Re: [PATCH net-next 4/6] net: hip04: Don't free the tx skb when the buffer is not ready for xmit

Ding Tianhong
2015-04-16T09:38:40Z

On 2015/4/16 16:41, Arnd Bergmann wrote:
> On Thursday 16 April 2015 14:26:21 Ding Tianhong wrote:
>> On 2015/4/15 22:19, Arnd Bergmann wrote:
>>> On Wednesday 15 April 2015 20:30:06 Ding Tianhong wrote:
>>>> @@ -489,6 +487,8 @@ static int hip04_mac_start_xmit(struct sk_buff *skb, struct net_device *ndev)
>>>>
>>>> /* Ensure tx_head update visible to tx reclaim */
>>>> smp_wmb();
>>>> + count++;
>>>> + priv->tx_head = TX_NEXT(tx_head);
>>>>
>>>>
>>>
>>> Something is wrong here, the comment does not match the code any more, because
>>> the smp_wmb() won't actually make the tx_head update visible.
>>>
>>> Arnd
>>>
>> Yes, the smp_wmb() could only make sure the tx buffer was ready to xmit.
>
> The problem you need to avoid is that the tx reclaim function thinks it's
> done with all outstanding packets and does not retrigger, while the
> start_xmit thinks it will still get to that. This means you
> need a barrier between the priv->tx_head update and the
> napi_schedule_prep() call.
>

But I still doubt about that, when the buffer is ready to xmit, it will add to fifo queue, but it
doesn't mean it is finished xmit yet, the tx reclaim function will free the skb only
depend on whether tx_head is updated.

Ding

> Arnd
>
> .