Subject: Re: [PATCH net 1/3] net: bcmgenet: fix off-by-one in incrementing read pointer

From: Florian Fainelli <[email-masked]>
Date: Fri, 10 Oct 2014 09:41:13 -0700

On 10/09/2014 11:01 PM, Petri Gynther wrote:
> Hi Florian,
> 
> On Thu, Oct 9, 2014 at 6:06 PM, Florian Fainelli <[email-masked]> wrote:
>> Commit b629be5c8399d7c423b92135eb43a86c924d1cbc ("net: bcmgenet: check
>> harder for out of memory conditions") moved the increment of the local
>> read pointer *before* reading from the hardware descriptor using
>> dmadesc_get_length_status(), which creates an off-by-one situation.
>>
>> Fix this by moving again the read_ptr increment after we have read the
>> hardware descriptor to get both the control block and the read pointer
>> back in sync.
>>
>> Fixes: b629be5c8399 ("net: bcmgenet: check harder for out of memory conditions")
>> Reported-by: Jaedon Shin <[email-masked]>
>> Signed-off-by: Florian Fainelli <[email-masked]>
>> ---
>>  drivers/net/ethernet/broadcom/genet/bcmgenet.c | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/broadcom/genet/bcmgenet.c b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
>> index fff2634b6f34..f1bcebcbba80 100644
>> --- a/drivers/net/ethernet/broadcom/genet/bcmgenet.c
>> +++ b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
>> @@ -1287,9 +1287,6 @@ static unsigned int bcmgenet_desc_rx(struct bcmgenet_priv *priv,
>>
>>                 rxpktprocessed++;
>>
>> -               priv->rx_read_ptr++;
>> -               priv->rx_read_ptr &= (priv->num_rx_bds - 1);
>> -
> 
> Wouldn't it be better to move the three lines:
> rxpktprocessed++;
> priv->rx_read_ptr++;
> priv->rx_read_ptr &= (priv->num_rx_bds - 1)
> 
> as the last lines of the while-loop, after the CB refill?

That's basically what Jaedon did in his first patch, I don't have strong
objections in doing that if you think that makes it look clearer.

> 
> -- Petri
> 
> 
>>                 /* We do not have a backing SKB, so we do not have a
>>                  * corresponding DMA mapping for this incoming packet since
>>                  * bcmgenet_rx_refill always either has both skb and mapping or
>> @@ -1332,6 +1329,9 @@ static unsigned int bcmgenet_desc_rx(struct bcmgenet_priv *priv,
>>                           __func__, p_index, priv->rx_c_index,
>>                           priv->rx_read_ptr, dma_length_status);
>>
>> +               priv->rx_read_ptr++;
>> +               priv->rx_read_ptr &= (priv->num_rx_bds - 1);
>> +
>>                 if (unlikely(!(dma_flag & DMA_EOP) || !(dma_flag & DMA_SOP))) {
>>                         netif_err(priv, rx_status, dev,
>>                                   "dropping fragmented packet!\n");
>> --
>> 1.9.1
>>