linux-kernel - Re: [PATCH net v2 3/3] net: nixge: fix tx queue handling

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Date:   Wed, 16 Nov 2022 16:55:58 +0800
From:   Zhang Changzhong <zhangchangzhong@...wei.com>
To:     Francois Romieu <romieu@...zoreil.com>
CC:     <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
        <pabeni@...hat.com>, <mdf@...nel.org>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net v2 3/3] net: nixge: fix tx queue handling

On 2022/11/16 7:04, Francois Romieu wrote:
> Zhang Changzhong <zhangchangzhong@...wei.com> :
>> Currently the driver check for available space at the beginning of
>> nixge_start_xmit(), and when there is not enough space for this packet,
>> it returns NETDEV_TX_OK, which casues packet loss and memory leak.
>>
>> Instead the queue should be stopped after the packet is added to the BD
>> when there may not be enough space for next packet. In addition, the
>> queue should be wakeup only if there is enough space for a packet with
>> max frags.
>>
>> Fixes: 492caffa8a1a ("net: ethernet: nixge: Add support for National Instruments XGE netdev")
>> Signed-off-by: Zhang Changzhong <zhangchangzhong@...wei.com>
>> ---
>>  drivers/net/ethernet/ni/nixge.c | 54 +++++++++++++++++++++++++++++------------
>>  1 file changed, 38 insertions(+), 16 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/ni/nixge.c b/drivers/net/ethernet/ni/nixge.c
>> index 91b7ebc..3776a03 100644
>> --- a/drivers/net/ethernet/ni/nixge.c
>> +++ b/drivers/net/ethernet/ni/nixge.c
> [...]
>>  static netdev_tx_t nixge_start_xmit(struct sk_buff *skb,
>> @@ -518,10 +523,15 @@ static netdev_tx_t nixge_start_xmit(struct sk_buff *skb,
>>  	cur_p = &priv->tx_bd_v[priv->tx_bd_tail];
>>  	tx_skb = &priv->tx_skb[priv->tx_bd_tail];
>>  
>> -	if (nixge_check_tx_bd_space(priv, num_frag + 1)) {
>> -		if (!netif_queue_stopped(ndev))
>> -			netif_stop_queue(ndev);
>> -		return NETDEV_TX_OK;
>> +	if (unlikely(nixge_check_tx_bd_space(priv, num_frag + 1))) {
>> +		/* Should not happen as last start_xmit call should have
>> +		 * checked for sufficient space and queue should only be
>> +		 * woken when sufficient space is available.
>> +		 */
> 
> Almost. IRQ triggering after nixge_start_xmit::netif_stop_queue and
> before nixge_start_xmit::smp_mb may wrongly wake queue.
> 

I don't know what you mean by "wronly wake queue". The queue is woken
only when there is sufficient for next packet.

> Call me timorous but I would feel more confortable if this code could
> be tested on real hardware before being fed into -net.
> 

I agree with you, hope someone can test and correct it.

Thanks,
Changzhong