Subject: Re: [PATCH net 2/2] net: stmmac: avoid rx queue overrun

From: Baruch Siach <[email-masked]>
Date: Tue, 14 Nov 2023 18:09:17 +0200

Hi Serge,

On Tue, Nov 14 2023, Serge Semin wrote:
> On Mon, Nov 13, 2023 at 07:42:50PM +0200, Baruch Siach wrote:
>> dma_rx_size can be set as low as 64. Rx budget might be higher than
>> that. Make sure to not overrun allocated rx buffers when budget is
>> larger.
>> 
>> Leave one descriptor unused to avoid wrap around of 'dirty_rx' vs
>> 'cur_rx'.
>
> Have you ever met the denoted problem? I am asking because what you
> say can happen only if the incoming traffic overruns the Rx-buffer,
> otherwise the loop will break on the first found DMA-own descriptor.
> But if that happens AFAICS the result will likely to be fatal because
> the stmmac_rx() method will try to handle the already handled and not
> yet recycled descriptor with no buffers assigned.

I have encountered this issue. When stmmac_rx() consumes all dma_rx_size
descriptors in one go, dirty_rx == cur_rx, which leads stmmac_rx_dirty()
to return zero. That in turn makes stmmac_rx_refill() skip
stmmac_set_rx_owner() so that Rx hangs completely.

> So after adding the Fixes tag feel tree to add:
> Reviewed-by: Serge Semin <[email-masked]>

Thanks,
baruch

> -Serge(y)
>
>> 
>> Signed-off-by: Baruch Siach <[email-masked]>
>> ---
>>  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 1 +
>>  1 file changed, 1 insertion(+)
>> 
>> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>> index f28838c8cdb3..2afb2bd25977 100644
>> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
>> @@ -5293,6 +5293,7 @@ static int stmmac_rx(struct stmmac_priv *priv, int limit, u32 queue)
>>  
>>  	dma_dir = page_pool_get_dma_dir(rx_q->page_pool);
>>  	buf_sz = DIV_ROUND_UP(priv->dma_conf.dma_buf_sz, PAGE_SIZE) * PAGE_SIZE;
>> +	limit = min(priv->dma_conf.dma_rx_size - 1, (unsigned int)limit);
>>  
>>  	if (netif_msg_rx_status(priv)) {
>>  		void *rx_head;
>> -- 
>> 2.42.0
>> 
>> 

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