Re: [PATCH iwl-net] igb: Always call igb_xdp_ring_update_tail() under Tx lock

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Thu, Aug 22, 2024 at 09:42:07AM +0200, Kurt Kanzenbach wrote:
> From: Sriram Yagnaraman <sriram.yagnaraman@est.tech>
> 
> Always call igb_xdp_ring_update_tail() under __netif_tx_lock, add a comment
> and lockdep assert to indicate that. This is needed to share the same TX
> ring between XDP, XSK and slow paths. Furthermore, the current XDP
> implementation is racy on tail updates.
> 
> Fixes: 9cbc948b5a20 ("igb: add XDP support")
> Signed-off-by: Sriram Yagnaraman <sriram.yagnaraman@est.tech>
> [Kurt: Add lockdep assert and fixes tag]
> Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>

Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>

Thanks!

> ---
>  drivers/net/ethernet/intel/igb/igb_main.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
> index 33a42b4c21e0..c71eb2bbb23d 100644
> --- a/drivers/net/ethernet/intel/igb/igb_main.c
> +++ b/drivers/net/ethernet/intel/igb/igb_main.c
> @@ -33,6 +33,7 @@
>  #include <linux/bpf_trace.h>
>  #include <linux/pm_runtime.h>
>  #include <linux/etherdevice.h>
> +#include <linux/lockdep.h>
>  #ifdef CONFIG_IGB_DCA
>  #include <linux/dca.h>
>  #endif
> @@ -2914,8 +2915,11 @@ static int igb_xdp(struct net_device *dev, struct netdev_bpf *xdp)
>  	}
>  }
>  
> +/* This function assumes __netif_tx_lock is held by the caller. */
>  static void igb_xdp_ring_update_tail(struct igb_ring *ring)
>  {
> +	lockdep_assert_held(&txring_txq(ring)->_xmit_lock);
> +
>  	/* Force memory writes to complete before letting h/w know there
>  	 * are new descriptors to fetch.
>  	 */
> @@ -3000,11 +3004,11 @@ static int igb_xdp_xmit(struct net_device *dev, int n,
>  		nxmit++;
>  	}
>  
> -	__netif_tx_unlock(nq);
> -
>  	if (unlikely(flags & XDP_XMIT_FLUSH))
>  		igb_xdp_ring_update_tail(tx_ring);
>  
> +	__netif_tx_unlock(nq);
> +
>  	return nxmit;
>  }
>  
> @@ -8854,12 +8858,14 @@ static void igb_put_rx_buffer(struct igb_ring *rx_ring,
>  
>  static int igb_clean_rx_irq(struct igb_q_vector *q_vector, const int budget)
>  {
> +	unsigned int total_bytes = 0, total_packets = 0;
>  	struct igb_adapter *adapter = q_vector->adapter;
>  	struct igb_ring *rx_ring = q_vector->rx.ring;
> -	struct sk_buff *skb = rx_ring->skb;
> -	unsigned int total_bytes = 0, total_packets = 0;
>  	u16 cleaned_count = igb_desc_unused(rx_ring);
> +	struct sk_buff *skb = rx_ring->skb;
> +	int cpu = smp_processor_id();
>  	unsigned int xdp_xmit = 0;
> +	struct netdev_queue *nq;
>  	struct xdp_buff xdp;
>  	u32 frame_sz = 0;
>  	int rx_buf_pgcnt;
> @@ -8987,7 +8993,10 @@ static int igb_clean_rx_irq(struct igb_q_vector *q_vector, const int budget)
>  	if (xdp_xmit & IGB_XDP_TX) {
>  		struct igb_ring *tx_ring = igb_xdp_tx_queue_mapping(adapter);
>  
> +		nq = txring_txq(tx_ring);
> +		__netif_tx_lock(nq, cpu);
>  		igb_xdp_ring_update_tail(tx_ring);
> +		__netif_tx_unlock(nq);
>  	}
>  
>  	u64_stats_update_begin(&rx_ring->rx_syncp);
> 
> ---
> base-commit: a0b4a80ed6ce2cf8140fe926303ba609884b5d9b
> change-id: 20240822-igb_xdp_tx_lock-b6846fddc758
> 
> Best regards,
> -- 
> Kurt Kanzenbach <kurt@linutronix.de>
>