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

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Date:   Thu, 06 Jun 2019 11:03:38 -0700
From:   "Jonathan Lemon" <jonathan.lemon@...il.com>
To:     "Ilya Maximets" <i.maximets@...sung.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, xdp-newbies@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        "Björn Töpel" <bjorn.topel@...el.com>,
        "Magnus Karlsson" <magnus.karlsson@...el.com>
Subject: Re: [PATCH] net: Fix hang while unregistering device bound to xdp
 socket

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@...sung.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