net: allow out-of-order netdev unregistration

Sprinkle for each loops to allow netdevices to be unregistered
out of order, as their refs are released.

This prevents problems caused by dependencies between netdevs
which want to release references in their ->priv_destructor.
See commit d6ff94a ("vlan: move dev_put into vlan_dev_uninit")
for example.

Eric has removed the only known ordering requirement in
commit c002496 ("Merge branch 'ipv6-loopback'")
so let's try this and see if anything explodes...

Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/20220215225310.3679266-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dev.c

@@ -9811,8 +9811,8 @@ int netdev_unregister_timeout_secs __read_mostly = 10;
 #define WAIT_REFS_MIN_MSECS 1
 #define WAIT_REFS_MAX_MSECS 250
 /**
- * netdev_wait_allrefs - wait until all references are gone.
- * @dev: target net_device
+ * netdev_wait_allrefs_any - wait until all references are gone.
+ * @list: list of net_devices to wait on
  *
  * This is called when unregistering network devices.
  *
@@ -9822,37 +9822,45 @@ int netdev_unregister_timeout_secs __read_mostly = 10;
  * We can get stuck here if buggy protocols don't correctly
  * call dev_put.
  */
-static void netdev_wait_allrefs(struct net_device *dev)
+static struct net_device *netdev_wait_allrefs_any(struct list_head *list)
 {
 	unsigned long rebroadcast_time, warning_time;
-	int wait = 0, refcnt;
+	struct net_device *dev;
+	int wait = 0;
 
-	linkwatch_forget_dev(dev);
+	list_for_each_entry(dev, list, todo_list)
+		linkwatch_forget_dev(dev);
 
 	rebroadcast_time = warning_time = jiffies;
-	refcnt = netdev_refcnt_read(dev);
 
-	while (refcnt != 1) {
+	list_for_each_entry(dev, list, todo_list)
+		if (netdev_refcnt_read(dev) == 1)
+			return dev;
+
+	while (true) {
 		if (time_after(jiffies, rebroadcast_time + 1 * HZ)) {
 			rtnl_lock();
 
 			/* Rebroadcast unregister notification */
-			call_netdevice_notifiers(NETDEV_UNREGISTER, dev);
+			list_for_each_entry(dev, list, todo_list)
+				call_netdevice_notifiers(NETDEV_UNREGISTER, dev);
 
 			__rtnl_unlock();
 			rcu_barrier();
 			rtnl_lock();
 
-			if (test_bit(__LINK_STATE_LINKWATCH_PENDING,
-				     &dev->state)) {
-				/* We must not have linkwatch events
-				 * pending on unregister. If this
-				 * happens, we simply run the queue
-				 * unscheduled, resulting in a noop
-				 * for this device.
-				 */
-				linkwatch_run_queue();
-			}
+			list_for_each_entry(dev, list, todo_list)
+				if (test_bit(__LINK_STATE_LINKWATCH_PENDING,
+					     &dev->state)) {
+					/* We must not have linkwatch events
+					 * pending on unregister. If this
+					 * happens, we simply run the queue
+					 * unscheduled, resulting in a noop
+					 * for this device.
+					 */
+					linkwatch_run_queue();
+					break;
+				}
 
 			__rtnl_unlock();
 
@@ -9867,14 +9875,18 @@ static void netdev_wait_allrefs(struct net_device *dev)
 			wait = min(wait << 1, WAIT_REFS_MAX_MSECS);
 		}
 
-		refcnt = netdev_refcnt_read(dev);
+		list_for_each_entry(dev, list, todo_list)
+			if (netdev_refcnt_read(dev) == 1)
+				return dev;
 
-		if (refcnt != 1 &&
-		    time_after(jiffies, warning_time +
+		if (time_after(jiffies, warning_time +
 			       netdev_unregister_timeout_secs * HZ)) {
-			pr_emerg("unregister_netdevice: waiting for %s to become free. Usage count = %d\n",
-				 dev->name, refcnt);
-			ref_tracker_dir_print(&dev->refcnt_tracker, 10);
+			list_for_each_entry(dev, list, todo_list) {
+				pr_emerg("unregister_netdevice: waiting for %s to become free. Usage count = %d\n",
+					 dev->name, netdev_refcnt_read(dev));
+				ref_tracker_dir_print(&dev->refcnt_tracker, 10);
+			}
+
 			warning_time = jiffies;
 		}
 	}
@@ -9942,11 +9954,9 @@ void netdev_run_todo(void)
 	}
 
 	while (!list_empty(&list)) {
-		dev = list_first_entry(&list, struct net_device, todo_list);
+		dev = netdev_wait_allrefs_any(&list);
 		list_del(&dev->todo_list);
 
-		netdev_wait_allrefs(dev);
-
 		/* paranoia */
 		BUG_ON(netdev_refcnt_read(dev) != 1);
 		BUG_ON(!list_empty(&dev->ptype_all));