Re: [PATCH net] net: openvswitch: Avoid needlessly taking the RTNL on vport destroy

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Adrián Moreno <amorenoz@redhat.com> writes:

>> Not fully sure I understand the code path you are referring to, but
>> if it’s through
>> ovs_dp_notify_wq()->dp_detach_port_notify()->ovs_dp_detach_port(), it
>> takes the ovs_lock().
>
> The codepath described by Toke is:
> (netdev gets unregistered outside of OVS) ->
> dp_device_event (under RTNL) -> ovs_netdev_detach_dev()
> (IFF_OVS_DATAPATH is cleared)
>
> Then dp_notify_work is scheduled and it does what you mention:
> ovs_dp_notify_wq (lock ovs_mutex) -> dp_detach_port_notify -> ovs_dp_detach_port
>     -> ovs_vport_del -> netdev_destroy (at this point
> netif_is_ovs_port is false)
>
> The first part of this codepath (NETDEV_UNREGISTER notification) happens
> under RTNL, not under ovs_mutex and it manipulates vport->dev->priv_flags.
>
> So in theory we could receive the netdev notification while we process a
> ovs_vport_cmd_del() command from userspace, which also ends up calling
> netdev_destroy.

Yeah, I agree, it's not guaranteed that reading the flags outside the
lock will be race free, so re-checking seems safer here (and is also
quite cheap).

There does seem to be other uses of netif_is_ovs_port() that are outside
the RTNL, though, so maybe not such a likely race in practice?

Anyway, I'll respin with a comment.

-Toke