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

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Date:   Fri, 7 Jun 2019 09:36:19 +0300
From:   Ilya Maximets <i.maximets@...sung.com>
To:     Jonathan Lemon <jonathan.lemon@...il.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 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@...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.
> 

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.