[PATCH AUTOSEL 6.8 16/30] net/sched: fix false lockdep warning on qdisc root lock

Sasha Levin
2024-05-27T14:14:49Z

From: Davide Caratti <dcaratti@redhat.com>

[ Upstream commit af0cb3fa3f9ed258d14abab0152e28a0f9593084 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/net/sch_generic.h | 1 +
net/sched/sch_generic.c | 3 +++
net/sched/sch_htb.c | 22 +++-------------------
3 files changed, 7 insertions(+), 19 deletions(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 41ca14e81d55f..0014b9ee5e381 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -128,6 +128,7 @@ 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 a498b5d7c5d60..7fd0167aa7ee1 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 =
@@ -1067,6 +1069,7 @@ 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(dev, &qdisc->dev_tracker);

diff --git a/net/sched/sch_htb.c b/net/sched/sch_htb.c
index 7349233eaa9b6..19a578c31a91f 100644
--- a/net/sched/sch_htb.c
+++ b/net/sched/sch_htb.c
@@ -1039,13 +1039,6 @@ static void htb_work_func(struct work_struct *work)
rcu_read_unlock();
}

-static void htb_set_lockdep_class_child(struct Qdisc *q)
-{
- static struct lock_class_key child_key;
-
- lockdep_set_class(qdisc_lock(q), &child_key);
-}
-
static int htb_offload(struct net_device *dev, struct tc_htb_qopt_offload *opt)
{
return dev->netdev_ops->ndo_setup_tc(dev, TC_SETUP_QDISC_HTB, opt);
@@ -1132,7 +1125,6 @@ static int htb_init(struct Qdisc *sch, struct nlattr *opt,
return -ENOMEM;
}

- htb_set_lockdep_class_child(qdisc);
q->direct_qdiscs[ntx] = qdisc;
qdisc->flags |= TCQ_F_ONETXQUEUE | TCQ_F_NOPARENT;
}
@@ -1468,7 +1460,6 @@ static int htb_graft(struct Qdisc *sch, unsigned long arg, struct Qdisc *new,
}

if (q->offload) {
- htb_set_lockdep_class_child(new);
/* One ref for cl->leaf.q, the other for dev_queue->qdisc. */
qdisc_refcount_inc(new);
old_q = htb_graft_helper(dev_queue, new);
@@ -1733,11 +1724,8 @@ static int htb_delete(struct Qdisc *sch, unsigned long arg,
new_q = qdisc_create_dflt(dev_queue, &pfifo_qdisc_ops,
cl->parent->common.classid,
NULL);
- if (q->offload) {
- if (new_q)
- htb_set_lockdep_class_child(new_q);
+ if (q->offload)
htb_parent_to_leaf_offload(sch, dev_queue, new_q);
- }
}

sch_tree_lock(sch);
@@ -1947,13 +1935,9 @@ static int htb_change_class(struct Qdisc *sch, u32 classid,
new_q = qdisc_create_dflt(dev_queue, &pfifo_qdisc_ops,
classid, NULL);
if (q->offload) {
- if (new_q) {
- htb_set_lockdep_class_child(new_q);
- /* One ref for cl->leaf.q, the other for
- * dev_queue->qdisc.
- */
+ /* One ref for cl->leaf.q, the other for dev_queue->qdisc. */
+ if (new_q)
qdisc_refcount_inc(new_q);
- }
old_q = htb_graft_helper(dev_queue, new_q);
/* No qdisc_put needed. */
WARN_ON(!(old_q->flags & TCQ_F_BUILTIN));
--
2.43.0