netdev - Re: [PATCH] net: broadcom: bcm4908_enet: report queued and transmitted bytes

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Date:   Wed, 26 Oct 2022 07:58:06 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Rafał Miłecki <zajec5@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, bcm-kernel-feedback-list@...adcom.com,
        Rafał Miłecki <rafal@...ecki.pl>
Subject: Re: [PATCH] net: broadcom: bcm4908_enet: report queued and
 transmitted bytes



On 10/26/2022 7:26 AM, Rafał Miłecki wrote:
> From: Rafał Miłecki <rafal@...ecki.pl>
> 
> This allows BQL to operate avoiding buffer bloat and reducing latency.
> 
> Signed-off-by: Rafał Miłecki <rafal@...ecki.pl>
> ---
>   drivers/net/ethernet/broadcom/bcm4908_enet.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/net/ethernet/broadcom/bcm4908_enet.c b/drivers/net/ethernet/broadcom/bcm4908_enet.c
> index 93ccf549e2ed..e672a9ef4444 100644
> --- a/drivers/net/ethernet/broadcom/bcm4908_enet.c
> +++ b/drivers/net/ethernet/broadcom/bcm4908_enet.c
> @@ -495,6 +495,7 @@ static int bcm4908_enet_stop(struct net_device *netdev)
>   	netif_carrier_off(netdev);
>   	napi_disable(&rx_ring->napi);
>   	napi_disable(&tx_ring->napi);
> +	netdev_reset_queue(netdev);
>   
>   	bcm4908_enet_dma_rx_ring_disable(enet, &enet->rx_ring);
>   	bcm4908_enet_dma_tx_ring_disable(enet, &enet->tx_ring);
> @@ -564,6 +565,8 @@ static netdev_tx_t bcm4908_enet_start_xmit(struct sk_buff *skb, struct net_devic
>   	enet->netdev->stats.tx_bytes += skb->len;
>   	enet->netdev->stats.tx_packets++;
>   
> +	netdev_sent_queue(enet->netdev, skb->len);

There is an opportunity for fixing an use after free here, after you 
call bcm4908_enet_dma_tx_ring_enable() the hardware can start 
transmission right away and also call the TX completion handler, so you 
could be de-referencing a freed skb reference at this point. Also, to 
ensure that DMA is actually functional, it is recommended to increase TX 
stats in the TX completion handler, since that indicates that you have a 
functional completion process.

So long story short, if you record the skb length *before* calling 
bcm4908_enet_dma_tx_ring_enable() and use that for reporting sent bytes, 
you should be good.
-- 
Florian