netdev - Re: alignment faults in 3.6

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Message-ID: <yw1xlifdpaxx.fsf@unicorn.mansr.com>
Date:	Thu, 11 Oct 2012 11:20:10 +0100
From:	Måns Rullgård <mans@...sr.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Måns Rullgård <mans@...sr.com>,
	David Laight <David.Laight@...LAB.COM>,
	Jon Masters <jonathan@...masters.org>,
	linux-arm-kernel@...ts.infradead.org, netdev@...r.kernel.org
Subject: Re: alignment faults in 3.6

Eric Dumazet <eric.dumazet@...il.com> writes:

> On Thu, 2012-10-11 at 10:45 +0100, Måns Rullgård wrote:
>> "David Laight" <David.Laight@...LAB.COM> writes:
>> 
>> >> -----Original Message-----
>> >> From: netdev-owner@...r.kernel.org [mailto:netdev-owner@...r.kernel.org] On Behalf Of Måns Rullgård
>> >> Sent: 11 October 2012 03:27
>> >> To: Jon Masters
>> >> Cc: linux-arm-kernel@...ts.infradead.org; netdev@...r.kernel.org
>> >> Subject: Re: alignment faults in 3.6
>> >> 
>> >> Jon Masters <jonathan@...masters.org> 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.

-- 
Måns Rullgård
mans@...sr.com
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