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

[Ilya Maximets <i.maximets@ovn.org>]

On 5/13/26 2:02 PM, Aaron Conole wrote:
> Hi Ilya,
> 
> Ilya Maximets <i.maximets@ovn.org> 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@ovn.org>
>> ---
>>  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?

Best regards, Ilya Maximets.