Re: [dpdk-dev] [PATCH 1/2] net/mlx4: fix memory barrier incorrectly placed

[fengchengwen <fengchengwen@huawei.com>]

On 2021/5/11 17:23, Slava Ovsiienko wrote:
>> -----Original Message-----
>> From: dev <dev-bounces@dpdk.org> On Behalf Of Chengwen Feng
>> Sent: Monday, May 10, 2021 15:06
>> To: NBU-Contact-Thomas Monjalon <thomas@monjalon.net>;
>> ferruh.yigit@intel.com
>> Cc: dev@dpdk.org
>> Subject: [dpdk-dev] [PATCH 1/2] net/mlx4: fix memory barrier incorrectly
>> placed
>>
>> The memory barrier is used to ensure that the response is returned only
>> after the Tx/Rx function is set, it should place after the Rx/Tx function is set.
>>
>> Fixes: 0203d33a1059 ("net/mlx4: support secondary process")
>> Cc: stable@dpdk.org
>>
>> Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
> Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>
> 
> 
> Hi, Chengwen
> 
> Nice catch, thank you for the patches.
> Just out of curiosity - did we meet the real issue with this ineffective barrier?
> 
> With best regards,
> Slava
> 

Hi, Slava

It's just a theoretical analysis that's possible, I think it should never
happen in practice.
We found this problem when checking the memory barrier of the hns3 driver
(ps: the implementation of hns3 mp is somewhat similar to that of mlx), this
memory barrier got us a little confused, so we fix it to make it just less
confusing.

Best Regards

>> ---
>>  drivers/net/mlx4/mlx4_mp.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/mlx4/mlx4_mp.c b/drivers/net/mlx4/mlx4_mp.c
>> index ddf7bdb..8fcfb54 100644
>> --- a/drivers/net/mlx4/mlx4_mp.c
>> +++ b/drivers/net/mlx4/mlx4_mp.c
>> @@ -126,7 +126,6 @@ mp_secondary_handle(const struct rte_mp_msg
>> *mp_msg, const void *peer)
>>  	switch (param->type) {
>>  	case MLX4_MP_REQ_START_RXTX:
>>  		INFO("port %u starting datapath", dev->data->port_id);
>> -		rte_mb();
>>  		dev->tx_pkt_burst = mlx4_tx_burst;
>>  		dev->rx_pkt_burst = mlx4_rx_burst;
>>  #ifdef HAVE_IBV_MLX4_UAR_MMAP_OFFSET
>> @@ -144,6 +143,7 @@ mp_secondary_handle(const struct rte_mp_msg
>> *mp_msg, const void *peer)
>>  			}
>>  		}
>>  #endif
>> +		rte_mb();
>>  		mp_init_msg(dev, &mp_res, param->type);
>>  		res->result = 0;
>>  		ret = rte_mp_reply(&mp_res, peer);
>> --
>> 2.8.1
> 
> 
> .
>