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Re: [PATCH] net/mlx4_en: ensure rx_desc updating reaches HW before prod db updating

On 01/12/2018 08:46 AM, Eric Dumazet wrote:
On Fri, 2018-01-12 at 09:32 -0700, Jason Gunthorpe wrote:
On Fri, Jan 12, 2018 at 11:42:22AM +0800, Jianchao Wang wrote:
Customer reported memory corruption issue on previous mlx4_en driver
version where the order-3 pages and multiple page reference counting
were still used.

Finally, find out one of the root causes is that the HW may see stale
rx_descs due to prod db updating reaches HW before rx_desc. Especially
when cross order-3 pages boundary and update a new one, HW may write
on the pages which may has been freed and allocated again by others.

To fix it, add a wmb between rx_desc and prod db updating to ensure
the order. Even thougth order-0 and page recycling has been introduced,
the disorder between rx_desc and prod db still could lead to corruption
on different inbound packages.

Signed-off-by: Jianchao Wang <jianchao.w.wang@xxxxxxxxxx>
  drivers/net/ethernet/mellanox/mlx4/en_rx.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
index 85e28ef..eefa82c 100644
+++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
@@ -555,7 +555,7 @@ static void mlx4_en_refill_rx_buffers(struct mlx4_en_priv *priv,
  	} while (likely(--missing));
+	wmb(); /* ensure rx_desc updating reaches HW before prod db updating */

Does this need to be dma_wmb(), and should it be in
mlx4_en_update_rx_prod_db ?

+1 on dma_wmb()

On what architecture bug was observed ?

In any case, the barrier should be moved in mlx4_en_update_rx_prod_db()
I think.

+1 on dma_wmb(), thanks Eric for reviewing this.

The barrier is also needed elsewhere in the code as well, but I wouldn't put it in mlx4_en_update_rx_prod_db(), just to allow batch filling of all rx rings and then hit the barrier only once. As a rule of thumb, mem barriers are the ring API caller responsibility.

e.g. in mlx4_en_activate_rx_rings():
between mlx4_en_fill_rx_buffers(priv); and the loop that updates rx prod for all rings ring, the dma_wmb is needed, see below.

diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
index b4d144e67514..65541721a240 100644
--- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
+++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
@@ -370,6 +370,8 @@ int mlx4_en_activate_rx_rings(struct mlx4_en_priv *priv)
        if (err)
                goto err_buffers;

+       dma_wmb();
        for (ring_ind = 0; ring_ind < priv->rx_ring_num; ring_ind++) {
                ring = priv->rx_ring[ring_ind];