[PATCH] net: remove delay at device dismantle

[Eric Dumazet <eric.dumazet@gmail.com>]

From: Eric Dumazet <edumazet@google.com>

I noticed extra one second delay in device dismantle, tracked down to
a call to dst_dev_event() while some call_rcu() are still in RCU queues.

These call_rcu() were posted by rt_free(struct rtable *rt) calls.

We then wait a little (but one second) in netdev_wait_allrefs() before
kicking again NETDEV_UNREGISTER.

As the call_rcu() are now completed, dst_dev_event() can do the needed
device swap on busy dst.

To solve this problem, add a new NETDEV_UNREGISTER_FINAL, called
in rollback_registered_many() after a rcu_barrier().

Change dst_dev_event() handler to react to NETDEV_UNREGISTER_FINAL

Also remove NETDEV_UNREGISTER_BATCH, as its not used anymore after
IP cache removal.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Mahesh Bandewar <maheshb@google.com>
---
 include/linux/netdevice.h |    2 +-
 net/core/dev.c            |   23 +++++++----------------
 net/core/dst.c            |    2 +-
 net/core/rtnetlink.c      |    2 +-
 net/ipv4/fib_frontend.c   |    2 --
 5 files changed, 10 insertions(+), 21 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index a9db4f3..e3a43fd 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -1550,7 +1550,7 @@ struct packet_type {
 #define NETDEV_PRE_TYPE_CHANGE	0x000E
 #define NETDEV_POST_TYPE_CHANGE	0x000F
 #define NETDEV_POST_INIT	0x0010
-#define NETDEV_UNREGISTER_BATCH 0x0011
+#define NETDEV_UNREGISTER_FINAL 0x0011
 #define NETDEV_RELEASE		0x0012
 #define NETDEV_NOTIFY_PEERS	0x0013
 #define NETDEV_JOIN		0x0014
diff --git a/net/core/dev.c b/net/core/dev.c
index a39354e..be3a5af 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1394,7 +1394,6 @@ rollback:
 				nb->notifier_call(nb, NETDEV_DOWN, dev);
 			}
 			nb->notifier_call(nb, NETDEV_UNREGISTER, dev);
-			nb->notifier_call(nb, NETDEV_UNREGISTER_BATCH, dev);
 		}
 	}
 
@@ -1436,7 +1435,6 @@ int unregister_netdevice_notifier(struct notifier_block *nb)
 				nb->notifier_call(nb, NETDEV_DOWN, dev);
 			}
 			nb->notifier_call(nb, NETDEV_UNREGISTER, dev);
-			nb->notifier_call(nb, NETDEV_UNREGISTER_BATCH, dev);
 		}
 	}
 unlock:
@@ -5307,14 +5305,14 @@ static void rollback_registered_many(struct list_head *head)
 		netdev_unregister_kobject(dev);
 	}
 
-	/* Process any work delayed until the end of the batch */
-	dev = list_first_entry(head, struct net_device, unreg_list);
-	call_netdevice_notifiers(NETDEV_UNREGISTER_BATCH, dev);
-
 	synchronize_net();
+	/* Wait for rcu callbacks to finish before next phase */
+	rcu_barrier();
 
-	list_for_each_entry(dev, head, unreg_list)
+	list_for_each_entry(dev, head, unreg_list) {
+		call_netdevice_notifiers(NETDEV_UNREGISTER_FINAL, dev);
 		dev_put(dev);
+	}
 }
 
 static void rollback_registered(struct net_device *dev)
@@ -5757,9 +5755,8 @@ static void netdev_wait_allrefs(struct net_device *dev)
 
 			/* Rebroadcast unregister notification */
 			call_netdevice_notifiers(NETDEV_UNREGISTER, dev);
-			/* don't resend NETDEV_UNREGISTER_BATCH, _BATCH users
-			 * should have already handle it the first time */
-
+			rcu_barrier();
+			call_netdevice_notifiers(NETDEV_UNREGISTER_FINAL, dev);
 			if (test_bit(__LINK_STATE_LINKWATCH_PENDING,
 				     &dev->state)) {
 				/* We must not have linkwatch events
@@ -5821,11 +5818,6 @@ void netdev_run_todo(void)
 
 	__rtnl_unlock();
 
-	/* Wait for rcu callbacks to finish before attempting to drain
-	 * the device list.  This usually avoids a 250ms wait.
-	 */
-	if (!list_empty(&list))
-		rcu_barrier();
 
 	while (!list_empty(&list)) {
 		struct net_device *dev
@@ -6226,7 +6218,6 @@ int dev_change_net_namespace(struct net_device *dev, struct net *net, const char
 	   the device is just moving and can keep their slaves up.
 	*/
 	call_netdevice_notifiers(NETDEV_UNREGISTER, dev);
-	call_netdevice_notifiers(NETDEV_UNREGISTER_BATCH, dev);
 	rtmsg_ifinfo(RTM_DELLINK, dev, ~0U);
 
 	/*
diff --git a/net/core/dst.c b/net/core/dst.c
index 56d6361..f6593d2 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -374,7 +374,7 @@ static int dst_dev_event(struct notifier_block *this, unsigned long event,
 	struct dst_entry *dst, *last = NULL;
 
 	switch (event) {
-	case NETDEV_UNREGISTER:
+	case NETDEV_UNREGISTER_FINAL:
 	case NETDEV_DOWN:
 		mutex_lock(&dst_gc_mutex);
 		for (dst = dst_busy_list; dst; dst = dst->next) {
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 2c5a0a0..30f5a06 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -2356,7 +2356,7 @@ static int rtnetlink_event(struct notifier_block *this, unsigned long event, voi
 	case NETDEV_PRE_TYPE_CHANGE:
 	case NETDEV_GOING_DOWN:
 	case NETDEV_UNREGISTER:
-	case NETDEV_UNREGISTER_BATCH:
+	case NETDEV_UNREGISTER_FINAL:
 	case NETDEV_RELEASE:
 	case NETDEV_JOIN:
 		break;
diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index c43ae3f..4fe50184 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -1071,8 +1071,6 @@ static int fib_netdev_event(struct notifier_block *this, unsigned long event, vo
 	case NETDEV_CHANGE:
 		rt_cache_flush(dev_net(dev), 0);
 		break;
-	case NETDEV_UNREGISTER_BATCH:
-		break;
 	}
 	return NOTIFY_DONE;
 }