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

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 27 Jul 2020 18:33:08 +0100
Ferruh Yigit <ferruh.yigit@intel.com> wrote:

> On 5/6/2020 1:14 AM, Stephen Hemminger wrote:
> > On Wed, 18 Mar 2020 16:17:57 +0100
> > Thomas Monjalon <thomas@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@dpdk.org
> >>>> Signed-off-by: Stephen Hemminger <stephen@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?

Still KNI driver is broken. Calling userspace with RTNL held is fundamentally
broken design. If KNI were to be incorporated in upstream kernel, then the netdev
developer would see this.

What ever solution you think is best.
I will continue to recommend against anyone using KNI.