Re: [dpdk-dev] [dpdk-stable] [PATCH v5 3/3] kni: fix kernel deadlock when using mlx devices

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

On 3/29/2021 3:36 PM, Ferruh Yigit wrote:
> KNI runs userspace callback with rtnl lock held, this is not working
> fine with some devices that needs to interact with kernel interface in
> the callback, like Mellanox devices.
> 
> The solution is releasing the rtnl lock before calling the userspace
> callback. But it requires two consideration:
> 
> 1. The rtnl lock needs to released before 'kni->sync_lock', otherwise it
>     causes deadlock with multiple KNI devices, please check below the A.
>     for the details of the deadlock condition.
> 
> 2. When rtnl lock is released for interface down event, it cause a
>     regression and deadlock, so can't release the rtnl lock for interface
>     down event, please check below B. for the details.
> 
> As a solution, interface down event is handled asynchronously and for
> all other events rtnl lock is released before processing the callback.
> 
> A. KNI sync lock is being locked while rtnl is held.
> If two threads are calling kni_net_process_request() ,
> then the first one will take the sync lock, release rtnl lock then sleep.
> The second thread will try to lock sync lock while holding rtnl.
> The first thread will wake, and try to lock rtnl, resulting in a
> deadlock.  The remedy is to release rtnl before locking the KNI sync
> lock.
> Since in between nothing is accessing Linux network-wise, no rtnl
> locking is needed.
> 
> B. There is a race condition in __dev_close_many() processing the
> close_list while the application terminates.
> It looks like if two KNI interfaces are terminating,
> and one releases the rtnl lock, the other takes it,
> updating the close_list in an unstable state,
> causing the close_list to become a circular linked list,
> hence list_for_each_entry() will endlessly loop inside
> __dev_close_many() .
> 
> To summarize:
> request != interface down : unlock rtnl, send request to user-space,
> wait for response, send the response error code to caller in user-space.
> 
> request == interface down: send request to user-space, return immediately
> with error code of 0 (success) to user-space.
> 
> Fixes: 3fc5ca2f6352 ("kni: initial import")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Elad Nachman <eladv6@gmail.com>
> ---
> Cc: Stephen Hemminger <stephen@networkplumber.org>
> Cc: Igor Ryzhov <iryzhov@nfware.com>
> Cc: Dan Gora <dg@adax.com>
> 

Hi Elad, Igor,

Can you please review/test this set when you have time?

Thanks,
ferruh