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Subject: Re: [PATCH] net/mlx4_en: ensure rx_desc updating reaches HW before
 prod db updating
To:     Tariq Toukan <tariqt@mellanox.com>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Jason Gunthorpe <jgg@ziepe.ca>
Cc:     junxiao.bi@oracle.com, netdev@vger.kernel.org,
        linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org,
        Saeed Mahameed <saeedm@mellanox.com>
References: <1515728542-3060-1-git-send-email-jianchao.w.wang@oracle.com>
 <20180112163247.GB15974@ziepe.ca> <1515775567.131759.42.camel@gmail.com>
 <a40e44f4-106b-1075-8f92-f7741508372c@oracle.com>
 <af562737-4a3e-0429-1fd6-fc25c3e3f867@mellanox.com>
 <fea0aa1c-b68e-9485-3826-2dfad7824911@oracle.com>
From:   "jianchao.wang" <jianchao.w.wang@oracle.com>
Message-ID: <53b1ac4d-a294-eb98-149e-65d7954243da@oracle.com>
Date:   Fri, 19 Jan 2018 23:16:09 +0800
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Hi Tariq

Very sad that the crash was reproduced again after applied the patch.

--- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
+++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
@@ -252,6 +252,7 @@ static inline bool mlx4_en_is_ring_empty(struct mlx4_en_rx_ring *ring)
 
 static inline void mlx4_en_update_rx_prod_db(struct mlx4_en_rx_ring *ring)
 {
+	dma_wmb();
 	*ring->wqres.db.db = cpu_to_be32(ring->prod & 0xffff);
 }

I analyzed the kdump, it should be a memory corruption.

Thanks
Jianchao
On 01/15/2018 01:50 PM, jianchao.wang wrote:
> Hi Tariq
> 
> Thanks for your kindly response.
> 
> On 01/14/2018 05:47 PM, Tariq Toukan wrote:
>> Thanks Jianchao for your patch.
>>
>> And Thank you guys for your reviews, much appreciated.
>> I was off-work on Friday and Saturday.
>>
>> On 14/01/2018 4:40 AM, jianchao.wang wrote:
>>> Dear all
>>>
>>> Thanks for the kindly response and reviewing. That's really appreciated.
>>>
>>> On 01/13/2018 12:46 AM, Eric Dumazet wrote:
>>>>> 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 ?
>>> This issue was observed on x86-64.
>>> And I will send a new patch, in which replace wmb() with dma_wmb(), to customer
>>> to confirm.
>>
>> +1 on dma_wmb, let us know once customer confirms.
>> Please place it within mlx4_en_update_rx_prod_db as suggested.
> Yes, I have recommended it to customer.
> Once I get the result, I will share it here.
>> All other calls to mlx4_en_update_rx_prod_db are in control/slow path so I prefer being on the safe side, and care less about bulking the barrier.
>>
>> Thanks,
>> Tariq
>>
>