigned long flags; > struct rcu_data *rdp; > > - WARN_ON_ONCE((unsigned long)head & 0x3); /* Misaligned rcu_head! */ > + WARN_ON_ONCE((unsigned long)head & (__alignof__(head) - 1)); /* Misaligned rcu_head! */ Hmmm... The purpose of the check is to reserve the low-order bits to allow RCU to classify callbacks as being time-critical or not. RCU can probably live with a single bit, but if there is some architecture out there that simply refuses to do alignment, I need to know about it. (See "git show 0bb7b59d6e2b8" for more info.) So how about this instead? - WARN_ON_ONCE((unsigned long)head & 0x1); /* Misaligned rcu_head! */ (Trying to remember if I have seen Linux kernel code that uses both the lower bits...) Thanx, Paul > if (debug_rcu_head_queue(head)) { > /* Probable double call_rcu(), so leak the callback. */ > ACCESS_ONCE(head->func) = rcu_leak_callback; > > Thanks > Greg > > > -- > 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/ > -- 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/