Re: [Intel-wired-lan] [PATCH net v2] igc: Prevent garbled TX queue with XDP ZEROCOPY

[Florian Kauer]

Hi Vinicius,

On 28.06.23 23:34, Vinicius Costa Gomes wrote:
> Florian Kauer <florian.kauer@xxxxxxxxxxxxx> writes:
> 
>> In normal operation, each populated queue item has
>> next_to_watch pointing to the last TX desc of the packet,
>> while each cleaned item has it set to 0. In particular,
>> next_to_use that points to the next (necessarily clean)
>> item to use has next_to_watch set to 0.
>>
>> When the TX queue is used both by an application using
>> AF_XDP with ZEROCOPY as well as a second non-XDP application
>> generating high traffic, the queue pointers can get in
>> an invalid state where next_to_use points to an item
>> where next_to_watch is NOT set to 0.
>>
>> However, the implementation assumes at several places
>> that this is never the case, so if it does hold,
>> bad things happen. In particular, within the loop inside
>> of igc_clean_tx_irq(), next_to_clean can overtake next_to_use.
>> Finally, this prevents any further transmission via
>> this queue and it never gets unblocked or signaled.
>> Secondly, if the queue is in this garbled state,
>> the inner loop of igc_clean_tx_ring() will never terminate,
>> completely hogging a CPU core.
>>
>> The reason is that igc_xdp_xmit_zc() reads next_to_use
>> before acquiring the lock, and writing it back
>> (potentially unmodified) later. If it got modified
>> before locking, the outdated next_to_use is written
>> pointing to an item that was already used elsewhere
>> (and thus next_to_watch got written).
>>
>> Fixes: 9acf59a752d4 ("igc: Enable TX via AF_XDP zero-copy")
>> Signed-off-by: Florian Kauer <florian.kauer@xxxxxxxxxxxxx>
>> Reviewed-by: Kurt Kanzenbach <kurt@xxxxxxxxxxxxx>
>> Tested-by: Kurt Kanzenbach <kurt@xxxxxxxxxxxxx>
>> ---
> 
> This patch doesn't directly apply because there's a small conflict with
> commit 95b681485563 ("igc: Avoid transmit queue timeout for XDP"),
> but really easy to solve.
> 
> Anyway, good catch:
> 
> Acked-by: Vinicius Costa Gomes <vinicius.gomes@xxxxxxxxx>

I am sorry, that was bad timing. I prepared the initial patch on Friday and overlooked the merge.
Shall I send a v3 or will someone else take care of the conflict resolution?

Greetings,
Florian