Re: [PATCH net-next] net/sched: fix false lockdep warning on qdisc root lock

[Eric Dumazet <edumazet@google.com>]

On Wed, Dec 6, 2023 at 10:04 AM Davide Caratti <dcaratti@redhat.com> wrote:
>
> Xiumei and Cristoph reported the following lockdep splat, it complains of
> the qdisc root being taken twice:
>
>  ============================================
>  WARNING: possible recursive locking detected
>  6.7.0-rc3+ #598 Not tainted
>  --------------------------------------------
>  swapper/2/0 is trying to acquire lock:
>  ffff888177190110 (&sch->q.lock){+.-.}-{2:2}, at: __dev_queue_xmit+0x1560/0x2e70
>
>  but task is already holding lock:
>  ffff88811995a110 (&sch->q.lock){+.-.}-{2:2}, at: __dev_queue_xmit+0x1560/0x2e70
>
>  other info that might help us debug this:
>   Possible unsafe locking scenario:
>
>         CPU0
>         ----
>    lock(&sch->q.lock);
>    lock(&sch->q.lock);
>
>   *** DEADLOCK ***
>
>   May be due to missing lock nesting notation
>
>  5 locks held by swapper/2/0:
>   #0: ffff888135a09d98 ((&in_dev->mr_ifc_timer)){+.-.}-{0:0}, at: call_timer_fn+0x11a/0x510
>   #1: ffffffffaaee5260 (rcu_read_lock){....}-{1:2}, at: ip_finish_output2+0x2c0/0x1ed0
>   #2: ffffffffaaee5200 (rcu_read_lock_bh){....}-{1:2}, at: __dev_queue_xmit+0x209/0x2e70
>   #3: ffff88811995a110 (&sch->q.lock){+.-.}-{2:2}, at: __dev_queue_xmit+0x1560/0x2e70
>   #4: ffffffffaaee5200 (rcu_read_lock_bh){....}-{1:2}, at: __dev_queue_xmit+0x209/0x2e70
>
>

Can you add a Fixes: tag ?

Also, what is the interaction with htb_set_lockdep_class_child(), have
you tried to use HTB after your patch ?

Could htb_set_lockdep_class_child() be removed ?


> CC: Xiumei Mu <xmu@redhat.com>
> Reported-by: Cristoph Paasch <cpaasch@apple.com>
> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/451
> Signed-off-by: Davide Caratti <dcaratti@redhat.com>
> ---
>  include/net/sch_generic.h | 1 +
>  net/sched/sch_generic.c   | 3 +++
>  2 files changed, 4 insertions(+)
>
> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
> index dcb9160e6467..a395ca76066c 100644
> --- a/include/net/sch_generic.h
> +++ b/include/net/sch_generic.h
> @@ -126,6 +126,7 @@ struct Qdisc {
>
>         struct rcu_head         rcu;
>         netdevice_tracker       dev_tracker;
> +       struct lock_class_key   root_lock_key;
>         /* private data */
>         long privdata[] ____cacheline_aligned;
>  };
> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
> index 8dd0e5925342..da3e1ea42852 100644
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -944,7 +944,9 @@ struct Qdisc *qdisc_alloc(struct netdev_queue *dev_queue,
>         __skb_queue_head_init(&sch->gso_skb);
>         __skb_queue_head_init(&sch->skb_bad_txq);
>         gnet_stats_basic_sync_init(&sch->bstats);
> +       lockdep_register_key(&sch->root_lock_key);
>         spin_lock_init(&sch->q.lock);
> +       lockdep_set_class(&sch->q.lock, &sch->root_lock_key);
>
>         if (ops->static_flags & TCQ_F_CPUSTATS) {
>                 sch->cpu_bstats =
> @@ -1064,6 +1066,7 @@ static void __qdisc_destroy(struct Qdisc *qdisc)
>         if (ops->destroy)
>                 ops->destroy(qdisc);
>
> +       lockdep_unregister_key(&qdisc->root_lock_key);
>         module_put(ops->owner);
>         netdev_put(qdisc_dev(qdisc), &qdisc->dev_tracker);
>
> --
> 2.41.0
>