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

[BjÃrn TÃpel]

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.maximets@xxxxxxxxxxx>
> > > ---
> > >
> > > 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!


BjÃrn