net/sched: fix false lockdep warning on qdisc root lock
Xiumei and Christoph reported the following lockdep splat, complaining of the qdisc root lock 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 stack backtrace: CPU: 2 PID: 0 Comm: swapper/2 Not tainted 6.7.0-rc3+ #598 Hardware name: Red Hat KVM, BIOS 1.13.0-2.module+el8.3.0+7353+9de0a3cc 04/01/2014 Call Trace: <IRQ> dump_stack_lvl+0x4a/0x80 __lock_acquire+0xfdd/0x3150 lock_acquire+0x1ca/0x540 _raw_spin_lock+0x34/0x80 __dev_queue_xmit+0x1560/0x2e70 tcf_mirred_act+0x82e/0x1260 [act_mirred] tcf_action_exec+0x161/0x480 tcf_classify+0x689/0x1170 prio_enqueue+0x316/0x660 [sch_prio] dev_qdisc_enqueue+0x46/0x220 __dev_queue_xmit+0x1615/0x2e70 ip_finish_output2+0x1218/0x1ed0 __ip_finish_output+0x8b3/0x1350 ip_output+0x163/0x4e0 igmp_ifc_timer_expire+0x44b/0x930 call_timer_fn+0x1a2/0x510 run_timer_softirq+0x54d/0x11a0 __do_softirq+0x1b3/0x88f irq_exit_rcu+0x18f/0x1e0 sysvec_apic_timer_interrupt+0x6f/0x90 </IRQ> This happens when TC does a mirred egress redirect from the root qdisc of device A to the root qdisc of device B. As long as these two locks aren't protecting the same qdisc, they can be acquired in chain: add a per-qdisc lockdep key to silence false warnings. This dynamic key should safely replace the static key we have in sch_htb: it was added to allow enqueueing to the device "direct qdisc" while still holding the qdisc root lock. v2: don't use static keys anymore in HTB direct qdiscs (thanks Eric Dumazet) CC: Maxim Mikityanskiy <maxim@isovalent.com> CC: Xiumei Mu <xmu@redhat.com> Reported-by: Christoph Paasch <cpaasch@apple.com> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/451 Signed-off-by: Davide Caratti <dcaratti@redhat.com> Link: https://lore.kernel.org/r/7dc06d6158f72053cf877a82e2a7a5bd23692faa.1713448007.git.dcaratti@redhat.com Signed-off-by: Paolo Abeni <pabeni@redhat.com>
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@ -128,6 +128,7 @@ struct Qdisc {
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struct rcu_head rcu;
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netdevice_tracker dev_tracker;
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struct lock_class_key root_lock_key;
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/* private data */
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long privdata[] ____cacheline_aligned;
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};
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@ -945,7 +945,9 @@ struct Qdisc *qdisc_alloc(struct netdev_queue *dev_queue,
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__skb_queue_head_init(&sch->gso_skb);
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__skb_queue_head_init(&sch->skb_bad_txq);
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gnet_stats_basic_sync_init(&sch->bstats);
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lockdep_register_key(&sch->root_lock_key);
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spin_lock_init(&sch->q.lock);
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lockdep_set_class(&sch->q.lock, &sch->root_lock_key);
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if (ops->static_flags & TCQ_F_CPUSTATS) {
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sch->cpu_bstats =
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@ -1068,6 +1070,7 @@ static void __qdisc_destroy(struct Qdisc *qdisc)
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if (ops->destroy)
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ops->destroy(qdisc);
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lockdep_unregister_key(&qdisc->root_lock_key);
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module_put(ops->owner);
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netdev_put(dev, &qdisc->dev_tracker);
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@ -1039,13 +1039,6 @@ static void htb_work_func(struct work_struct *work)
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rcu_read_unlock();
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}
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static void htb_set_lockdep_class_child(struct Qdisc *q)
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{
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static struct lock_class_key child_key;
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lockdep_set_class(qdisc_lock(q), &child_key);
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}
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static int htb_offload(struct net_device *dev, struct tc_htb_qopt_offload *opt)
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{
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return dev->netdev_ops->ndo_setup_tc(dev, TC_SETUP_QDISC_HTB, opt);
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@ -1132,7 +1125,6 @@ static int htb_init(struct Qdisc *sch, struct nlattr *opt,
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return -ENOMEM;
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}
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htb_set_lockdep_class_child(qdisc);
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q->direct_qdiscs[ntx] = qdisc;
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qdisc->flags |= TCQ_F_ONETXQUEUE | TCQ_F_NOPARENT;
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}
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@ -1468,7 +1460,6 @@ static int htb_graft(struct Qdisc *sch, unsigned long arg, struct Qdisc *new,
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}
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if (q->offload) {
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htb_set_lockdep_class_child(new);
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/* One ref for cl->leaf.q, the other for dev_queue->qdisc. */
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qdisc_refcount_inc(new);
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old_q = htb_graft_helper(dev_queue, new);
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@ -1733,11 +1724,8 @@ static int htb_delete(struct Qdisc *sch, unsigned long arg,
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new_q = qdisc_create_dflt(dev_queue, &pfifo_qdisc_ops,
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cl->parent->common.classid,
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NULL);
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if (q->offload) {
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if (new_q)
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htb_set_lockdep_class_child(new_q);
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if (q->offload)
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htb_parent_to_leaf_offload(sch, dev_queue, new_q);
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}
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}
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sch_tree_lock(sch);
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@ -1947,13 +1935,9 @@ static int htb_change_class(struct Qdisc *sch, u32 classid,
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new_q = qdisc_create_dflt(dev_queue, &pfifo_qdisc_ops,
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classid, NULL);
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if (q->offload) {
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if (new_q) {
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htb_set_lockdep_class_child(new_q);
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/* One ref for cl->leaf.q, the other for
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* dev_queue->qdisc.
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*/
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/* One ref for cl->leaf.q, the other for dev_queue->qdisc. */
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if (new_q)
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qdisc_refcount_inc(new_q);
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}
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old_q = htb_graft_helper(dev_queue, new_q);
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/* No qdisc_put needed. */
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WARN_ON(!(old_q->flags & TCQ_F_BUILTIN));
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