Re: [PATCH net] net: dsa: fix flow dissection on Tx path

[Alexander Lobakin]

David Miller wrote 07.12.2019 07:19:
> From: Alexander Lobakin <alobakin@xxxxxxxx>
> Date: Thu,  5 Dec 2019 13:02:35 +0300
>
> > Commit 43e665287f93 ("net-next: dsa: fix flow dissection") added an
> > ability to override protocol and network offset during flow dissection
> > for DSA-enabled devices (i.e. controllers shipped as switch CPU ports)
> > in order to fix skb hashing for RPS on Rx path.
> >
> > However, skb_hash() and added part of code can be invoked not only on
> > Rx, but also on Tx path if we have a multi-queued device and:
> >  - kernel is running on UP system or
> >  - XPS is not configured.
> >
> > The call stack in this two cases will be like: dev_queue_xmit() ->
> > __dev_queue_xmit() -> netdev_core_pick_tx() -> netdev_pick_tx() ->
> > skb_tx_hash() -> skb_get_hash().
> >
> > The problem is that skbs queued for Tx have both network offset and
> > correct protocol already set up even after inserting a CPU tag by DSA
> > tagger, so calling tag_ops->flow_dissect() on this path actually only
> > breaks flow dissection and hashing.
> >
> > This can be observed by adding debug prints just before and right 
> > after
> > tag_ops->flow_dissect() call to the related block of code:
>   ...
> > In order to fix that we can add the check 'proto == htons(ETH_P_XDSA)'
> > to prevent code from calling tag_ops->flow_dissect() on Tx.
> > I also decided to initialize 'offset' variable so tagger callbacks can
> > now safely leave it untouched without provoking a chaos.
> >
> > Fixes: 43e665287f93 ("net-next: dsa: fix flow dissection")
> > Signed-off-by: Alexander Lobakin <alobakin@xxxxxxxx>
>
> Applied and queued up for -stable.

David, Andrew, Florian, Rainer,
Thank you!

Regards,
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