[PATCH net] net_sched: fix pfifo_head_drop behavior vs backlog

From: Eric Dumazet <edumazet@google.com>

When the qdisc is full, we drop a packet at the head of the queue,
queue the current skb and return NET_XMIT_CN

Now we track backlog on upper qdiscs, we need to call
qdisc_tree_reduce_backlog(), even if the qlen did not change.

Fixes: 2ccccf5fb43f ("net_sched: update hierarchical backlog too")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: WANG Cong <redacted>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
---
 net/sched/sch_fifo.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/sched/sch_fifo.c b/net/sched/sch_fifo.c
index 2177eac0a61e..2e4bd2c0a50c 100644
--- a/net/sched/sch_fifo.c
+++ b/net/sched/sch_fifo.c

@@ -37,14 +37,18 @@ static int pfifo_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 
 static int pfifo_tail_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 {
+	unsigned int prev_backlog;
+
 	if (likely(skb_queue_len(&sch->q) < sch->limit))
 		return qdisc_enqueue_tail(skb, sch);
 
+	prev_backlog = sch->qstats.backlog;
 	/* queue full, remove one skb to fulfill the limit */
 	__qdisc_queue_drop_head(sch, &sch->q);
 	qdisc_qstats_drop(sch);
 	qdisc_enqueue_tail(skb, sch);
 
+	qdisc_tree_reduce_backlog(sch, 0, prev_backlog - sch->qstats.backlog);
 	return NET_XMIT_CN;
 }
 

On Sun, Jun 12, 2016 at 8:01 PM, Eric Dumazet [off-list ref] wrote:

From: Eric Dumazet <edumazet@google.com>

When the qdisc is full, we drop a packet at the head of the queue,
queue the current skb and return NET_XMIT_CN

Now we track backlog on upper qdiscs, we need to call
qdisc_tree_reduce_backlog(), even if the qlen did not change.

Fixes: 2ccccf5fb43f ("net_sched: update hierarchical backlog too")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: WANG Cong <redacted>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>

Acked-by: Cong Wang <redacted>

From: Eric Dumazet <redacted>
Date: Sun, 12 Jun 2016 20:01:25 -0700

From: Eric Dumazet <edumazet@google.com>

When the qdisc is full, we drop a packet at the head of the queue,
queue the current skb and return NET_XMIT_CN

Now we track backlog on upper qdiscs, we need to call
qdisc_tree_reduce_backlog(), even if the qlen did not change.

Fixes: 2ccccf5fb43f ("net_sched: update hierarchical backlog too")
Signed-off-by: Eric Dumazet <edumazet@google.com>

Applied and queued up for -stable, thanks.