net_sched: sch_fifo: implement lockless __fifo_dump()

Instead of relying on RTNL, __fifo_dump() can use READ_ONCE()
annotations, paired with WRITE_ONCE() ones in __fifo_init().

Also add missing READ_ONCE(sh->limit) in bfifo_enqueue(),
pfifo_enqueue() and pfifo_tail_enqueue().
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_fifo.c

@@ -19,7 +19,8 @@
 static int bfifo_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 			 struct sk_buff **to_free)
 {
-	if (likely(sch->qstats.backlog + qdisc_pkt_len(skb) <= sch->limit))
+	if (likely(sch->qstats.backlog + qdisc_pkt_len(skb) <=
+		   READ_ONCE(sch->limit)))
 		return qdisc_enqueue_tail(skb, sch);
 
 	return qdisc_drop(skb, sch, to_free);
@@ -28,7 +29,7 @@ static int bfifo_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 static int pfifo_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 			 struct sk_buff **to_free)
 {
-	if (likely(sch->q.qlen < sch->limit))
+	if (likely(sch->q.qlen < READ_ONCE(sch->limit)))
 		return qdisc_enqueue_tail(skb, sch);
 
 	return qdisc_drop(skb, sch, to_free);
@@ -39,7 +40,7 @@ static int pfifo_tail_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 {
 	unsigned int prev_backlog;
 
-	if (likely(sch->q.qlen < sch->limit))
+	if (likely(sch->q.qlen < READ_ONCE(sch->limit)))
 		return qdisc_enqueue_tail(skb, sch);
 
 	prev_backlog = sch->qstats.backlog;
@@ -105,14 +106,14 @@ static int __fifo_init(struct Qdisc *sch, struct nlattr *opt,
 		if (is_bfifo)
 			limit *= psched_mtu(qdisc_dev(sch));
 
-		sch->limit = limit;
+		WRITE_ONCE(sch->limit, limit);
 	} else {
 		struct tc_fifo_qopt *ctl = nla_data(opt);
 
 		if (nla_len(opt) < sizeof(*ctl))
 			return -EINVAL;
 
-		sch->limit = ctl->limit;
+		WRITE_ONCE(sch->limit, ctl->limit);
 	}
 
 	if (is_bfifo)
@@ -154,7 +155,7 @@ static void fifo_destroy(struct Qdisc *sch)
 
 static int __fifo_dump(struct Qdisc *sch, struct sk_buff *skb)
 {
-	struct tc_fifo_qopt opt = { .limit = sch->limit };
+	struct tc_fifo_qopt opt = { .limit = READ_ONCE(sch->limit) };
 
 	if (nla_put(skb, TCA_OPTIONS, sizeof(opt), &opt))
 		goto nla_put_failure;