taprio: Fix kernel panic in taprio_destroy

taprio_init may fail earlier than this line:

	list_add(&q->taprio_list, &taprio_list);

i.e. due to the net device not being multi queue.

Attempting to remove q from the global taprio_list when it is not part
of it will result in a kernel panic.

Fix it by matching list_add and list_del better to one another in the
order of operations. This way we can keep the deletion unconditional
and with lower complexity - O(1).

Cc: Leandro Dorileo <[email protected]>
Fixes: 7b9eba7 ("net/sched: taprio: fix picos_per_byte miscalculation")
Signed-off-by: Vladimir Oltean <[email protected]>
Acked-by: Vinicius Costa Gomes <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

net/sched/sch_taprio.c

@@ -1249,6 +1249,10 @@ static int taprio_init(struct Qdisc *sch, struct nlattr *opt,
 	 */
 	q->clockid = -1;
 
+	spin_lock(&taprio_list_lock);
+	list_add(&q->taprio_list, &taprio_list);
+	spin_unlock(&taprio_list_lock);
+
 	if (sch->parent != TC_H_ROOT)
 		return -EOPNOTSUPP;
 
@@ -1266,10 +1270,6 @@ static int taprio_init(struct Qdisc *sch, struct nlattr *opt,
 	if (!opt)
 		return -EINVAL;
 
-	spin_lock(&taprio_list_lock);
-	list_add(&q->taprio_list, &taprio_list);
-	spin_unlock(&taprio_list_lock);
-
 	for (i = 0; i < dev->num_tx_queues; i++) {
 		struct netdev_queue *dev_queue;
 		struct Qdisc *qdisc;