From: Stephen Hemminger <shemminger@vyatta•com>
To: Ben Hutchings <bhutchings@solarflare•com>
Cc: Arvid Brodin <arvid.brodin@enea•com>,
David Miller <davem@davemloft•net>, <netdev@vger•kernel.org>,
<balferreira@googlemail•com>, <arvid.brodin@xdin•com>
Subject: Re: [RFC] net/hsr: Add support for IEC 62439-3 High-availability Seamless Redundancy
Date: Fri, 6 Apr 2012 11:19:12 -0700 [thread overview]
Message-ID: <20120406111912.172bb1fb@nehalam.linuxnetplumber.net> (raw)
In-Reply-To: <1333731991.3282.17.camel@deadeye>
On Fri, 6 Apr 2012 18:06:31 +0100
Ben Hutchings <bhutchings@solarflare•com> wrote:
> On Fri, 2012-04-06 at 17:51 +0200, Arvid Brodin wrote:
> > David Miller wrote:
> > > From: Stephen Hemminger <shemminger@vyatta•com>
> > > 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.
next prev parent reply other threads:[~2012-04-06 18:19 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-03-27 13:20 [RFC] net/hsr: Add support for IEC 62439-3 High-availability Seamless Redundancy Arvid Brodin
2012-04-03 18:37 ` Stephen Hemminger
2012-04-04 23:09 ` Arvid Brodin
2012-04-04 23:55 ` Stephen Hemminger
2012-04-05 0:21 ` David Miller
2012-04-06 15:51 ` Arvid Brodin
2012-04-06 16:43 ` David Miller
2012-04-06 17:08 ` Arvid Brodin
2012-04-06 17:06 ` Ben Hutchings
2012-04-06 18:19 ` Stephen Hemminger [this message]
2012-04-11 0:00 ` Arvid Brodin
2012-04-11 1:28 ` Stephen Hemminger
2012-04-11 14:39 ` Arvid Brodin
2012-04-05 0:17 ` David Miller
2012-04-05 19:21 ` Ben Hutchings
2012-04-05 22:31 ` David Miller
2012-05-14 18:11 ` Arvid Brodin
2012-05-14 18:28 ` Stephen Hemminger
2012-05-24 17:09 ` Arvid Brodin
2012-05-24 17:16 ` Stephen Hemminger
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20120406111912.172bb1fb@nehalam.linuxnetplumber.net \
--to=shemminger@vyatta$(echo .)com \
--cc=arvid.brodin@enea$(echo .)com \
--cc=arvid.brodin@xdin$(echo .)com \
--cc=balferreira@googlemail$(echo .)com \
--cc=bhutchings@solarflare$(echo .)com \
--cc=davem@davemloft$(echo .)net \
--cc=netdev@vger$(echo .)kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox