Re: [PATCH net-next V1 5/6] net/mlx4_en: Fix a race when closing TX queue

[Eric Dumazet <eric.dumazet@gmail.com>]

On Thu, 2013-01-17 at 13:47 +0200, Amir Vadai wrote:
> There is a possible race where the TX completion handler can clean the
> entire TX queue between the decision that the queue is full and actually
> closing it. To avoid this situation, check again if the queue is really
> full, if not, reopen the transmit and continue with sending the packet.
> 
> Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.com>
> Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
> Signed-off-by: Amir Vadai <amirv@mellanox.com>
> ---
>  drivers/net/ethernet/mellanox/mlx4/en_tx.c |    9 ++++++++-
>  1 files changed, 8 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx4/en_tx.c b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> index 2b799f4..1d17f5f 100644
> --- a/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> +++ b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> @@ -592,7 +592,14 @@ netdev_tx_t mlx4_en_xmit(struct sk_buff *skb, struct net_device *dev)
>  		netif_tx_stop_queue(ring->tx_queue);
>  		priv->port_stats.queue_stopped++;
>  
> -		return NETDEV_TX_BUSY;
> +		/* Check again whether the queue was cleaned */
> +		if (unlikely(((int)(ring->prod - ring->cons)) <=
> +				ring->size - HEADROOM - MAX_DESC_TXBBS)) {
> +			netif_tx_wake_queue(ring->tx_queue);
> +			priv->port_stats.wake_queue++;
> +		} else {
> +			return NETDEV_TX_BUSY;
> +		}
>  	}
>  
>  	/* Track current inflight packets for performance analysis */

This looks racy to me. You probably want explicit memory barriers ?