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 <bro...@redhat.com> --- net/sched/sch_api.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c index 206dc24add3a..f337f1bdd1d4 100644 --- a/net/sched/sch_api.c +++ b/net/sched/sch_api.c @@ -960,6 +960,17 @@ static struct Qdisc *qdisc_create(struct net_device *dev, sch->handle = handle; + /* This exist to keep backward compatible with a userspace + * loophole, what allowed userspace to get IFF_NO_QUEUE + * facility on older kernels by setting tx_queue_len=0 (prior + * to qdisc init), and then forgot to reinit tx_queue_len + * before again attaching a qdisc. + */ + if ((dev->priv_flags & IFF_NO_QUEUE) && (dev->tx_queue_len == 0)) { + dev->tx_queue_len = DEFAULT_TX_QUEUE_LEN; + netdev_info(dev, "Caught tx_queue_len zero misconfig\n"); + } + if (!ops->init || (err = ops->init(sch, tca[TCA_OPTIONS])) == 0) { if (qdisc_is_percpu_stats(sch)) { sch->cpu_bstats =
