net_sched: sch_hfsc: implement lockless accesses to q->defcls

Instead of relying on RTNL, hfsc_dump_qdisc() can use READ_ONCE()
annotation, paired with WRITE_ONCE() one in hfsc_change_qdisc().

Use READ_ONCE(q->defcls) in hfsc_classify() to
no longer acquire qdisc lock from hfsc_change_qdisc().
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_hfsc.c

@@ -1174,7 +1174,8 @@ hfsc_classify(struct sk_buff *skb, struct Qdisc *sch, int *qerr)
 	}
 
 	/* classification failed, try default class */
-	cl = hfsc_find_class(TC_H_MAKE(TC_H_MAJ(sch->handle), q->defcls), sch);
+	cl = hfsc_find_class(TC_H_MAKE(TC_H_MAJ(sch->handle),
+				       READ_ONCE(q->defcls)), sch);
 	if (cl == NULL || cl->level > 0)
 		return NULL;
 
@@ -1443,9 +1444,7 @@ hfsc_change_qdisc(struct Qdisc *sch, struct nlattr *opt,
 		return -EINVAL;
 	qopt = nla_data(opt);
 
-	sch_tree_lock(sch);
-	q->defcls = qopt->defcls;
-	sch_tree_unlock(sch);
+	WRITE_ONCE(q->defcls, qopt->defcls);
 
 	return 0;
 }
@@ -1525,7 +1524,7 @@ hfsc_dump_qdisc(struct Qdisc *sch, struct sk_buff *skb)
 	unsigned char *b = skb_tail_pointer(skb);
 	struct tc_hfsc_qopt qopt;
 
-	qopt.defcls = q->defcls;
+	qopt.defcls = READ_ONCE(q->defcls);
 	if (nla_put(skb, TCA_OPTIONS, sizeof(qopt), &qopt))
 		goto nla_put_failure;
 	return skb->len;