Re: [dpdk-dev] [PATCH] kni: fix kni rx fifo producer synchronization

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 8/16/2018 7:52 AM, Jerin Jacob wrote:
> -----Original Message-----
>> Date: Thu, 9 Aug 2018 12:30:57 +0100
>> From: Ferruh Yigit <ferruh.yigit@intel.com>
>> To: Kiran Kumar <kkokkilagadda@caviumnetworks.com>
>> CC: dev@dpdk.org, Jerin Jacob <jerin.jacob@caviumnetworks.com>, Konstantin
>>  Ananyev <konstantin.ananyev@intel.com>, Bruce Richardson
>>  <bruce.richardson@intel.com>, Santosh Shukla
>>  <santosh.shukla@caviumnetworks.com>
>> Subject: Re: [dpdk-dev] [PATCH] kni: fix kni rx fifo producer
>>  synchronization
>> User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101
>>  Thunderbird/52.9.1
>>
>>
>> On 8/9/2018 11:23 AM, Kiran Kumar wrote:
>>> With existing code in kni_fifo_put, rx_q values are not being updated
>>> before updating fifo_write. While reading rx_q in kni_net_rx_normal,
>>> This is causing the sync issue on other core. So adding a write
>>> barrier to make sure the values being synced before updating fifo_write.
>>>
>>> Fixes: 3fc5ca2f6352 ("kni: initial import")
>>>
>>> Signed-off-by: Kiran Kumar <kkokkilagadda@caviumnetworks.com>
>>> ---
>>>  lib/librte_kni/rte_kni_fifo.h | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/lib/librte_kni/rte_kni_fifo.h b/lib/librte_kni/rte_kni_fifo.h
>>> index ac26a8c..4d6b33e 100644
>>> --- a/lib/librte_kni/rte_kni_fifo.h
>>> +++ b/lib/librte_kni/rte_kni_fifo.h
>>> @@ -39,6 +39,7 @@ kni_fifo_put(struct rte_kni_fifo *fifo, void **data, unsigned num)
>>>               fifo->buffer[fifo_write] = data[i];
>>>               fifo_write = new_write;
>>>       }
>>> +     rte_smp_wmb();
>>>       fifo->write = fifo_write;
>>>       return i;
>>
>> For Intel this is just a compiler barrier so no issue but not sure if a memory
>> barrier is required here,
>>
>> Related code block is:
>>
>> |-          for (i = 0; i < num; i++) {
>> ||                  new_write = (new_write + 1) & (fifo->len - 1);
>> ||
>> ||                  if (new_write == fifo_read)
>> ||                          break;
>> ||                  fifo->buffer[fifo_write] = data[i];
>> ||                  fifo_write = new_write;
>> ||          }
>> |           fifo->write = fifo_write;
>>
>> "fifo_write" is updated in the loop, so there is a dependency to it for
>> "fifo->write". Can memory writes be reordered when there is a dependency?
> 
> From CPU0 compiler instruction generation perspective, It will take care of the
> dependency. In weakly ordered machine, it does NOT grandees CPU1 sees
> fifo->write as final write.
> 
> So, IMO, This patch make sense. We are able to reproduce the problem on
> arm64 machine with certain traffic.

OK, thanks for clarification.

> 
>>
>> Cc'ed a few more people for comment.