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Date:   Fri, 17 Jan 2020 02:04:10 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     xiyou.wangcong@...il.com
Cc:     netdev@...r.kernel.org,
        syzbot+4ec99438ed7450da6272@...kaller.appspotmail.com,
        ap420073@...il.com
Subject: Re: [Patch net] net: avoid updating qdisc_xmit_lock_key in
 netdev_update_lockdep_key()

From: Cong Wang <xiyou.wangcong@...il.com>
Date: Wed, 15 Jan 2020 13:02:38 -0800

> syzbot reported some bogus lockdep warnings, for example bad unlock
> balance in sch_direct_xmit(). They are due to a race condition between
> slow path and fast path, that is qdisc_xmit_lock_key gets re-registered
> in netdev_update_lockdep_key() on slow path, while we could still
> acquire the queue->_xmit_lock on fast path in this small window:
> 
> CPU A						CPU B
> 						__netif_tx_lock();
> lockdep_unregister_key(qdisc_xmit_lock_key);
> 						__netif_tx_unlock();
> lockdep_register_key(qdisc_xmit_lock_key);
> 
> In fact, unlike the addr_list_lock which has to be reordered when
> the master/slave device relationship changes, queue->_xmit_lock is
> only acquired on fast path and only when NETIF_F_LLTX is not set,
> so there is likely no nested locking for it.
> 
> Therefore, we can just get rid of re-registration of
> qdisc_xmit_lock_key.
> 
> Reported-by: syzbot+4ec99438ed7450da6272@...kaller.appspotmail.com
> Fixes: ab92d68fc22f ("net: core: add generic lockdep keys")
> Cc: Taehee Yoo <ap420073@...il.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>

Applied and queued up for -stable, thanks.

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