net_sched: sch_ets: implement lockless ets_dump()

Instead of relying on RTNL, ets_dump() can use READ_ONCE()
annotations, paired with WRITE_ONCE() ones in ets_change().
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_ets.c

@@ -646,7 +646,7 @@ static int ets_qdisc_change(struct Qdisc *sch, struct nlattr *opt,
 
 	sch_tree_lock(sch);
 
-	q->nbands = nbands;
+	WRITE_ONCE(q->nbands, nbands);
 	for (i = nstrict; i < q->nstrict; i++) {
 		if (q->classes[i].qdisc->q.qlen) {
 			list_add_tail(&q->classes[i].alist, &q->active);
@@ -658,11 +658,11 @@ static int ets_qdisc_change(struct Qdisc *sch, struct nlattr *opt,
 			list_del(&q->classes[i].alist);
 		qdisc_tree_flush_backlog(q->classes[i].qdisc);
 	}
-	q->nstrict = nstrict;
+	WRITE_ONCE(q->nstrict, nstrict);
 	memcpy(q->prio2band, priomap, sizeof(priomap));
 
 	for (i = 0; i < q->nbands; i++)
-		q->classes[i].quantum = quanta[i];
+		WRITE_ONCE(q->classes[i].quantum, quanta[i]);
 
 	for (i = oldbands; i < q->nbands; i++) {
 		q->classes[i].qdisc = queues[i];
@@ -676,7 +676,7 @@ static int ets_qdisc_change(struct Qdisc *sch, struct nlattr *opt,
 	for (i = q->nbands; i < oldbands; i++) {
 		qdisc_put(q->classes[i].qdisc);
 		q->classes[i].qdisc = NULL;
-		q->classes[i].quantum = 0;
+		WRITE_ONCE(q->classes[i].quantum, 0);
 		q->classes[i].deficit = 0;
 		gnet_stats_basic_sync_init(&q->classes[i].bstats);
 		memset(&q->classes[i].qstats, 0, sizeof(q->classes[i].qstats));
@@ -733,6 +733,7 @@ static int ets_qdisc_dump(struct Qdisc *sch, struct sk_buff *skb)
 	struct ets_sched *q = qdisc_priv(sch);
 	struct nlattr *opts;
 	struct nlattr *nest;
+	u8 nbands, nstrict;
 	int band;
 	int prio;
 	int err;
@@ -745,21 +746,22 @@ static int ets_qdisc_dump(struct Qdisc *sch, struct sk_buff *skb)
 	if (!opts)
 		goto nla_err;
 
-	if (nla_put_u8(skb, TCA_ETS_NBANDS, q->nbands))
+	nbands = READ_ONCE(q->nbands);
+	if (nla_put_u8(skb, TCA_ETS_NBANDS, nbands))
 		goto nla_err;
 
-	if (q->nstrict &&
-	    nla_put_u8(skb, TCA_ETS_NSTRICT, q->nstrict))
+	nstrict = READ_ONCE(q->nstrict);
+	if (nstrict && nla_put_u8(skb, TCA_ETS_NSTRICT, nstrict))
 		goto nla_err;
 
-	if (q->nbands > q->nstrict) {
+	if (nbands > nstrict) {
 		nest = nla_nest_start(skb, TCA_ETS_QUANTA);
 		if (!nest)
 			goto nla_err;
 
-		for (band = q->nstrict; band < q->nbands; band++) {
+		for (band = nstrict; band < nbands; band++) {
 			if (nla_put_u32(skb, TCA_ETS_QUANTA_BAND,
-					q->classes[band].quantum))
+					READ_ONCE(q->classes[band].quantum)))
 				goto nla_err;
 		}
 
@@ -771,7 +773,8 @@ static int ets_qdisc_dump(struct Qdisc *sch, struct sk_buff *skb)
 		goto nla_err;
 
 	for (prio = 0; prio <= TC_PRIO_MAX; prio++) {
-		if (nla_put_u8(skb, TCA_ETS_PRIOMAP_BAND, q->prio2band[prio]))
+		if (nla_put_u8(skb, TCA_ETS_PRIOMAP_BAND,
+			       READ_ONCE(q->prio2band[prio])))
 			goto nla_err;
 	}