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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 16 Jan 2020 06:25:46 +0100
Message-ID: <CACT4Y+YuM55YUT37jwRP163J7ha25cN03sZ5WqTUPkz3e43Ggw@mail.gmail.com>
Subject: Re: BUG: MAX_LOCKDEP_CHAINS too low!
To:     Cong Wang <xiyou.wangcong@gmail.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Taehee Yoo <ap420073@gmail.com>,
        syzbot <syzbot+aaa6fa4949cc5d9b7b25@syzkaller.appspotmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        Will Deacon <will.deacon@arm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Thomas Gleixner <tglx@linutronix.de>
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On Wed, Jan 15, 2020 at 10:53 PM Cong Wang <xiyou.wangcong@gmail.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.

And these are not freed/reused, right? So with dynamic keys LOCKDEP
inherently can't handle prolonged running, only O(1) work?


> 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.