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SubjectRe: [PATCH bpf v2] xdp: fix race on generic receive path
On Wed, Jul 3, 2019 at 2:09 PM Ilya Maximets <i.maximets@samsung.com> wrote:
>
> Unlike driver mode, generic xdp receive could be triggered
> by different threads on different CPU cores at the same time
> leading to the fill and rx queue breakage. For example, this
> could happen while sending packets from two processes to the
> first interface of veth pair while the second part of it is
> open with AF_XDP socket.
>
> Need to take a lock for each generic receive to avoid race.

I measured the performance degradation of rxdrop on my local machine
and it went from 2.19 to 2.08, so roughly a 5% drop. I think we can
live with this in XDP_SKB mode. If we at some later point in time need
to boost performance in this mode, let us look at it then from a
broader perspective and find the most low hanging fruit.

Thanks Ilya for this fix.

Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>

> Fixes: c497176cb2e4 ("xsk: add Rx receive functions and poll support")
> Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
> ---
>
> Version 2:
> * spin_lock_irqsave --> spin_lock_bh.
>
> include/net/xdp_sock.h | 2 ++
> net/xdp/xsk.c | 31 ++++++++++++++++++++++---------
> 2 files changed, 24 insertions(+), 9 deletions(-)
>
> diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
> index d074b6d60f8a..ac3c047d058c 100644
> --- a/include/net/xdp_sock.h
> +++ b/include/net/xdp_sock.h
> @@ -67,6 +67,8 @@ struct xdp_sock {
> * in the SKB destructor callback.
> */
> spinlock_t tx_completion_lock;
> + /* Protects generic receive. */
> + spinlock_t rx_lock;
> u64 rx_dropped;
> };
>
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index a14e8864e4fa..5e0637db92ea 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -123,13 +123,17 @@ int xsk_generic_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
> u64 addr;
> int err;
>
> - if (xs->dev != xdp->rxq->dev || xs->queue_id != xdp->rxq->queue_index)
> - return -EINVAL;
> + spin_lock_bh(&xs->rx_lock);
> +
> + if (xs->dev != xdp->rxq->dev || xs->queue_id != xdp->rxq->queue_index) {
> + err = -EINVAL;
> + goto out_unlock;
> + }
>
> if (!xskq_peek_addr(xs->umem->fq, &addr) ||
> len > xs->umem->chunk_size_nohr - XDP_PACKET_HEADROOM) {
> - xs->rx_dropped++;
> - return -ENOSPC;
> + err = -ENOSPC;
> + goto out_drop;
> }
>
> addr += xs->umem->headroom;
> @@ -138,13 +142,21 @@ int xsk_generic_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
> memcpy(buffer, xdp->data_meta, len + metalen);
> addr += metalen;
> err = xskq_produce_batch_desc(xs->rx, addr, len);
> - if (!err) {
> - xskq_discard_addr(xs->umem->fq);
> - xsk_flush(xs);
> - return 0;
> - }
> + if (err)
> + goto out_drop;
> +
> + xskq_discard_addr(xs->umem->fq);
> + xskq_produce_flush_desc(xs->rx);
>
> + spin_unlock_bh(&xs->rx_lock);
> +
> + xs->sk.sk_data_ready(&xs->sk);
> + return 0;
> +
> +out_drop:
> xs->rx_dropped++;
> +out_unlock:
> + spin_unlock_bh(&xs->rx_lock);
> return err;
> }
>
> @@ -765,6 +777,7 @@ static int xsk_create(struct net *net, struct socket *sock, int protocol,
>
> xs = xdp_sk(sk);
> mutex_init(&xs->mutex);
> + spin_lock_init(&xs->rx_lock);
> spin_lock_init(&xs->tx_completion_lock);
>
> mutex_lock(&net->xdp.lock);
> --
> 2.17.1
>

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