Re: [PATCH net v2] net: stmmac: do not cache the new TSO MSS before it reaches the DMA

[Lorenzo Bianconi]

> From: Linkui Xiao <xiaolinkui@xxxxxxxxxx>
> 
> stmmac_tso_xmit() fills the MSS context descriptor and stores the new MSS
> in tx_q->mss right away, but the descriptor only gets its OWN bit much
> later, right before the frame is handed to the DMA. Every error path in
> between - the dma_map_single() of the linear part and the
> skb_frag_dma_map() of each fragment - returns with tx_q->mss already
> updated while the MAC is still programmed with the previous MSS; the
> abandoned context descriptor is later reclaimed by stmmac_tx_clean().
> 
> The next skb carrying the same MSS then compares equal to the cached
> value, so no context descriptor is emitted and the hardware segments the
> TCP stream with a stale MSS, generating frames whose payload size does
> not match what the stack accounted for.
> 
> Update tx_q->mss only once the context descriptor has been given to the
> DMA, so that the cached value always describes what the hardware is
> actually programmed with.
> 
> The context descriptor is now handled like the data descriptors are:
> tx_q->cur_tx is not advanced while it is being filled. Whether the frame
> can be queued is only known after every dma_map_single() and
> skb_frag_dma_map() has succeeded, so the descriptor stays at the slot
> tx_q->cur_tx points to and the index moves past it later, together with
> the data descriptors. That also keeps the context descriptor outside the
> range stmmac_tx_clean() walks when the ring is cleaned after a failure,
> so the error paths have to release it explicitly.
> 
> Fixes: f748be531d70 ("stmmac: support new GMAC4")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Linkui Xiao <xiaolinkui@xxxxxxxxxx>

Acked-by: Lorenzo Bianconi <lorenzo.bianconi@xxxxxxxxxxxxxxxx>

> ---
> Changes in v2:
> - Do not advance tx_q->cur_tx while the context descriptor is filled, as
>   for the data descriptors, and release the context descriptor on the
>   error paths instead of leaving it to stmmac_tx_clean(). (Lorenzo Bianconi)
> 
>  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> index af2d38a2bb3d..e2e680dd980c 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -4563,10 +4563,6 @@ static netdev_tx_t stmmac_tso_xmit(struct sk_buff *skb, struct net_device *dev)
>  			mss_desc = &tx_q->dma_tx[tx_q->cur_tx];
>  
>  		stmmac_set_mss(priv, mss_desc, mss);
> -		tx_q->mss = mss;
> -		tx_q->cur_tx = STMMAC_NEXT_ENTRY(tx_q->cur_tx,
> -						priv->dma_conf.dma_tx_size);
> -		WARN_ON(tx_q->tx_skbuff[tx_q->cur_tx]);
>  	}
>  
>  	if (netif_msg_tx_queued(priv)) {
> @@ -4577,6 +4573,9 @@ static netdev_tx_t stmmac_tso_xmit(struct sk_buff *skb, struct net_device *dev)
>  	}
>  
>  	first_entry = tx_q->cur_tx;
> +	if (mss_desc)
> +		first_entry = STMMAC_NEXT_ENTRY(first_entry,
> +						priv->dma_conf.dma_tx_size);
>  	entry = first_entry;
>  
>  	WARN_ON(tx_q->tx_skbuff[entry]);
> @@ -4714,6 +4713,7 @@ static netdev_tx_t stmmac_tso_xmit(struct sk_buff *skb, struct net_device *dev)
>  		 */
>  		dma_wmb();
>  		stmmac_set_tx_owner(priv, mss_desc);
> +		tx_q->mss = mss;
>  	}
>  
>  	if (netif_msg_pktdata(priv)) {
> @@ -4745,6 +4745,9 @@ static netdev_tx_t stmmac_tso_xmit(struct sk_buff *skb, struct net_device *dev)
>  						priv->dma_conf.dma_tx_size);
>  	}
>  error:
> +	if (mss_desc)
> +		stmmac_release_tx_desc(priv, mss_desc, priv->descriptor_mode);
> +
>  	dev_err(priv->device, "Tx dma map failed\n");
>  	dev_kfree_skb(skb);
>  	priv->xstats.tx_dropped++;
> -- 
> 2.25.1
> 
>

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