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 <edumazet@google.com>
Link: https://patch.msgid.link/20250109171850.2871194-1-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index bb7dd351bd651d4aa16945816ee26df3c4a48645..14ab2f4c190a1e201dd1788b413a06e799a829f2 100644 (file)
--- a/net/sched/sch_generic.c
+++ b/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);