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

[Saeed Mahameed]

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,
> > >   			break;
> > >   		ring->prod++;
> > >   	} while (likely(--missing));
> > > -
> > > +	wmb(); /* ensure rx_desc updating reaches HW before prod db updating */
> > >   	mlx4_en_update_rx_prod_db(ring);
> > >   }
> > >   
> >
> > 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];