net: sched: calls synchronize_net() only when needed

dev_deactivate_many() role is to remove the qdiscs
of a network device.

When/if a qdisc is dismantled, an rcu grace period
is needed to make sure all outstanding qdisc enqueue
are done before we proceed with a qdisc reset.

Most virtual devices do not have a qdisc.

We can call the expensive synchronize_net() only
if needed.

Note that dev_deactivate_many() does not have to deal
with qdisc-less dev_queue_xmit, as an old comment
was claiming.

Signed-off-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: edumazet

net/sched/sch_generic.c

@@ -1277,15 +1277,17 @@ static void qdisc_deactivate(struct Qdisc *qdisc)
 
 static void dev_deactivate_queue(struct net_device *dev,
 				 struct netdev_queue *dev_queue,
-				 void *_qdisc_default)
+				 void *_sync_needed)
 {
-	struct Qdisc *qdisc_default = _qdisc_default;
+	bool *sync_needed = _sync_needed;
 	struct Qdisc *qdisc;
 
 	qdisc = rtnl_dereference(dev_queue->qdisc);
 	if (qdisc) {
+		if (qdisc->enqueue)
+			*sync_needed = true;
 		qdisc_deactivate(qdisc);
-		rcu_assign_pointer(dev_queue->qdisc, qdisc_default);
+		rcu_assign_pointer(dev_queue->qdisc, &noop_qdisc);
 	}
 }
 
@@ -1352,24 +1354,22 @@ static bool some_qdisc_is_busy(struct net_device *dev)
  */
 void dev_deactivate_many(struct list_head *head)
 {
+	bool sync_needed = false;
 	struct net_device *dev;
 
 	list_for_each_entry(dev, head, close_list) {
 		netdev_for_each_tx_queue(dev, dev_deactivate_queue,
-					 &noop_qdisc);
+					 &sync_needed);
 		if (dev_ingress_queue(dev))
 			dev_deactivate_queue(dev, dev_ingress_queue(dev),
-					     &noop_qdisc);
+					     &sync_needed);
 
 		netdev_watchdog_down(dev);
 	}
 
-	/* Wait for outstanding qdisc-less dev_queue_xmit calls or
-	 * outstanding qdisc enqueuing calls.
-	 * This is avoided if all devices are in dismantle phase :
-	 * Caller will call synchronize_net() for us
-	 */
-	synchronize_net();
+	/* Wait for outstanding qdisc enqueuing calls. */
+	if (sync_needed)
+		synchronize_net();
 
 	list_for_each_entry(dev, head, close_list) {
 		netdev_for_each_tx_queue(dev, dev_reset_queue, NULL);