Re: bridge/netfilter: regression in 2.6.39.1

[Eric Dumazet]

Le lundi 06 juin 2011 Ã 14:12 +0200, Eric Dumazet a Ãcrit :
> Le lundi 06 juin 2011 Ã 13:48 +0200, Alexander Holler a Ãcrit :
> > Am 06.06.2011 13:15, schrieb Neil Horman:
> > > On Fri, Jun 03, 2011 at 09:21:06PM +0200, Alexander Holler wrote:
> > >> Hello,
> > >>
> > >> I'm getting a oops in the bridge code in br_change_mtu() with
> > >> 2.6.39.1. The patch below seems to fix that.
> > >>
> > >> I'm not sure about the usage of dst_cow_metrics_generic() in
> > >> fake_dst_ops, but after having a quick look at it seems to be ok to
> > >> use that here.
> > >>
> > >> Regards,
> > >>
> > >> Alexander
> > >>
> > > How did the flags of the dst entry on which we're callnig dst_entry_write_ptr
> > > wind up getting the  READ_ONLY flag set on it?  I don't see how we'er falling
> > > into that clause in which we call cow_metrics when we call dst_metric_set.  It
> > > seems like that flag is set erroneously.  perhaps we should just update
> > > fake_rtable.dst to have the correct flags?
> > > Neil
> > 
> > It is set by that change:
> > 
> > --------
> > @@ -124,7 +128,7 @@ void br_netfilter_rtable_init(struct net_bridge *br)
> >   	atomic_set(&rt->dst.__refcnt, 1);
> >   	rt->dst.dev = br->dev;
> >   	rt->dst.path = &rt->dst;
> > -	dst_metric_set(&rt->dst, RTAX_MTU, 1500);
> > +	dst_init_metrics(&rt->dst, br_dst_default_metrics, true);
> >   	rt->dst.flags	= DST_NOXFRM;
> >   	rt->dst.ops = &fake_dst_ops;
> >   }
> > --------
> > 
> > The true in dst_init_metrics() is responsible for that flag.
> > 
> 
> You are aware this change fixed an oops ?
> 
> read_only in this context means : In case this must be written, we make
> a COW first
> (allocate a piece of memory, copy the source in it before applying any
> change)
> 
> It would be nice you send us the stack trace, so that we can have a clue
> of whats going on.
> 

Alexander, you should take a look at :

git show 0972ddb2

To get an idea of how to deal with this problem 
(See how Held Bernhard included a backtrace to help us make a
diagnostic)

We dont want to even allocate a piece of memory to copy
br_dst_default_metric for a fake dst.


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