Re: [dpdk-dev] [PATCH 2/2] kni: fix rtnl deadlocks and race conditions v3

[Igor Ryzhov <iryzhov@nfware.com>]

Stephen's idea was to fix the deadlock when working with the bifurcated
driver.
Your rework breaks this because you still send link down requests under
rtnl_lock.
Did you test your patch with Mellanox devices?

On Wed, Feb 24, 2021 at 5:56 PM Elad Nachman <eladv6@gmail.com> wrote:

> The deadlock scenarios are explained below:
>
> It is described in Stephen Hemminger's original patch:
>
> "
>
> 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.
>
> "
>
> And also in my patch:
>
> "
> 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.
> "
>
> FYI,
>
> Elad.
>
> On Wed, Feb 24, 2021 at 4:41 PM Igor Ryzhov <iryzhov@nfware.com> wrote:
> >
> > Both link up and link down also work for me without this patch.
> > So what's the point in merging it?
> >
> > Just to clarify - I am not against the idea of this patch.
> > Talking to userspace under rtnl_lock is a bad idea.
> > I just think that any patch should fix some specified problem.
> >
> > If this patch is trying to solve the overall "userspace request under
> rtnl_lock" problem,
> > then it doesn't solve it correctly, because we still send link down
> requests under the lock.
> >
> > If this patch is trying to solve some other issue, for example, some
> "KNI deadlocks"
> > you're talking about, then you should explain what these deadlocks are,
> how to reproduce
> > them and why this patch solves the issue.
> >
> > On Wed, Feb 24, 2021 at 5:07 PM Elad Nachman <eladv6@gmail.com> wrote:
> >>
> >> I tested both link up and link down many times without any problems on
> >> 100 restarts of the application.
> >>
> >> Having KNI deadlock frequently for real life applications is far worst,
> IMHO.
> >>
> >> FYI
> >>
> >> Elad.
> >>
> >> On Wed, Feb 24, 2021 at 4:04 PM Igor Ryzhov <iryzhov@nfware.com> wrote:
> >> >
> >> > Elad,
> >> >
> >> > I understand your point.
> >> > But the fact that this fix works for you doesn't mean that it will
> work for all DPDK users.
> >> >
> >> > For example, I provided two simple commands: "ip link set up" and "ip
> link set down".
> >> > Your fix works for only one of them. For me, this is not a proper fix.
> >> > It may work for you because you don't disable interfaces, but it will
> fail for users who do.
> >> >
> >> > On Wed, Feb 24, 2021 at 4:33 PM Elad Nachman <eladv6@gmail.com>
> wrote:
> >> >>
> >> >> Currently KNI has a lot of issues with deadlocks locking the code,
> >> >> after this commit, they are gone, and the code runs properly without
> >> >> crashing.
> >> >> That was tested with over 100 restarts of the application, which
> >> >> previously required a hard reset of the board.
> >> >>
> >> >> I think this benefit overweights the complication of the code.
> >> >>
> >> >> The function is called with rtnl locked because this is how the Linux
> >> >> kernel is designed to work - it is not designed to work with deferral
> >> >> to user-space mid-function.
> >> >>
> >> >> To fix all such requests you need to reach an agreement with Linux
> >> >> netdev, which is unlikely.
> >> >>
> >> >> Calling user-space can be done asynchronously, as Ferruh asked, but
> >> >> then you will always have to return success, even on failure, as
> Linux
> >> >> kernel does not have a mechanism to asynchronously report on failure
> >> >> for such system calls.
> >> >>
> >> >> IMHO - weighting the non-reporting of failure versus how the code
> >> >> looks (as it functions perfectly OK), I decided to go with
> >> >> functionality.
> >> >>
> >> >> FYI,
> >> >>
> >> >> Elad.
> >> >>
> >> >> On Wed, Feb 24, 2021 at 2:50 PM Igor Ryzhov <iryzhov@nfware.com>
> wrote:
> >> >> >
> >> >> > This looks more like a hack than an actual fix to me.
> >> >> >
> >> >> > After this commit:
> >> >> > "ip link set up" is sent to the userspace with unlocked rtnl_lock
> >> >> > "ip link set down" is sent to the userspace with locked rtnl_lock
> >> >> >
> >> >> > How is this really fixing anything? IMHO it only complicates the
