From: Alexander Lobakin <[email protected]>
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 <[email protected]>
Applied and queued up for -stable.
David Miller wrote 07.12.2019 07:19:
> From: Alexander Lobakin <[email protected]>
> 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 <[email protected]>
>
> Applied and queued up for -stable.
David, Andrew, Florian, Rainer,
Thank you!
Regards,
ᚷ ᛖ ᚢ ᚦ ᚠ ᚱ