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SubjectRe: [PATCH] net: Fix hang while unregistering device bound to xdp socket
Date
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.maximets@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.
--
Jonathan

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