netdev - Re: WARNING: bad unlock balance in sch_direct

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Date:   Tue, 7 Jan 2020 16:34:11 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Taehee Yoo <ap420073@...il.com>
Cc:     syzbot <syzbot+4ec99438ed7450da6272@...kaller.appspotmail.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: WARNING: bad unlock balance in sch_direct_xmit

On Tue, Jan 7, 2020 at 3:31 AM Taehee Yoo <ap420073@...il.com> wrote:
> After "ip link set team0 master team1", the "team1 -> team0" locking path
> will be recorded in lockdep key of both team1 and team0.
> Then, if "ip link set team1 master team0" is executed, "team0 -> team1"
> locking path also will be recorded in lockdep key. At this moment,
> lockdep will catch possible deadlock situation and it prints the above
> warning message. But, both "team0 -> team1" and "team1 -> team0"
> will not be existing concurrently. so the above message is actually wrong.
> In order to avoid this message, a recorded locking path should be
> removed. So, both lockdep_unregister_key() and lockdep_register_key()
> are needed.
>

So, after you move the key down to each netdevice, they are now treated
as different locks. Is this stacked device scenario the reason why you
move it to per-netdevice? If so, I wonder why not just use nested locks?
Like:

netif_addr_nested_lock(upper, 0);
netif_addr_nested_lock(lower, 1);
netif_addr_nested_unlock(lower);
netif_addr_nested_unlock(upper);

For this case, they could still share a same key.

Thanks for the details!