From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arvid Brodin Subject: Re: [RFC] net/hsr: Add support for IEC 62439-3 High-availability Seamless Redundancy Date: Wed, 11 Apr 2012 00:00:13 +0000 Message-ID: <4F84CA36.7020209@xdin.com> References: <20120403113751.21fd0b17@s6510.linuxnetplumber.net> <4F7CD4BC.4000006@enea.com> <20120404165559.5223ab95@s6510.linuxnetplumber.net> <20120404.202109.2046106039992811660.davem@davemloft.net> <4F7F10FC.3020308@enea.com> <1333731991.3282.17.camel@deadeye> <20120406111912.172bb1fb@nehalam.linuxnetplumber.net> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT Cc: "netdev@vger.kernel.org" , "balferreira@googlemail.com" To: Stephen Hemminger , Ben Hutchings , David Miller Return-path: Received: from spam1.webland.se ([91.207.112.90]:49594 "EHLO spam1.webland.se" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759841Ab2DKAOZ convert rfc822-to-8bit (ORCPT ); Tue, 10 Apr 2012 20:14:25 -0400 In-Reply-To: <20120406111912.172bb1fb@nehalam.linuxnetplumber.net> Content-Language: en-US Content-ID: <0ECA069770C4AF4382233EF46BAF10CB@redbull.xdin.com> Sender: netdev-owner@vger.kernel.org List-ID: On 2012-04-06 20:19, Stephen Hemminger wrote: > On Fri, 6 Apr 2012 18:06:31 +0100 > Ben Hutchings wrote: > >> On Fri, 2012-04-06 at 17:51 +0200, Arvid Brodin wrote: >>> David Miller wrote: >>>> From: Stephen Hemminger >>>> Date: Wed, 4 Apr 2012 16:55:59 -0700 >>>> >>>>> That isn't so bad, doing a memcpy versus a structure copy. >>>> >>>> GCC is going to inline the memcpy and thus we'll still do the >>>> unaligned accesses. This change therefore won't fix the problem. >>> >>> Well, it does work for me, with gcc-4.2.2-compiled linux-2.6.37 running >>> on an AVR32 board. >>> >>> Just out of curiosity, what's the mechanism behind this inline >>> assignment that turns the memcpy into an unaligned access? If gcc is >>> "smart" enough to detect a bunch of char * accesses and turn them >>> into unaligned 32-bit accesses, isn't that a bug in gcc? >> >> If I remember correctly, casting a char* pointer to foo* where the >> original pointer isn't properly aligned for type foo results in >> undefined behaviour. And that is what icmp_hdr() is doing, so there is >> no requirement that the compiler does anything reasonable with the >> result. Removing that cast (using skb_transport_header() instead of >> icmp_hdr()) should avoid that. >> >> (We do generally assume, however, that if the processor can handle >> unaligned accesses in a useful way then the compiler will be reasonable >> and not break them.) >> >> Ben. >> >>> Or will this only happen on archs which __HAVE_ARCH_MEMCPY? (But looking >>> at a couple of arch/xxx/lib/string.c, these too seem to take alignment >>> into account.) >>> >> > > Since icmp_hdr is 64 bits you might be able to use get_unaligned64 > in some way. > I'm sorry, I have no idea how to do this. Besides, get_unaligned64 seems to be implemented for the "c6x" arch only? So far we have: 1) icmp_hdr() casts an unaligned char * to a wider type, which is bad (undefined). 2) We cannot use skb_transport_header(): On 2012-04-06 00:31, David Miller wrote: > From: Ben Hutchings > Date: Thu, 5 Apr 2012 20:21:08 +0100 >> >> So presumably icmp_hdr() should be changed to skb_transport_header(). > > I would not say so. Otherwise we introduce ugly casts everywhere. > 3) My feeble suggestion to cast icmp_hdr() to (char *) is of course even worse (it doesn't even avoid the erroneous cast in the first place). So what do we do? -- Arvid Brodin Enea Services Stockholm AB - since February 16 a part of Xdin in the Alten Group. Soon we will be working under the common brand name Xdin. Read more at www.xdin.com.