Subject: Re: [PATCH net-next v3 8/9] xsk: support generic batch xmit in copy mode

From: Stanislav Fomichev <[email-masked]>
Date: Fri, 24 Oct 2025 11:52:02 -0700

On 10/21, Jason Xing wrote:
> From: Jason Xing <[email-masked]>
> 
> - Move xs->mutex into xsk_generic_xmit to prevent race condition when
>   application manipulates generic_xmit_batch simultaneously.
> - Enable batch xmit eventually.
> 
> Make the whole feature work eventually.
> 
> Signed-off-by: Jason Xing <[email-masked]>
> ---
>  net/xdp/xsk.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 1fa099653b7d..3741071c68fd 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -891,8 +891,6 @@ static int __xsk_generic_xmit_batch(struct xdp_sock *xs)
>  	struct sk_buff *skb;
>  	int err = 0;
>  
> -	mutex_lock(&xs->mutex);
> -
>  	/* Since we dropped the RCU read lock, the socket state might have changed. */
>  	if (unlikely(!xsk_is_bound(xs))) {
>  		err = -ENXIO;
> @@ -982,21 +980,17 @@ static int __xsk_generic_xmit_batch(struct xdp_sock *xs)
>  	if (sent_frame)
>  		__xsk_tx_release(xs);
>  
> -	mutex_unlock(&xs->mutex);
>  	return err;
>  }
>  
> -static int __xsk_generic_xmit(struct sock *sk)
> +static int __xsk_generic_xmit(struct xdp_sock *xs)
>  {
> -	struct xdp_sock *xs = xdp_sk(sk);
>  	bool sent_frame = false;
>  	struct xdp_desc desc;
>  	struct sk_buff *skb;
>  	u32 max_batch;
>  	int err = 0;
>  
> -	mutex_lock(&xs->mutex);
> -
>  	/* Since we dropped the RCU read lock, the socket state might have changed. */
>  	if (unlikely(!xsk_is_bound(xs))) {
>  		err = -ENXIO;
> @@ -1071,17 +1065,22 @@ static int __xsk_generic_xmit(struct sock *sk)
>  	if (sent_frame)
>  		__xsk_tx_release(xs);
>  
> -	mutex_unlock(&xs->mutex);
>  	return err;
>  }
>  
>  static int xsk_generic_xmit(struct sock *sk)
>  {
> +	struct xdp_sock *xs = xdp_sk(sk);
>  	int ret;
>  
>  	/* Drop the RCU lock since the SKB path might sleep. */
>  	rcu_read_unlock();
> -	ret = __xsk_generic_xmit(sk);
> +	mutex_lock(&xs->mutex);
> +	if (xs->batch.generic_xmit_batch)
> +		ret = __xsk_generic_xmit_batch(xs);
> +	else
> +		ret = __xsk_generic_xmit(xs);

What's the point of keeping __xsk_generic_xmit? Can we have batch=1 by
default and always use __xsk_generic_xmit_batch?