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

[Elad Nachman <eladv6@gmail.com>]

Hi,

Regarding the asynchronous call - thought about it, but then the
request will always return OK to user-space and I will have no way to
return failure error codes back to user-space.

If the above explanation is acceptable, per your other comments - I
can send a new patch without the parameter change , without the empty
line, and with the comment moved to the proper place in the code.

Waiting for your decision,

Elad.

On Fri, Feb 19, 2021 at 8:42 PM Ferruh Yigit <ferruh.yigit@intel.com> wrote:
>
> On 11/26/2020 2:46 PM, Elad Nachman wrote:
> > This patch leverages on Stephen Hemminger's 64106 patch from Dec 2019,
> > and fixes the issues reported by Ferruh and Igor:
> >
> > A. KNI sync lock is being locked while rtnl is held.
> > If two threads are calling kni_net_process_request() ,
> > then the first one wil 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.
>
> Hi Elad,
>
> Thanks for explanation, that clarifies the issue.
> Also I confirm I don't see the hang, at least as much as I test.
>
> >
> > 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.
> >
>
> I can't reproduce this case, I see the protection in the code, but better to get
> confirmation from Igor.
>
>
>
> Overall the issue seems calling a function pointed by 'rte_kni_ops' which
> requires to acquire the rtnl lock.
> So I wonder if this can't be handled in the ops function, by processing the
> request asynchronously,
> like recording the request, return from 'rte_kni_ops', and process the request
> afterwards?
>
> I assume the application we mention is not kni sample application.
>
> >
> >
> > Signed-off-by: Elad Nachman <eladv6@gmail.com>
> > ---
> >   kernel/linux/kni/kni_net.c | 47 +++++++++++++++++++++++++-------------
> >   1 file changed, 31 insertions(+), 16 deletions(-)
> >
> > diff --git a/kernel/linux/kni/kni_net.c b/kernel/linux/kni/kni_net.c
> > index 4b752083d..cf5b0845d 100644
> > --- a/kernel/linux/kni/kni_net.c
> > +++ b/kernel/linux/kni/kni_net.c
> > @@ -17,6 +17,7 @@
> >   #include <linux/skbuff.h>
> >   #include <linux/kthread.h>
> >   #include <linux/delay.h>
> > +#include <linux/rtnetlink.h>
> >
> >   #include <rte_kni_common.h>
> >   #include <kni_fifo.h>
> > @@ -102,18 +103,26 @@ get_data_kva(struct kni_dev *kni, void *pkt_kva)
> >    * It can be called to process the request.
> >    */
> >   static int
> > -kni_net_process_request(struct kni_dev *kni, struct rte_kni_request *req)
> > +kni_net_process_request(struct net_device *dev, struct rte_kni_request *req)
> >   {
> > +     struct kni_dev *kni = netdev_priv(dev);
> >       int ret = -1;
> >       void *resp_va;
> >       uint32_t num;
> >       int ret_val;
> > +     int req_is_dev_stop = 0;
> >
> > -     if (!kni || !req) {
> > -             pr_err("No kni instance or request\n");
> > -             return -EINVAL;
> > -     }
> > +     if (req->req_id == RTE_KNI_REQ_CFG_NETWORK_IF &&
> > +                     req->if_up == 0)
> > +             req_is_dev_stop = 1;
> >
> > +     ASSERT_RTNL();
> > +
> > +     if (!req_is_dev_stop) {
> > +             dev_hold(dev);
> > +             rtnl_unlock();
> > +     }
> > +
> >       mutex_lock(&kni->sync_lock);
> >
> >       /* Construct data */
> > @@ -125,8 +134,13 @@ kni_net_process_request(struct kni_dev *kni, 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
> > +      */
> > +
>
> Comment seems left here, need to go up. s/refernce/reference
>
> >       ret_val = wait_event_interruptible_timeout(kni->wq,
> >                       kni_fifo_count(kni->resp_q), 3 * HZ);
> > +
> >       if (signal_pending(current) || ret_val <= 0) {
> >               ret = -ETIME;
> >               goto fail;
> > @@ -144,6 +158,13 @@ kni_net_process_request(struct kni_dev *kni, struct rte_kni_request *req)
> >
> >   fail:
> >       mutex_unlock(&kni->sync_lock);
> > +
> > +
>
> extra empty line
>
> > +     if (!req_is_dev_stop) {
> > +             rtnl_lock();
> > +             dev_put(dev);
> > +     }
> > +
> >       return ret;
> >   }
> >
> > @@ -155,7 +176,6 @@ kni_net_open(struct net_device *dev)
> >   {
> >       int ret;
> >       struct rte_kni_request req;
> > -     struct kni_dev *kni = netdev_priv(dev);
> >
> >       netif_start_queue(dev);
> >       if (kni_dflt_carrier == 1)
> > @@ -168,7 +188,7 @@ kni_net_open(struct net_device *dev)
> >
> >       /* Setting if_up to non-zero means up */
> >       req.if_up = 1;
> > -     ret = kni_net_process_request(kni, &req);
> > +     ret = kni_net_process_request(dev, &req);
> >
>
> Althoug it is not soo confusing, these lines and following ones are noise for
> this patch, they are just for 'kni_net_process_request' paramter change.
>
> What do you think do the 'kni_net_process_request' parameter change in first
> patch, and fix the issue in second, this way second patch can contain only the
> actual changes required for fix.