From mboxrd@z Thu Jan 1 00:00:00 1970 From: mans@mansr.com (=?iso-8859-1?Q?M=E5ns_Rullg=E5rd?=) Date: Thu, 11 Oct 2012 11:20:10 +0100 Subject: alignment faults in 3.6 In-Reply-To: <1349949638.21172.8445.camel@edumazet-glaptop> (Eric Dumazet's message of "Thu, 11 Oct 2012 12:00:38 +0200") References: <506E1762.3010601@gmail.com> <506E3E58.80703@gmail.com> <20121005071216.GD4625@n2100.arm.linux.org.uk> <20121005082439.GF4625@n2100.arm.linux.org.uk> <506ED18C.3010009@gmail.com> <20121005140556.GQ4625@n2100.arm.linux.org.uk> <506EEFBB.3060705@gmail.com> <507619FA.6080001@jonmasters.org> <1349949638.21172.8445.camel@edumazet-glaptop> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Eric Dumazet writes: > On Thu, 2012-10-11 at 10:45 +0100, M?ns Rullg?rd wrote: >> "David Laight" writes: >> >> >> -----Original Message----- >> >> From: netdev-owner at vger.kernel.org [mailto:netdev-owner at vger.kernel.org] On Behalf Of M?ns Rullg?rd >> >> Sent: 11 October 2012 03:27 >> >> To: Jon Masters >> >> Cc: linux-arm-kernel at lists.infradead.org; netdev at vger.kernel.org >> >> Subject: Re: alignment faults in 3.6 >> >> >> >> Jon Masters writes: >> >> >> >> > Hi everyone, >> >> > >> >> > On 10/05/2012 10:33 AM, Rob Herring wrote: >> >> >> On 10/05/2012 09:05 AM, Russell King - ARM Linux wrote: >> >> >>> On Fri, Oct 05, 2012 at 07:24:44AM -0500, Rob Herring wrote: >> >> >>>> On 10/05/2012 03:24 AM, Russell King - ARM Linux wrote: >> >> >>>>> Does it matter? I'm just relaying the argument against adding __packed >> >> >>>>> which was used before we were forced (by the networking folk) to implement >> >> >>>>> the alignment fault handler. >> >> >>>> >> >> >>>> It doesn't really matter what will be accepted or not as adding __packed >> >> >>>> to struct iphdr doesn't fix the problem anyway. >> > ... >> >> There are exactly two possible solutions: >> >> >> >> 1. Change the networking code so those structs are always aligned. This >> >> might not be (easily) possible. >> >> 2. Mark the structs __packed and fix any typecasts like the ones seen in >> >> this thread. This will have an adverse effect in cases where the >> >> structs are in fact aligned. >> >> >> >> Both solutions lie squarely in the networking code. It's time to >> >> involve that list, or we'll never get anywhere. >> > >> > It might be enough to use __attribute__((aligned(2))) on some structure >> > members (actually does 'ldm' need 8 byte alignment?? - in which case >> > aligned(4) is enough). >> >> The aligned attribute can only increase alignment. > > I have no idea what is the problem, > > -ENOTENOUGHCONTEXT The thread starts here: http://marc.info/?l=linux-arm-kernel&m=134939228120020 Summary: a pointer to "struct iphdr" is not 4-byte aligned as required by the ARM ABI rules, and this causes traps to the unaligned access fault handler. A recent change makes the kernel print "scheduling while atomic" warnings on some of these traps, which may or may not be benign. Either way, this is bad for performance and should be fixed one way or another. -- M?ns Rullg?rd mans at mansr.com