Re: [PATCH net v3] net: ks8851: Fix TX stall caused by TX buffer overrun

[Simon Horman <horms@kernel.org>]

On Thu, Dec 14, 2023 at 07:11:12PM +0100, Ronald Wahl wrote:
> From: Ronald Wahl <ronald.wahl@raritan.com>
> 
> There is a bug in the ks8851 Ethernet driver that more data is written
> to the hardware TX buffer than actually available. This is caused by
> wrong accounting of the free TX buffer space.
> 
> The driver maintains a tx_space variable that represents the TX buffer
> space that is deemed to be free. The ks8851_start_xmit_spi() function
> adds an SKB to a queue if tx_space is large enough and reduces tx_space
> by the amount of buffer space it will later need in the TX buffer and
> then schedules a work item. If there is not enough space then the TX
> queue is stopped.
> 
> The worker function ks8851_tx_work() dequeues all the SKBs and writes
> the data into the hardware TX buffer. The last packet will trigger an
> interrupt after it was send. Here it is assumed that all data fits into
> the TX buffer.
> 
> In the interrupt routine (which runs asynchronously because it is a
> threaded interrupt) tx_space is updated with the current value from the
> hardware. Also the TX queue is woken up again.
> 
> Now it could happen that after data was sent to the hardware and before
> handling the TX interrupt new data is queued in ks8851_start_xmit_spi()
> when the TX buffer space had still some space left. When the interrupt
> is actually handled tx_space is updated from the hardware but now we
> already have new SKBs queued that have not been written to the hardware
> TX buffer yet. Since tx_space has been overwritten by the value from the
> hardware the space is not accounted for.
> 
> Now we have more data queued then buffer space available in the hardware
> and ks8851_tx_work() will potentially overrun the hardware TX buffer. In
> many cases it will still work because often the buffer is written out
> fast enough so that no overrun occurs but for example if the peer
> throttles us via flow control then an overrun may happen.
> 
> This can be fixed in different ways. The most simple way would be to set
> tx_space to 0 before writing data to the hardware TX buffer preventing
> the queuing of more SKBs until the TX interrupt has been handled. I have
> chosen a slightly more efficient (and still rather simple) way and
> track the amount of data that is already queued and not yet written to
> the hardware. When new SKBs are to be queued the already queued amount
> of data is honoured when checking free TX buffer space.
> 
> I tested this with a setup of two linked KS8851 running iperf3 between
> the two in bidirectional mode. Before the fix I got a stall after some
> minutes. With the fix I saw now issues anymore after hours.
> 
> Fixes: 3ba81f3ece3c ("net: Micrel KS8851 SPI network driver")
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: Ben Dooks <ben.dooks@codethink.co.uk>
> Cc: Tristram Ha <Tristram.Ha@microchip.com>
> Cc: netdev@vger.kernel.org
> Cc: stable@vger.kernel.org # 5.10+
> Signed-off-by: Ronald Wahl <ronald.wahl@raritan.com>
> ---
> V3: - Add missing kdoc of structure fields
>     - Avoid potential NULL pointer dereference
>     - Fix stack variable declaration order
> 
> V2: - Added Fixes: tag (issue actually present from the beginning)
>     - cosmetics reported by checkpatch

Thanks for the updates.

This change looks good to me, and I agree that
the problem was introduced in the cited commit.

Reviewed-by: Simon Horman <horms@kernel.org>