[PATCH net 2/2] net: core: split unregister_netdevice list into smaller chunks

[Florian Westphal]

Since blamed commit, unregister_netdevice_many_notify() takes the netdev
mutex if the device needs it.

This isn't a problem in itself, the problem is that the list can be
very long, so it may lock a LOT of mutexes, but lockdep engine can only
deal with MAX_LOCK_DEPTH held locks:

unshare -n bash -c 'for i in $(seq 1 100);do  ip link add foo$i type dummy;done'
BUG: MAX_LOCK_DEPTH too low!
turning off the locking correctness validator.
depth: 48  max: 48!
48 locks held by kworker/u16:1/69:
 #0: ffff8880010b7148 ((wq_completion)netns){+.+.}-{0:0}, at: process_one_work+0x7ed/0x1350
 #1: ffffc900004a7d40 (net_cleanup_work){+.+.}-{0:0}, at: process_one_work+0xcf3/0x1350
 #2: ffffffff8bc6fbd0 (pernet_ops_rwsem){++++}-{4:4}, at: cleanup_net+0xab/0x7f0
 #3: ffffffff8bc8daa8 (rtnl_mutex){+.+.}-{4:4}, at: default_device_exit_batch+0x7e/0x2e0
 #4: ffff88800b5e9cb0 (&dev_instance_lock_key#3){+.+.}-{4:4}, at: unregister_netdevice_many_notify+0x1056/0x1b00
[..]

Work around this limitation by chopping the list into smaller chunks
and process them individually for LOCKDEP enabled kernels.

Fixes: 7e4d784f5810 ("net: hold netdev instance lock during rtnetlink operations")
Signed-off-by: Florian Westphal <fw@xxxxxxxxx>
---
 net/core/dev.c | 34 +++++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 9a09b48c9371..7e35aa4ebc74 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -12208,6 +12208,38 @@ static void unregister_netdevice_close_many(struct list_head *head)
 	}
 }
 
+static void unregister_netdevice_close_many_lockdep(struct list_head *head)
+{
+#ifdef CONFIG_LOCKDEP
+	unsigned int lock_depth = lockdep_depth(current);
+	unsigned int lock_count = lock_depth;
+	struct net_device *dev, *tmp;
+	LIST_HEAD(done_head);
+
+	list_for_each_entry_safe(dev, tmp, head, unreg_list) {
+		if (netdev_need_ops_lock(dev))
+			lock_count++;
+
+		/* we'll run out of lockdep keys, reduce size. */
+		if (lock_count >= MAX_LOCK_DEPTH - 1) {
+			LIST_HEAD(tmp_head);
+
+			list_cut_before(&tmp_head, head, &dev->unreg_list);
+			unregister_netdevice_close_many(&tmp_head);
+			lock_count = lock_depth;
+			list_splice_tail(&tmp_head, &done_head);
+		}
+	}
+
+	unregister_netdevice_close_many(head);
+
+	list_for_each_entry_safe_reverse(dev, tmp, &done_head, unreg_list)
+		list_move(&dev->unreg_list, head);
+#else
+	unregister_netdevice_close_many(head);
+#endif
+}
+
 void unregister_netdevice_many_notify(struct list_head *head,
 				      u32 portid, const struct nlmsghdr *nlh)
 {
@@ -12237,7 +12269,7 @@ void unregister_netdevice_many_notify(struct list_head *head,
 		BUG_ON(dev->reg_state != NETREG_REGISTERED);
 	}
 
-	unregister_netdevice_close_many(head);
+	unregister_netdevice_close_many_lockdep(head);
 
 	flush_all_backlogs();
 
-- 
2.49.1