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

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Message-Id: <20191206.201950.100960973648804142.davem@davemloft.net>
Date:   Fri, 06 Dec 2019 20:19:50 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     alobakin@...nk.ru
Cc:     muciri@...nmesh.com, shashidhar.lakkavalli@...nmesh.com,
        john@...ozen.org, andrew@...n.ch, vivien.didelot@...il.com,
        f.fainelli@...il.com, sdf@...gle.com, daniel@...earbox.net,
        songliubraving@...com, ast@...nel.org, mcroce@...hat.com,
        jakub@...udflare.com, edumazet@...gle.com, paulb@...lanox.com,
        komachi.yoshiki@...il.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: dsa: fix flow dissection on Tx path

From: Alexander Lobakin <alobakin@...nk.ru>
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@...nk.ru>

Applied and queued up for -stable.