net/sched: make stab available before ops->init() call

Some qdiscs like taprio turn out to be actually pretty reliant on a well
configured stab, to not underestimate the skb transmission time (by
properly accounting for L1 overhead).

In a future change, taprio will need the stab, if configured by the
user, to be available at ops->init() time. It will become even more
important in upcoming work, when the overhead will be used for the
queueMaxSDU calculation that is passed to an offloading driver.

However, rcu_assign_pointer(sch->stab, stab) is called right after
ops->init(), making it unavailable, and I don't really see a good reason
for that.

Move it earlier, which nicely seems to simplify the error handling path
as well.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_api.c

@@ -1282,12 +1282,6 @@ static struct Qdisc *qdisc_create(struct net_device *dev,
 	if (err)
 		goto err_out3;
 
-	if (ops->init) {
-		err = ops->init(sch, tca[TCA_OPTIONS], extack);
-		if (err != 0)
-			goto err_out5;
-	}
-
 	if (tca[TCA_STAB]) {
 		stab = qdisc_get_stab(tca[TCA_STAB], extack);
 		if (IS_ERR(stab)) {
@@ -1296,11 +1290,18 @@ static struct Qdisc *qdisc_create(struct net_device *dev,
 		}
 		rcu_assign_pointer(sch->stab, stab);
 	}
+
+	if (ops->init) {
+		err = ops->init(sch, tca[TCA_OPTIONS], extack);
+		if (err != 0)
+			goto err_out5;
+	}
+
 	if (tca[TCA_RATE]) {
 		err = -EOPNOTSUPP;
 		if (sch->flags & TCQ_F_MQROOT) {
 			NL_SET_ERR_MSG(extack, "Cannot attach rate estimator to a multi-queue root qdisc");
-			goto err_out4;
+			goto err_out5;
 		}
 
 		err = gen_new_estimator(&sch->bstats,
@@ -1311,7 +1312,7 @@ static struct Qdisc *qdisc_create(struct net_device *dev,
 					tca[TCA_RATE]);
 		if (err) {
 			NL_SET_ERR_MSG(extack, "Failed to generate new estimator");
-			goto err_out4;
+			goto err_out5;
 		}
 	}
 
@@ -1321,6 +1322,8 @@ static struct Qdisc *qdisc_create(struct net_device *dev,
 	return sch;
 
 err_out5:
+	qdisc_put_stab(rtnl_dereference(sch->stab));
+err_out4:
 	/* ops->init() failed, we call ->destroy() like qdisc_create_dflt() */
 	if (ops->destroy)
 		ops->destroy(sch);
@@ -1332,16 +1335,6 @@ err_out2:
 err_out:
 	*errp = err;
 	return NULL;
-
-err_out4:
-	/*
-	 * Any broken qdiscs that would require a ops->reset() here?
-	 * The qdisc was never in action so it shouldn't be necessary.
-	 */
-	qdisc_put_stab(rtnl_dereference(sch->stab));
-	if (ops->destroy)
-		ops->destroy(sch);
-	goto err_out3;
 }
 
 static int qdisc_change(struct Qdisc *sch, struct nlattr **tca,