Re: [PATCH net 1/2] net: ethernet: oa_tc6: fix infinite loop error when tx credits becomes 0

[Jacob Keller <jacob.e.keller@intel.com>]

On 11/20/2024 5:51 AM, Parthiban Veerasooran wrote:
> SPI thread wakes up to perform SPI transfer whenever there is an TX skb
> from n/w stack or interrupt from MAC-PHY. Ethernet frame from TX skb is
> transferred based on the availability tx credits in the MAC-PHY which is
> reported from the previous SPI transfer. Sometimes there is a possibility
> that TX skb is available to transmit but there is no tx credits from
> MAC-PHY. In this case, there will not be any SPI transfer but the thread
> will be running in an endless loop until tx credits available again.
> 
> So checking the availability of tx credits along with TX skb will prevent
> the above infinite loop. When the tx credits available again that will be
> notified through interrupt which will trigger the SPI transfer to get the
> available tx credits.
> 
> Fixes: 53fbde8ab21e ("net: ethernet: oa_tc6: implement transmit path to transfer tx ethernet frames")
> Signed-off-by: Parthiban Veerasooran <parthiban.veerasooran@microchip.com>
> ---
>  drivers/net/ethernet/oa_tc6.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/oa_tc6.c b/drivers/net/ethernet/oa_tc6.c
> index f9c0dcd965c2..4c8b0ca922b7 100644
> --- a/drivers/net/ethernet/oa_tc6.c
> +++ b/drivers/net/ethernet/oa_tc6.c
> @@ -1111,8 +1111,9 @@ static int oa_tc6_spi_thread_handler(void *data)
>  		/* This kthread will be waken up if there is a tx skb or mac-phy
>  		 * interrupt to perform spi transfer with tx chunks.
>  		 */
> -		wait_event_interruptible(tc6->spi_wq, tc6->waiting_tx_skb ||
> -					 tc6->int_flag ||
> +		wait_event_interruptible(tc6->spi_wq, tc6->int_flag ||
> +					 (tc6->waiting_tx_skb &&
> +					 tc6->tx_credits) ||
>  					 kthread_should_stop());
>  

Ok, so previously we check:

waiting_tx_skb || int_flag

Now we check:

int_flag || (waiting_tx_skb && tx_credits) || kthread_should_stop.

We didn't check kthread_should_stop before and this isn't mentioned in
the commit message, (or at least its not clear to me).

Whats the purpose behind that? I guess you want to wake up immediately
when kthread_should_stop() so that we can shutdown the kthread ASAP? Is
the condition "waiting_tx_skb && tx_credits" such that we might
otherwise not wake up, but with just "waiting_tx_skb" we definitely wake
up and stop earlier?

I think that change makes sense but I don't like that it was not called
out in the commit message.

The code seems correct to me otherwise.

>  		if (kthread_should_stop())