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

[Dan Gora <dg@adax.com>]

Hi All,

Sorry to butt in on this, but I fixed this same issue about 3 years
ago in my application, but I was never able to get the changes
integrated and eventually just gave up trying.

The rule with KNI is:
1) The app should have a separate control thread per rte_kni which
just spins calling rte_kni_handle_request().  This ensures that other
threads calling rte_kni_XXX functions will always get a response.

2) In order to deal with lockups and timeouts when closing the device, I sent
patches which separated the closing process into two steps:
rte_kni_release() which would unregister the underlying netdev, then
rte_kni_free() which would free the KNI portions of the KNI device.
When rte_kni_release() is called the kernel netdev is unregistered and
a response is sent back to the application, the control thread calling
rte_kni_handle_request() is still running, so the application will
still get a response back from the kernel and not lock up, the
application then kills the control thread so that
rte_kni_handle_request() is not called again, then the application
calls rte_kni_free() which frees all of the FIFOs and closes the
device.

If anyone is interested the patches are probably still floating around
patchwork.  If not you can check them out here:

https://github.com/danielgora/dpdk.git

thanks-
dan

On Mon, Mar 1, 2021 at 5:10 AM Igor Ryzhov <iryzhov@nfware.com> wrote:
>
> Stephen,
>
> No, I don't have a better proposal, but I think it is not correct to change
> the behavior of KNI (making link down without a real response).
> Even though we know that communicating with userspace under rtnl_lock is a
> bad idea, it works as it is for many years already.
>
> Elad,
>
> I agree with you that KNI should be removed from the main tree if it is not
> possible to fix this __dev_close_many issue.
> There were discussions about this multiple times already, but no one is
> working on this AFAIK.
> Last time the discussion was a month ago:
> https://www.mail-archive.com/dev@dpdk.org/msg196033.html
>
> Igor
>
> On Fri, Feb 26, 2021 at 8:43 PM Elad Nachman <eladv6@gmail.com> wrote:
>
> > 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 .
> >lockups
> > 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.
> >lockups
> > Currently, the rtnl lock is the only thing keeping the close_list from
> > corruption.
> >
> > 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
> > repository.
> >
> > 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,
> >
> > Elad.
> >
> > On Fri, Feb 26, 2021 at 5:49 PM Stephen Hemminger
> > <stephen@networkplumber.org> wrote:
> > >
> > > On Fri, 26 Feb 2021 00:01:01 +0300
> > > Igor Ryzhov <iryzhov@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.
> >