Re: [PATCH] xsk: Fix race condition in AF_XDP generic RX path

[Magnus Karlsson]

On Wed, 9 Apr 2025 at 14:56, e.kubanski <e.kubanski@xxxxxxxxxxxxxxxxxxx> wrote:
>
> rx_lock moved from xsk_socket to xsk_buff_pool.
> Previous synchronization didn't take care of
> shared umem mode in generic RX path where sockets
> share the same xsk_buff_pool.
>
> RX queue is exclusive to xsk_socket, while FILL
> queue can be shared between multiple sockets.
> This could result in race condition where two
> CPU cores access RX path of two different sockets
> sharing the same umem.

I do not fully understand what you are doing in user space. Could you
please provide a user-space code example that will trigger this
problem?

Please note that if you share an Rx ring or the fill ring between
processes/threads, then you have to take care about mutual exclusion
in user space. If you really want to do this, it is usually a better
idea to use the other shared umem mode in which each process gets its
own rx and fill ring, removing the need for mutual exclusion.

> Now both queues are protected by acquiring spinlock
> in shared xsk_buff_pool.
>
> Lock contention may be minimized in the future by some
> per-thread FQ buffering.
>
> It's safe and necessary to move spin_lock_bh(rx_lock)
> after xsk_rcv_check():
> * xs->pool and spinlock_init is synchronized by
>   xsk_bind() -> xsk_is_bound() memory barriers.
> * xsk_rcv_check() may return true at the moment
>   of xsk_release() or xsk_unbind_dev(),
>   however this will not cause any data races or
>   race conditions. xsk_unbind_dev() removes xdp
>   socket from all maps and waits for completion
>   of all outstanding rx operations. Packets in
>   RX path will either complete safely or drop.
>
> Signed-off-by: Eryk Kubanski <e.kubanski@xxxxxxxxxxxxxxxxxxx>
> ---
>  include/net/xdp_sock.h      | 3 ---
>  include/net/xsk_buff_pool.h | 2 ++
>  net/xdp/xsk.c               | 6 +++---
>  net/xdp/xsk_buff_pool.c     | 1 +
>  4 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
> index bfe625b55d55..df3f5f07bc7c 100644
> --- a/include/net/xdp_sock.h
> +++ b/include/net/xdp_sock.h
> @@ -71,9 +71,6 @@ struct xdp_sock {
>          */
>         u32 tx_budget_spent;
>
> -       /* Protects generic receive. */
> -       spinlock_t rx_lock;
> -
>         /* Statistics */
>         u64 rx_dropped;
>         u64 rx_queue_full;
> diff --git a/include/net/xsk_buff_pool.h b/include/net/xsk_buff_pool.h
> index 50779406bc2d..7f0a75d6563d 100644
> --- a/include/net/xsk_buff_pool.h
> +++ b/include/net/xsk_buff_pool.h
> @@ -53,6 +53,8 @@ struct xsk_buff_pool {
>         refcount_t users;
>         struct xdp_umem *umem;
>         struct work_struct work;
> +       /* Protects generic receive in shared and non-shared umem mode. */
> +       spinlock_t rx_lock;
>         struct list_head free_list;
>         struct list_head xskb_list;
>         u32 heads_cnt;
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 89d2bef96469..e2a75f3be237 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -337,13 +337,14 @@ int xsk_generic_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
>         u32 len = xdp_get_buff_len(xdp);
>         int err;
>
> -       spin_lock_bh(&xs->rx_lock);
>         err = xsk_rcv_check(xs, xdp, len);
>         if (!err) {
> +               spin_lock_bh(&xs->pool->rx_lock);
>                 err = __xsk_rcv(xs, xdp, len);
>                 xsk_flush(xs);
> +               spin_unlock_bh(&xs->pool->rx_lock);
>         }
> -       spin_unlock_bh(&xs->rx_lock);
> +
>         return err;
>  }
>
> @@ -1724,7 +1725,6 @@ static int xsk_create(struct net *net, struct socket *sock, int protocol,
>         xs = xdp_sk(sk);
>         xs->state = XSK_READY;
>         mutex_init(&xs->mutex);
> -       spin_lock_init(&xs->rx_lock);
>
>         INIT_LIST_HEAD(&xs->map_list);
>         spin_lock_init(&xs->map_list_lock);
> diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
> index 1f7975b49657..3a5f16f53178 100644
> --- a/net/xdp/xsk_buff_pool.c
> +++ b/net/xdp/xsk_buff_pool.c
> @@ -87,6 +87,7 @@ struct xsk_buff_pool *xp_create_and_assign_umem(struct xdp_sock *xs,
>         pool->addrs = umem->addrs;
>         pool->tx_metadata_len = umem->tx_metadata_len;
>         pool->tx_sw_csum = umem->flags & XDP_UMEM_TX_SW_CSUM;
> +       spin_lock_init(&pool->rx_lock);
>         INIT_LIST_HEAD(&pool->free_list);
>         INIT_LIST_HEAD(&pool->xskb_list);
>         INIT_LIST_HEAD(&pool->xsk_tx_list);
> --
> 2.34.1
>
>