[dpdk-dev] [PATCH 2/2] kni: fix rtnl deadlocks and race conditions v4

Elad Nachman eladv6 at gmail.com
Fri Feb 26 18:43:39 CET 2021

The way the kernel handles its locks and lists for the dev close many
path, there is no way you can go around this with rtnl unlocked :

There is a race condition in __dev_close_many() processing the
close_list while the application terminates.
It looks like if two vEth devices 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() .

And I don't expect David Miller will bend the kernel networking for DPDK or KNI.

But - Stephen - if you can personally convince David to accept a
kernel patch which will separate the close_list locking mechanism to a
separate (RCU?) lock, then I can introduce first a patch to the kernel
which will add a lock for the close_list, this way rtnl can be
unlocked for the if down case.

After that kernel patch, your original patch + relocation of the sync
mutex locking will do the job .

Otherwise, rtnl has to be kept locked all of the way for the if down
case in order to prevent corruption causing a circular linked list out
of the close_list, causing a hang in the kernel.

Currently, the rtnl lock is the only thing keeping the close_list from

If you doubt rtnl cannot be unlocked for dev close path, you can
consult David for his opinion, as I think it is critical to understand
what the kernel can or cannot do, or expects to be done before we can
unlock its locks as we wish inside rte_kni.ko .

Otherwise, if we are still in disagreement on how to patch this set of
problems, I think the responsible way around it is to completely
remove kni from the main dpdk tree and move it to dpdk-kmods

I know BSD style open-source does not carry legal responsibility from
the developers, but I think when a bunch of developers know a piece of
code is highly buggy, they should not leave it for countless new users
to bounce their head desperately against, if they cannot agree on a
correct way to solve the bunch of problems, of which I think we all
agree exist (we just do not agree on the proper solution or patch)...

That's my two cents,


On Fri, Feb 26, 2021 at 5:49 PM Stephen Hemminger
<stephen at networkplumber.org> wrote:
> On Fri, 26 Feb 2021 00:01:01 +0300
> Igor Ryzhov <iryzhov at nfware.com> wrote:
> > Hi Elad,
> >
> > Thanks for the patch, but this is still NACK from me.
> >
> > The only real advantage of KNI over other exceptional-path techniques
> > like virtio-user is the ability to configure DPDK-managed interfaces
> > directly
> > from the kernel using well-known utils like iproute2. A very important part
> > of this is getting responses from the DPDK app and knowing the actual
> > result of command execution.
> > If you're making async requests to the application and you don't know
> > the result, then what's the point of using KNI at all?
> >
> > Igor
> Do you have a better proposal that keeps the request result but does not
> call userspace with lock held.
> PS: I also have strong dislike of KNI, as designed it would have been rejected
> by Linux kernel developers.  A better solution would be userspace version of
> something like devlink devices. But doing control operations by proxy is
> a locking nightmare.

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