linux-kernel - Re: BUG: MAX_LOCKDEP

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Date:   Wed, 15 Jan 2020 13:53:28 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Taehee Yoo <ap420073@...il.com>,
        syzbot <syzbot+aaa6fa4949cc5d9b7b25@...kaller.appspotmail.com>,
        Ingo Molnar <mingo@...nel.org>,
        Will Deacon <will.deacon@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: BUG: MAX_LOCKDEP_CHAINS too low!

On Mon, Jan 13, 2020 at 3:11 AM Dmitry Vyukov <dvyukov@...gle.com> 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!