It is a clear misconfiguration to attach a qdisc to a device with
tx_queue_len zero, because some qdisc's (namely, pfifo, bfifo, gred,
htb, plug and sfb) inherit/copy this value as their queue length.
Why should the kernel catch such a misconfiguration? Because prior to
introducing the IFF_NO_QUEUE device flag, userspace found a loophole
in the qdisc config system that allowed them to achieve the equivalent
of IFF_NO_QUEUE, which is to remove the qdisc code path entirely from
a device. The loophole on older kernels is setting tx_queue_len=0,
*prior* to device qdisc init (the config time is significant, simply
setting tx_queue_len=0 doesn't trigger the loophole).
This loophole is currently used by Docker[1] to get better performance
and scalability out of the veth device. The Docker developers were
warned[1] that they needed to adjust the tx_queue_len if ever
attaching a qdisc. The OpenShift project didn't remember this warning
and attached a qdisc, this were caught and fixed in[2].
[1] https://github.com/docker/libcontainer/pull/193
[2] https://github.com/openshift/origin/pull/11126
Instead of fixing every userspace program that used this loophole, and
forgot to reset the tx_queue_len, prior to attaching a qdisc. Let's
catch the misconfiguration on the kernel side.
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
84c46dd865