Re: BUG: MAX_LOCKDEP_CHAINS too low!

[Cong Wang]

On Mon, Jan 13, 2020 at 3:11 AM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> +Taehee, Cong,
>
> In the other thread Taehee mentioned the creation of dynamic keys for
> net devices that was added recently and that they are subject to some
> limits.
> syzkaller creates lots of net devices for isolation (several dozens
> per test process, but then these can be created and destroyed
> periodically). I wonder if it's the root cause of the lockdep limits
> problems?

Very possibly. In current code base, there are 4 lockdep keys
per netdev:

        struct lock_class_key   qdisc_tx_busylock_key;
        struct lock_class_key   qdisc_running_key;
        struct lock_class_key   qdisc_xmit_lock_key;
        struct lock_class_key   addr_list_lock_key;

so the number of lockdep keys is at least 4x number of network
devices.

I think only addr_list_lock_key is necessary as it has a nested
locking use case, all the rest are not. Taehee, do you agree?

I plan to remove at least qdisc_xmit_lock_key for net-next
after the fix for net gets merged.

Thanks!