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 08:46:07 -0800

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.

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