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

Florian Westphal
2025-10-10T13:54:26Z

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@strlen.de>
---
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