net: sched: generalize check for no-queue qdisc on TX queue

The "noqueue" qdisc can either be directly attached, or get default
attached if net_device priv_flags has IFF_NO_QUEUE. In both cases, the
allocated Qdisc structure gets it's enqueue function pointer reset to
NULL by noqueue_init() via noqueue_qdisc_ops.

This is a common case for software virtual net_devices. For these devices
with no-queue, the transmission path in __dev_queue_xmit() will bypass
the qdisc layer. Directly invoking device drivers ndo_start_xmit (via
dev_hard_start_xmit).  In this mode the device driver is not allowed to
ask for packets to be queued (either via returning NETDEV_TX_BUSY or
stopping the TXQ).

The simplest and most reliable way to identify this no-queue case is by
checking if enqueue == NULL.

The vrf driver currently open-codes this check (!qdisc->enqueue). While
functionally correct, this low-level detail is better encapsulated in a
dedicated helper for clarity and long-term maintainability.

To make this behavior more explicit and reusable, this patch introduce a
new helper: qdisc_txq_has_no_queue(). Helper will also be used by the
veth driver in the next patch, which introduces optional qdisc-based
backpressure.

This is a non-functional change.

Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Link: https://patch.msgid.link/174559293172.827981.7583862632045264175.stgit@firesoul
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/vrf.c b/drivers/net/vrf.c
index 7168b33adadb0dc799b8b7e3bea739ae95315218..9a4beea6ee0c292ab987e6b76e247caaa3ba41ea 100644 (file)
--- a/drivers/net/vrf.c
+++ b/drivers/net/vrf.c
@@ -343,15 +343,13 @@ unlock:
 static bool qdisc_tx_is_default(const struct net_device *dev)
 {
        struct netdev_queue *txq;
-       struct Qdisc *qdisc;
 
        if (dev->num_tx_queues > 1)
                return false;
 
        txq = netdev_get_tx_queue(dev, 0);
-       qdisc = rcu_access_pointer(txq->qdisc);
 
-       return !qdisc->enqueue;
+       return qdisc_txq_has_no_queue(txq);
 }
 
 /* Local traffic destined to local address. Reinsert the packet to rx

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index d48c657191cd01e4d278c2dd6e162d14ed45e2d6..b6c177f7141c008d2d80760c344acc1e9d5d8a54 100644 (file)
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -803,6 +803,14 @@ static inline bool qdisc_tx_changing(const struct net_device *dev)
        return false;
 }
 
+/* "noqueue" qdisc identified by not having any enqueue, see noqueue_init() */
+static inline bool qdisc_txq_has_no_queue(const struct netdev_queue *txq)
+{
+       struct Qdisc *qdisc = rcu_access_pointer(txq->qdisc);
+
+       return qdisc->enqueue == NULL;
+}
+
 /* Is the device using the noop qdisc on all queues?  */
 static inline bool qdisc_tx_is_noop(const struct net_device *dev)
 {