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

[Elad Nachman <eladv6@gmail.com>]

Hi,

The new patch is fine by me.

Tested several dozens restarts of our proprietary application without
apparent problem.

FYI,

Elad.

בתאריך יום ו׳, 9 באפר׳ 2021, 17:56, מאת 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
>
>