buf_ring in HEAD is racy

[Ryan Stone]

On Tue, Dec 17, 2013 at 11:49 AM, Adrian Chadd <adrian at freebsd.org> wrote:
> Try:
>
> Index: sys/dev/ixgbe/ixgbe.c
> ===================================================================
> --- sys/dev/ixgbe/ixgbe.c       (revision 2995)
> +++ sys/dev/ixgbe/ixgbe.c       (working copy)
> @@ -5178,6 +5178,7 @@
>         struct ixgbe_hw *hw = &adapter->hw;
>         u32  missed_rx = 0, bprc, lxon, lxoff, total;
>         u64  total_missed_rx = 0;
> +       u64  odrops = 0;
>
>         adapter->stats.crcerrs += IXGBE_READ_REG(hw, IXGBE_CRCERRS);
>         adapter->stats.illerrc += IXGBE_READ_REG(hw, IXGBE_ILLERRC);
> @@ -5308,6 +5309,11 @@
>                 adapter->stats.fcoedwtc += IXGBE_READ_REG(hw, IXGBE_FCOEDWTC);
>         }
>
> +       /* TX drops */
> +       for (int i = 0; i < adapter->num_queues; i++) {
> +               odrops += adapter->tx_rings[i].br->br_drops;
> +       }
> +
>         /* Fill out the OS statistics structure */
>         ifp->if_ipackets = adapter->stats.gprc;
>         ifp->if_opackets = adapter->stats.gptc;
> @@ -5317,6 +5323,9 @@
>         ifp->if_omcasts = adapter->stats.mptc;
>         ifp->if_collisions = 0;
>
> +       /* TX drops are stored in if_snd for now, not the top level counters */
> +       ifp->if_snd.ifq_drops = odrops;
> +
>         /* Rx Errors */
>         ifp->if_iqdrops = total_missed_rx;
>         ifp->if_ierrors = adapter->stats.crcerrs + adapter->stats.rlec;
>
>
> -adrian

The trick with this is that if ALTQ is enabled then we will use if_snd
queue instead of the buf ring, and if that queue drops then ifq_drops
will be incremented.  With this patch if ALTQ is in use then we will
not count any drops (as you will be overwriting the drop count from
the queue).