Re: [RFC PATCH net-next 05/13] net: bonding: hold the netdev lists lock when retrieving device statistics

Vladimir Oltean
2020-12-07T15:23:32Z

On Mon, Dec 07, 2020 at 03:00:40AM +0200, Vladimir Oltean wrote:
> There is a very obvious deadlock here which happens when we have
> bond-over-bond and the upper calls dev_get_stats from the lower.
>
> Conceptually, the same can happen even in any number of stacking
> combinations between bonding, net_failover, [ insert any other driver
> that takes net->netdev_lists_lock here ].
>
> There would be two approaches trying to solve this issue:
> - using mutex_lock_nested where we aren't sure that we are top level
> - ensuring through convention that user space always takes
> net->netdev_lists_lock when calling dev_get_stats, and documenting
> that, and therefore making it unnecessary to lock in bonding.
>
> I took neither of the two approaches (I don't really like either one too
> much), hence [ one of ] the reasons for the RFC. Comments?

And there are also issues which are more subtle (or maybe just to me, at
the time I wrote the patch). Like the fact that the netdev adjacency
lists are not protected by net->netdev_lists_lock, but still by the RTNL
mutex and RCU. I think that in order for the iteration through lower
interfaces to capture a consistent state of the adjancency lists of all
interfaces, the __netdev_adjacent_dev_link_lists and
__netdev_adjacent_dev_unlink_lists functions would need to be run under
the net->netdev_lists_lock, and not just under some lock per-netdev.
But this is raising the locking domain covered by net->netdev_lists_lock
to more than I initially intended. I'll try to do this and see how it
works.