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

[Saeed Mahameed <saeedm@mellanox.com>]

On 01/12/2018 12:16 PM, Eric Dumazet wrote:
> 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...
> 

Simply putting a memory barrier on the top or the bottom of a functions,
means nothing unless you are looking at the whole picture, of all the
callers of that function to understand why is it there.

which is better to grasp ?:

update_doorbell() {
    dma_wmb();
    ring->db = prod;
}

or

fill buffers();
dma_wmb();
update_doorbell();

I simply like the 2nd one since with one look you can understand what this dma_wmb is protecting.

Anyway this is truly a nit, Tariq can decide what is better for him :).

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

+1, Tariq will you handle ?