Hi Tariq and all
Many thanks for your kindly and detailed response and comment.
On 01/22/2018 12:24 AM, Tariq Toukan wrote:
>
>
> On 21/01/2018 11:31 AM, Tariq Toukan wrote:
>>
>>
>> On 19/01/2018 5:49 PM, Eric Dumazet wrote:
>>> On Fri, 2018-01-19 at 23:16 +0800, jianchao.wang wrote:
>>>> Hi Tariq
>>>>
>>>> Very sad that the crash was reproduced again after applied the patch.
>
> Memory barriers vary for different Archs, can you please share more details
> regarding arch and repro steps?The hardware is HP ProLiant DL380
> Gen9/ProLiant DL380 Gen9, BIOS P89 12/27/2015
The xen is installed. The crash occurred in DOM0.
Regarding to the repro steps, it is a customer's test which does heavy disk I/O
over NFS storage without any guest.
The patch that can fix this issue is as follow:
--- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
+++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
@@ -1005,6 +1005,7 @@ out:
wmb(); /* ensure HW sees CQ consumer before we post new buffers */
ring->cons = cq->mcq.cons_index;
mlx4_en_refill_rx_buffers(priv, ring);
+ wmb();
mlx4_en_update_rx_prod_db(ring);
return polled;
}
Thanks
Jianchao
>
>>>>
>>>> --- 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();
>>>
>>> So... is wmb() here fixing the issue ?
>>>
>>>> *ring->wqres.db.db = cpu_to_be32(ring->prod & 0xffff);
>>>> }
>>>>
>>>> I analyzed the kdump, it should be a memory corruption.
>>>>
>>>> Thanks
>>>> Jianchao
>>
>> Hmm, this is actually consistent with the example below [1].
>>
>> AIU from the example, it seems that the dma_wmb/dma_rmb barriers are good
>> for synchronizing cpu/device accesses to the "Streaming DMA mapped" buffers
>> (the descriptors, went through the dma_map_page() API), but not for the
>> doorbell (a coherent memory, typically allocated via dma_alloc_coherent)
>> that requires using the stronger wmb() barrier.
>>
>>
>> [1] Documentation/memory-barriers.txt
>>
>> (*) dma_wmb();
>> (*) dma_rmb();
>>
>> These are for use with consistent memory to guarantee the ordering
>> of writes or reads of shared memory accessible to both the CPU and a
>> DMA capable device.
>>
>> For example, consider a device driver that shares memory with a device
>> and uses a descriptor status value to indicate if the descriptor
>> belongs
>> to the device or the CPU, and a doorbell to notify it when new
>> descriptors are available:
>>
>> if (desc->status != DEVICE_OWN) {
>> /* do not read data until we own descriptor */
>> dma_rmb();
>>
>> /* read/modify data */
>> read_data = desc->data;
>> desc->data = write_data;
>>
>> /* flush modifications before status update */
>> dma_wmb();
>>
>> /* assign ownership */
>> desc->status = DEVICE_OWN;
>>
>> /* force memory to sync before notifying device via MMIO */
>> wmb();
>>
>> /* notify device of new descriptors */
>> writel(DESC_NOTIFY, doorbell);
>> }
>>
>> The dma_rmb() allows us guarantee the device has released ownership
>> before we read the data from the descriptor, and the dma_wmb() allows
>> us to guarantee the data is written to the descriptor before the device
>> can see it now has ownership. The wmb() is needed to guarantee that
>> the
>> cache coherent memory writes have completed before attempting a write
>> to
>> the cache incoherent MMIO region.
>>
>> See Documentation/DMA-API.txt for more information on consistent
>> memory.
>>
>>
>>>> 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
>>>>>>
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>
