Re: [PATCH net] openvswitch: vport: fix race between linking and the device notifier

[Aaron Conole]

Ilya Maximets <i.maximets@xxxxxxx> writes:

> On 5/13/26 2:02 PM, Aaron Conole wrote:
>> Hi Ilya,
>> 
>> Ilya Maximets <i.maximets@xxxxxxx> writes:
>> 
>>> Sashiko reports that it is technically possible that we got the device
>>> reference, but by the time we're linking it to the OVS datapath, it
>>> may be already in the process of being deleted.  In this case if the
>>> notifier wins the race for RTNL, it will see that the device is not
>>> yet in the OVS datapath (ovs_netdev_get_vport() will fail in the
>>> dp_device_event()) and will do nothing.  Then the ovs_netdev_link()
>>> will take the RTNL and link the unregistering device to OVS datapath.
>>>
>>> Eventually, netdev_wait_allrefs_any() will re-broadcast the event and
>>> the device will be properly detached, but it will take at least a
>>> second before that happens, so it's not something we should rely on.
>>>
>>> Let's avoid linking the non-registered device in the first place.
>>>
>>> Note: As per documentation, RTNL doesn't protect the reg_state, but
>>> it actually does for all the state transitions we care about here,
>>> so it should not be necessary to use READ_ONCE or taking the instance
>>> lock.  We can still do that, but we have a few more places even in
>>> this file where the reg_state is accessed without those while under
>>> RTNL, and many more places like this across the kernel code, so it
>>> might make more sense to change all of them in a more centralized
>>> fashion in the future, if necessary.
>>>
>>> Fixes: ccb1352e76cf ("net: Add Open vSwitch kernel components.")
>>> Signed-off-by: Ilya Maximets <i.maximets@xxxxxxx>
>>> ---
>>>  net/openvswitch/vport-netdev.c | 5 +++++
>>>  1 file changed, 5 insertions(+)
>>>
>>> diff --git a/net/openvswitch/vport-netdev.c b/net/openvswitch/vport-netdev.c
>>> index c42642075685d..de90d0541e172 100644
>>> --- a/net/openvswitch/vport-netdev.c
>>> +++ b/net/openvswitch/vport-netdev.c
>>> @@ -83,6 +83,11 @@ struct vport *ovs_netdev_link(struct vport *vport, bool tunnel)
>>>  	}
>>>  
>>>  	rtnl_lock();
>> 
>> As noted in your commit, this shouldn't cause any kind of issues, since
>> the next netdev_wait_allrefs_any() will make sure things look correct to
>> the users again.
>> 
>> That said, I agree this is good to do to prevent some confusion going to
>> the users.  I wonder if it makes sense to add a comment here noting
>> that.  Otherwise, if I were just freshly reading through the code it
>> wouldn't follow (all the places where ovs_netdev_link get called are in
>> the 'create' path).
>> 
>> WDYT?
>
> I'm not sure if the comment is necessary.  We're not creating a device here
> and it seems clear enough that we shouldn't be linking devices that are not
> registered, even if there are no races.  But I could add something like:
>
> /* Do not link devices that are not registered to avoid a potential
>  * race with the NETDEV_UNREGISTER notification in dp_device_event().
>  */
>
> WDYT?

That comment makes sense to me.

> Best regards, Ilya Maximets.