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    SubjectRe: [PATCH] net/mlx4_en: ensure rx_desc updating reaches HW before prod db updating
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    On Fri, 2018-01-12 at 11:53 -0800, Saeed Mahameed wrote:
    >
    > 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@oracle.com>
    > > > > 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];


    Why bother, considering dma_wmb() is a nop on x86,
    simply a compiler barrier.

    Putting it in mlx4_en_update_rx_prod_db() and have no obscure bugs...

    Also we might change the existing wmb() in mlx4_en_process_rx_cq() by
    dma_wmb(), that would help performance a bit.


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