> code.
> >> >> > If talking with userspace under rtnl_lock is a problem, then we
> should fix all such requests, not only part of them.
> >> >> > If it is not a problem, then I don't see any point in merging this.
> >> >> >
> >> >> > On Tue, Feb 23, 2021 at 4:45 PM Elad Nachman <eladv6@gmail.com>
> wrote:
> >> >> >>
> >> >> >> This part of the series includes my fixes for the issues reported
> >> >> >> by Ferruh and Igor on top of part 1 of the patch series:
> >> >> >>
> >> >> >> 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 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() .
> >> >> >> Since the description for the original patch indicate the
> >> >> >> original motivation was bringing the device up,
> >> >> >> I have changed kni_net_process_request() to hold the rtnl mutex
> >> >> >> in case of bringing the device down since this is the path called
> >> >> >> from __dev_close_many() , causing the corruption of the
> close_list.
> >> >> >>
> >> >> >> Signed-off-by: Elad Nachman <eladv6@gmail.com>
> >> >> >> ---
> >> >> >> v3:
> >> >> >> * Include original patch and new patch as a series of patch,
> added a
> >> >> >>   comment to the new patch
> >> >> >> v2:
> >> >> >> * rebuild the patch as increment from patch 64106
> >> >> >> * fix comment and blank lines
> >> >> >> ---
> >> >> >>  kernel/linux/kni/kni_net.c | 29 +++++++++++++++++++++--------
> >> >> >>  1 file changed, 21 insertions(+), 8 deletions(-)
> >> >> >>
> >> >> >> diff --git a/kernel/linux/kni/kni_net.c
> b/kernel/linux/kni/kni_net.c
> >> >> >> index f0b6e9a8d..017e44812 100644
> >> >> >> --- a/kernel/linux/kni/kni_net.c
> >> >> >> +++ b/kernel/linux/kni/kni_net.c
> >> >> >> @@ -110,9 +110,26 @@ kni_net_process_request(struct net_device
> *dev, struct rte_kni_request *req)
> >> >> >>         void *resp_va;
> >> >> >>         uint32_t num;
> >> >> >>         int ret_val;
> >> >> >> +       int req_is_dev_stop = 0;
> >> >> >> +
> >> >> >> +       /* For configuring the interface to down,
> >> >> >> +        * rtnl must be held all the way to prevent race condition
> >> >> >> +        * inside __dev_close_many() between two netdev instances
> of KNI
> >> >> >> +        */
> >> >> >> +       if (req->req_id == RTE_KNI_REQ_CFG_NETWORK_IF &&
> >> >> >> +                       req->if_up == 0)
> >> >> >> +               req_is_dev_stop = 1;
> >> >> >>
> >> >> >>         ASSERT_RTNL();
> >> >> >>
> >> >> >> +       /* Since we need to wait and RTNL mutex is held
> >> >> >> +        * drop the mutex and hold reference to keep device
> >> >> >> +        */
> >> >> >> +       if (!req_is_dev_stop) {
> >> >> >> +               dev_hold(dev);
> >> >> >> +               rtnl_unlock();
> >> >> >> +       }
> >> >> >> +
> >> >> >>         mutex_lock(&kni->sync_lock);
> >> >> >>
> >> >> >>         /* Construct data */
> >> >> >> @@ -124,16 +141,8 @@ kni_net_process_request(struct net_device
> *dev, struct rte_kni_request *req)
> >> >> >>                 goto fail;
> >> >> >>         }
> >> >> >>
> >> >> >> -       /* Since we need to wait and RTNL mutex is held
> >> >> >> -        * drop the mutex and hold refernce to keep device
> >> >> >> -        */
> >> >> >> -       dev_hold(dev);
> >> >> >> -       rtnl_unlock();
> >> >> >> -
> >> >> >>         ret_val = wait_event_interruptible_timeout(kni->wq,
> >> >> >>                         kni_fifo_count(kni->resp_q), 3 * HZ);
> >> >> >> -       rtnl_lock();
> >> >> >> -       dev_put(dev);
> >> >> >>
> >> >> >>         if (signal_pending(current) || ret_val <= 0) {
> >> >> >>                 ret = -ETIME;
> >> >> >> @@ -152,6 +161,10 @@ kni_net_process_request(struct net_device
> *dev, struct rte_kni_request *req)
> >> >> >>
> >> >> >>  fail:
> >> >> >>         mutex_unlock(&kni->sync_lock);
> >> >> >> +       if (!req_is_dev_stop) {
> >> >> >> +               rtnl_lock();
> >> >> >> +               dev_put(dev);
> >> >> >> +       }
> >> >> >>         return ret;
> >> >> >>  }
> >> >> >>
> >> >> >> --
> >> >> >> 2.17.1
> >> >> >>
>