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

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Date:   Wed, 16 Nov 2022 00:04:29 +0100
From:   Francois Romieu <romieu@...zoreil.com>
To:     Zhang Changzhong <zhangchangzhong@...wei.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

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.

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

-- 
Ueimor