[dpdk-stable] [dpdk-dev] [PATCH] kni: fix kernel deadlock when using mlx devices

[Ferruh Yigit]

On 5/6/2020 1:14 AM, Stephen Hemminger wrote:
> On Wed, 18 Mar 2020 16:17:57 +0100
> Thomas Monjalon <thomas at monjalon.net> wrote:
> 
>> 17/01/2020 17:43, Ferruh Yigit:
>>> On 12/22/2019 5:55 PM, Stephen Hemminger wrote:  
>>>> This fixes a deadlock when using KNI with bifurcated drivers.
>>>> Bringing kni device up always times out when using Mellanox
>>>> devices.
>>>>
>>>> The kernel KNI driver sends message to userspace to complete
>>>> the request. For the case of bifurcated driver, this may involve
>>>> an additional request to kernel to change state. This request
>>>> would deadlock because KNI was holding the RTNL mutex.
>>>>
>>>> This was a bad design which goes back to the original code.
>>>> A workaround is for KNI driver to drop RTNL while waiting.
>>>> To prevent the device from disappearing while the operation
>>>> is in progress, it needs to hold reference to network device
>>>> while waiting.
>>>>
>>>> As an added benefit, an useless error check can also be removed.
>>>>
>>>> Fixes: 3fc5ca2f6352 ("kni: initial import")
>>>> Cc: stable at dpdk.org
>>>> Signed-off-by: Stephen Hemminger <stephen at networkplumber.org>
>>>> ---  
>>>
>>> This patch cause a hang on my server, not sure what exactly was the problem but
>>> kernel log was continuously printing "Cannot send to req_q". Will dig more.  
>>
>> Ferruh, did you have a chance to check what is hanging?
>> Stephen, is there any news on your side?
>>
>>
> 
> It did not hang when I tested it. The bug report is still open
> 

Sorry for the delay, since I am working remotely I was worried about loosing the
connection to my server, finally I did create a virtual environment to test again.

I confirm the hang observed %100 when two different process updates the kni
interface, like two different process sets the mtu. Without this patch this
works fine.

I understand the motivation of the patch, but with change there is a possibility
to hang the server, which we can't allow, need to find another way. Can updating
mlx interface wait KNI interface operation to complete?