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

From: Stanislav Fomichev <[email-masked]>
Date: Fri, 10 Oct 2025 15:38:30 -0700

On 10/10, Florian Westphal wrote:
> 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 <[email-masked]>
> ---
>  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

Any reason not to morph the original code to add this 'no more than 8 at a
time' constraint? Having a separate lockdep path with list juggling
seems a bit fragile.

1. add all ops locked devs to the list
2. for each MAX_LOCK_DEPTH (or 'infinity' in the case of non-lockdep)
  2.1 lock N devs
  2.2 netif_close_many
  2.3 unlock N devs
3. ... do the non-ops-locked ones

This way the code won't diverge too much I hope.