Re: [PATCH net v2 2/2] net: stmmac: Stop Tx queue when (en|dis)abling XSK pools

[Nicolai Buchwitz <nb@tipi-net.de>]

Hi Kurt

On 5.10.2026 09:09, Kurt Kanzenbach wrote:
> When enabling or disabling XSK pools in parallel to Tx traffic, kernel
> crashes occur. For VLAN tagged frames that happens in stmmac_xmit() ->
> dwmac4_set_vlan_tag() and for normal frames in stmmac_xmit() ->
> dwmac4_set_addr(). Both of these functions access the Tx DMA 
> descriptors.
> 
> The XDP pool (en|dis)ablement frees and reallocates the Tx DMA 
> resources:
> 
>  stmmac_disable_tx_queue:
>    __free_dma_tx_desc_resources
> 
>  stmmac_enable_tx_queue:
>    __alloc_dma_tx_desc_resources
>    __init_dma_tx_desc_rings
> 
> NAPI is disabled during that allocation window, but the Tx queue is not
> stopped. Therefore, add the stopping of the Tx queue during the 
> enabling
> and disabling of XSK pools. Update trans_start when stopping the queue
> to avoid spurious watchdog timeouts.
> 
> The issue can be easily reproduced by:
> 
>  1. Run iperf
>  2. Run application which opens an AF_XDP/ZC socket
> 
> Fixes: 132c32ee5bc0 ("net: stmmac: Add TX via XDP zero-copy socket")
> Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
> ---
>  drivers/net/ethernet/stmicro/stmmac/stmmac_xdp.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_xdp.c 
> b/drivers/net/ethernet/stmicro/stmmac/stmmac_xdp.c
> index d7e4db7224b0..883bd3fe8089 100644
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_xdp.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_xdp.c
> @@ -6,6 +6,16 @@
>  #include "stmmac.h"
>  #include "stmmac_xdp.h"
> 
> +static void stmmac_xdp_stop_tx_queue(struct stmmac_priv *priv, u16 
> queue)
> +{
> +	struct netdev_queue *nq = netdev_get_tx_queue(priv->dev, queue);
> +
> +	__netif_tx_lock_bh(nq);
> +	txq_trans_cond_update(nq);
> +	netif_tx_stop_queue(nq);
> +	__netif_tx_unlock_bh(nq);
> +}
> +
>  static int stmmac_xdp_enable_pool(struct stmmac_priv *priv,
>  				  struct xsk_buff_pool *pool, u16 queue)
>  {
> @@ -36,6 +46,7 @@ static int stmmac_xdp_enable_pool(struct stmmac_priv 
> *priv,
>  	if (need_update) {
>  		napi_disable(&ch->rx_napi);
>  		napi_disable(&ch->tx_napi);
> +		stmmac_xdp_stop_tx_queue(priv, queue);

Unfortunately XDP_TX and ndo_xdp_xmit() ignore the stopped queue and 
still
hit the freed ring. I can reproduce this on STM32MP215 with a veth 
redirect
into the port while toggling the pool:

   pc : dwmac4_set_addr+0x8/0x18
   lr : stmmac_xdp_xmit_xdpf+0x1d0/0x3f0
    stmmac_xdp_xmit+0xe4/0x1a8
    bq_xmit_all+0xa0/0x208
    __dev_flush+0x60/0xc0
    xdp_do_flush+0x134/0x198
    veth_poll+0x258/0x340

Both go through stmmac_xdp_xmit_xdpf() with the queue lock held, so this
fixes it for me:

--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ static int stmmac_xdp_xmit_xdpf(struct stmmac_priv *priv, int queue,
       dma_addr_t dma_addr;
       bool set_ic;

+     /* Ring may be torn down for an XSK pool switch */
+     if (netif_tx_queue_stopped(netdev_get_tx_queue(priv->dev, queue)))
+             return STMMAC_XDP_CONSUMED;
+
       if (stmmac_tx_avail(priv, queue) < STMMAC_TX_THRESH(priv))
               return STMMAC_XDP_CONSUMED;

This is older than your patch, but could you fold it in / add a oatch?

> [...]

Thanks,
Nicolai