net/sched: sch_ets: properly init all active DRR list handles

leaf classes of ETS qdiscs are served in strict priority or deficit round
robin (DRR), depending on the value of 'nstrict'. Since this value can be
changed while traffic is running, we need to be sure that the active list
of DRR classes can be updated at any time, so:

1) call INIT_LIST_HEAD(&alist) on all leaf classes in .init(), before the
   first packet hits any of them.
2) ensure that 'alist' is not overwritten with zeros when a leaf class is
   no more strict priority nor DRR (i.e. array elements beyond 'nbands').

Link: https://lore.kernel.org/netdev/YS%2FoZ+f0Nr8eQkzH@dcaratti.users.ipa.redhat.comSuggested-by: NCong Wang <cong.wang@bytedance.com>
Signed-off-by: NDavide Caratti <dcaratti@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_ets.c

@@ -661,7 +661,6 @@ static int ets_qdisc_change(struct Qdisc *sch, struct nlattr *opt,
 
 	q->nbands = nbands;
 	for (i = nstrict; i < q->nstrict; i++) {
-		INIT_LIST_HEAD(&q->classes[i].alist);
 		if (q->classes[i].qdisc->q.qlen) {
 			list_add_tail(&q->classes[i].alist, &q->active);
 			q->classes[i].deficit = quanta[i];
@@ -687,7 +686,11 @@ static int ets_qdisc_change(struct Qdisc *sch, struct nlattr *opt,
 	ets_offload_change(sch);
 	for (i = q->nbands; i < oldbands; i++) {
 		qdisc_put(q->classes[i].qdisc);
-		memset(&q->classes[i], 0, sizeof(q->classes[i]));
+		q->classes[i].qdisc = NULL;
+		q->classes[i].quantum = 0;
+		q->classes[i].deficit = 0;
+		memset(&q->classes[i].bstats, 0, sizeof(q->classes[i].bstats));
+		memset(&q->classes[i].qstats, 0, sizeof(q->classes[i].qstats));
 	}
 	return 0;
 }
@@ -696,7 +699,7 @@ static int ets_qdisc_init(struct Qdisc *sch, struct nlattr *opt,
 			  struct netlink_ext_ack *extack)
 {
 	struct ets_sched *q = qdisc_priv(sch);
-	int err;
+	int err, i;
 
 	if (!opt)
 		return -EINVAL;
@@ -706,6 +709,9 @@ static int ets_qdisc_init(struct Qdisc *sch, struct nlattr *opt,
 		return err;
 
 	INIT_LIST_HEAD(&q->active);
+	for (i = 0; i < TCQ_ETS_MAX_BANDS; i++)
+		INIT_LIST_HEAD(&q->classes[i].alist);
+
 	return ets_qdisc_change(sch, opt, extack);
 }