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

From: Eric Dumazet <[email-masked]>
Date: Fri, 12 Jan 2018 12:16:31 -0800

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 <[email-masked]>
> > > >   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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