netdev - Re: [PATCH net] net: xilinx: axienet: Increment Rx skb ring head pointer after BD is successfully allocated in dmaengine flow

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Message-ID: <08bd431d-c887-4f69-9c1e-4b40a301ecf7@linux.dev>
Date: Tue, 5 Aug 2025 15:32:41 -0400
From: Sean Anderson <sean.anderson@...ux.dev>
To: Suraj Gupta <suraj.gupta2@....com>, andrew+netdev@...n.ch,
 davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, michal.simek@....com, radhey.shyam.pandey@....com,
 horms@...nel.org
Cc: netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, harini.katakam@....com
Subject: Re: [PATCH net] net: xilinx: axienet: Increment Rx skb ring head
 pointer after BD is successfully allocated in dmaengine flow

On 8/5/25 15:19, Suraj Gupta wrote:
> In DMAengine flow, AXI DMA driver invokes callback before freeing BD in
> irq handling path.
> In Rx callback (axienet_dma_rx_cb()), axienet driver tries to allocate
> new BD after processing skb.
> This will be problematic if both AXI-DMA and AXI ethernet have same
> BD count as all Rx BDs will be allocated initially and it won't be
> able to allocate new one after Rx irq. Incrementing head pointer w/o
> checking for BD allocation will result in garbage values in skb BD and
> cause the below kernel crash:
> 
> Unable to handle kernel paging request at virtual address fffffffffffffffa
> <snip>
> Internal error: Oops: 0000000096000006 [#1]  SMP
> pc : axienet_dma_rx_cb+0x78/0x150
> lr : axienet_dma_rx_cb+0x78/0x150
>  Call trace:
>   axienet_dma_rx_cb+0x78/0x150 (P)
>   xilinx_dma_do_tasklet+0xdc/0x290
>   tasklet_action_common+0x12c/0x178
>   tasklet_action+0x30/0x3c
>   handle_softirqs+0xf8/0x230
> <snip>
> 
> Fixes: 6a91b846af85 ("net: axienet: Introduce dmaengine support")
> Signed-off-by: Suraj Gupta <suraj.gupta2@....com>
> ---
>  drivers/net/ethernet/xilinx/xilinx_axienet_main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> index 6011d7eae0c7..acd5be60afec 100644
> --- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> +++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> @@ -1457,7 +1457,6 @@ static void axienet_rx_submit_desc(struct net_device *ndev)
>  	if (!skbuf_dma)
>  		return;
>  
> -	lp->rx_ring_head++;
>  	skb = netdev_alloc_skb(ndev, lp->max_frm_size);
>  	if (!skb)
>  		return;
> @@ -1482,6 +1481,7 @@ static void axienet_rx_submit_desc(struct net_device *ndev)
>  	skbuf_dma->desc = dma_rx_desc;
>  	dma_rx_desc->callback_param = lp;
>  	dma_rx_desc->callback_result = axienet_dma_rx_cb;
> +	lp->rx_ring_head++;
>  	dmaengine_submit(dma_rx_desc);
>  
>  	return;

Reviewed-by: Sean Anderson <sean.anderson@...ux.dev>