[net PATCH] net: sched, fix OOO packets with pfifo_fast

[John Fastabend <john.fastabend@gmail.com>]

After the qdisc lock was dropped in pfifo_fast we allow multiple
enqueue threads and dequeue threads to run in parallel. On the
enqueue side the skb bit ooo_okay is used to ensure all related
skbs are enqueued in-order. On the dequeue side though there is
no similar logic. What we observe is with fewer queues than CPUs
it is possible to re-order packets when two instances of
__qdisc_run() are running in parallel. Each thread will dequeue
a skb and then whichever thread calls the ndo op first will
be sent on the wire. This doesn't typically happen because
qdisc_run() is usually triggered by the same core that did the
enqueue. However, drivers will trigger __netif_schedule()
when queues are transitioning from stopped to awake using the
netif_tx_wake_* APIs. When this happens netif_schedule() calls
qdisc_run() on the same CPU that did the netif_tx_wake_* which
is usually done in the interrupt completion context. This CPU
is selected with the irq affinity which is unrelated to the
enqueue operations.

To resolve this we add a RUNNING bit to the qdisc to ensure
only a single dequeue per qdisc is running. Enqueue and dequeue
operations can still run in parallel and also on multi queue
NICs we can still have a dequeue in-flight per qdisc, which
is typically per CPU.

Fixes: c5ad119fb6c0 ("net: sched: pfifo_fast use skb_array")
Reported-by: Jakob Unterwurzacher <jakob.unterwurzacher@theobroma-systems.com>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
---
 include/net/sch_generic.h |    1 +
 net/sched/sch_generic.c   |   13 ++++++++++---
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 2092d33..8da3267 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -30,6 +30,7 @@ struct qdisc_rate_table {
 enum qdisc_state_t {
 	__QDISC_STATE_SCHED,
 	__QDISC_STATE_DEACTIVATED,
+	__QDISC_STATE_RUNNING,
 };
 
 struct qdisc_size_table {
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 7e3fbe9..29a1b47 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -377,12 +377,17 @@ static inline bool qdisc_restart(struct Qdisc *q, int *packets)
 	struct netdev_queue *txq;
 	struct net_device *dev;
 	struct sk_buff *skb;
-	bool validate;
+	bool more, validate;
 
 	/* Dequeue packet */
+	if (test_and_set_bit(__QDISC_STATE_RUNNING, &q->state))
+		return false;
+
 	skb = dequeue_skb(q, &validate, packets);
-	if (unlikely(!skb))
+	if (unlikely(!skb)) {
+		clear_bit(__QDISC_STATE_RUNNING, &q->state);
 		return false;
+	}
 
 	if (!(q->flags & TCQ_F_NOLOCK))
 		root_lock = qdisc_lock(q);
@@ -390,7 +395,9 @@ static inline bool qdisc_restart(struct Qdisc *q, int *packets)
 	dev = qdisc_dev(q);
 	txq = skb_get_tx_queue(dev, skb);
 
-	return sch_direct_xmit(skb, q, dev, txq, root_lock, validate);
+	more = sch_direct_xmit(skb, q, dev, txq, root_lock, validate);
+	clear_bit(__QDISC_STATE_RUNNING, &q->state);
+	return more;
 }
 
 void __qdisc_run(struct Qdisc *q)