Re: [PATCH bpf 1/4] xsk: fix potential lost completion message in SKB path

[Song Liu <liu.song.a23@gmail.com>]

On Wed, Jun 27, 2018 at 7:02 AM, Magnus Karlsson
<magnus.karlsson@intel.com> wrote:
> The code in xskq_produce_addr erroneously checked if there
> was up to LAZY_UPDATE_THRESHOLD amount of space in the completion
> queue. It only needs to check if there is one slot left in the
> queue. This bug could under some circumstances lead to a WARN_ON_ONCE
> being triggered and the completion message to user space being lost.
>
> Fixes: 35fcde7f8deb ("xsk: support for Tx")
> Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
> Reported-by: Pavel Odintsov <pavel@fastnetmon.com>

Acked-by: Song Liu <songliubraving@fb.com>

> ---
>  net/xdp/xsk_queue.h | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
>
> diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h
> index ef6a6f0ec949..52ecaf770642 100644
> --- a/net/xdp/xsk_queue.h
> +++ b/net/xdp/xsk_queue.h
> @@ -62,14 +62,9 @@ static inline u32 xskq_nb_avail(struct xsk_queue *q, u32 dcnt)
>         return (entries > dcnt) ? dcnt : entries;
>  }
>
> -static inline u32 xskq_nb_free_lazy(struct xsk_queue *q, u32 producer)
> -{
> -       return q->nentries - (producer - q->cons_tail);
> -}
> -
>  static inline u32 xskq_nb_free(struct xsk_queue *q, u32 producer, u32 dcnt)
>  {
> -       u32 free_entries = xskq_nb_free_lazy(q, producer);
> +       u32 free_entries = q->nentries - (producer - q->cons_tail);
>
>         if (free_entries >= dcnt)
>                 return free_entries;
> @@ -129,7 +124,7 @@ static inline int xskq_produce_addr(struct xsk_queue *q, u64 addr)
>  {
>         struct xdp_umem_ring *ring = (struct xdp_umem_ring *)q->ring;
>
> -       if (xskq_nb_free(q, q->prod_tail, LAZY_UPDATE_THRESHOLD) == 0)
> +       if (xskq_nb_free(q, q->prod_tail, 1) == 0)
>                 return -ENOSPC;
>
>         ring->desc[q->prod_tail++ & q->ring_mask] = addr;
> --
> 2.7.4
>