[PATCH net] net_sched: fix pfifo_head_drop behavior vs backlog

[Eric Dumazet]

From: Eric Dumazet <eduma...@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 <eduma...@google.com>
Cc: WANG Cong <xiyou.wangc...@gmail.com>
Cc: Jamal Hadi Salim <j...@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;
 }