Re: [PATCH] net: Fix hang while unregistering device bound to xdp socket

[Björn Töpel]

On 2019-06-07 08:36, Ilya Maximets wrote:

On 06.06.2019 21:03, Jonathan Lemon wrote:

On 6 Jun 2019, at 5:40, Ilya Maximets wrote:

Device that bound to XDP socket will not have zero refcount until the
userspace application will not close it. This leads to hang inside
'netdev_wait_allrefs()' if device unregistering requested:

   # ip link del p1
   < hang on recvmsg on netlink socket >

   # ps -x | grep ip
   5126  pts/0    D+   0:00 ip link del p1

   # journalctl -b

   Jun 05 07:19:16 kernel:
   unregister_netdevice: waiting for p1 to become free. Usage count = 1

   Jun 05 07:19:27 kernel:
   unregister_netdevice: waiting for p1 to become free. Usage count = 1
   ...

Fix that by counting XDP references for the device and failing
RTM_DELLINK with EBUSY if device is still in use by any XDP socket.

With this change:

   # ip link del p1
   RTNETLINK answers: Device or resource busy

Fixes: 965a99098443 ("xsk: add support for bind for Rx")
Signed-off-by: Ilya Maximets <i.maxim...@samsung.com>
---

Another option could be to force closing all the corresponding AF_XDP
sockets, but I didn't figure out how to do this properly yet.

  include/linux/netdevice.h | 25 +++++++++++++++++++++++++
  net/core/dev.c            | 10 ++++++++++
  net/core/rtnetlink.c      |  6 ++++++
  net/xdp/xsk.c             |  7 ++++++-
  4 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 44b47e9df94a..24451cfc5590 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1705,6 +1705,7 @@ enum netdev_priv_flags {
   *    @watchdog_timer:        List of timers
   *
   *    @pcpu_refcnt:           Number of references to this device
+ *     @pcpu_xdp_refcnt:       Number of XDP socket references to this device
   *    @todo_list:             Delayed register/unregister
   *    @link_watch_list:       XXX: need comments on this one
   *
@@ -1966,6 +1967,7 @@ struct net_device {
        struct timer_list       watchdog_timer;

        int __percpu            *pcpu_refcnt;
+       int __percpu            *pcpu_xdp_refcnt;
        struct list_head        todo_list;

I understand the intention here, but don't think that putting a XDP reference
into the generic netdev structure is the right way of doing this.  Likely the
NETDEV_UNREGISTER notifier should be used so the socket and umem unbinds from
the device.

Thanks for the pointer! That is exactly what I looked for.
I'll make a new version that will unbind resources using netdevice notifier.

Best regards, Ilya Maximets.

Thanks for working on this.

This would open up for supporting killing sockets via ss(8) (-K,
--kill), as well!


Björn