=============================
WARNING: suspicious RCU usage
syzkaller #0 Not tainted
-----------------------------
net/sched/sch_api.c:304 suspicious rcu_dereference_protected() usage!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
8 locks held by ksoftirqd/0/14:
 #0: ffffffff8c11c760 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313
 #1: ffffffff8c11c760 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x5/0x30 include/linux/rcupdate.h:312
 #2: ffff88807729a4a0 (slock-AF_INET/1){+.-.}-{2:2}, at: tcp_v4_rcv+0x261d/0x2cb0 net/ipv4/tcp_ipv4.c:2137
 #3: ffffffff8c11c760 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313
 #4: ffffffff8c11c7c0 (rcu_read_lock_bh){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313
 #5: ffffffff8c11c7c0 (rcu_read_lock_bh){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313
 #6: ffff888024c63908 (&sch->q.lock){+.-.}-{2:2}, at: spin_lock include/linux/spinlock.h:364 [inline]
 #6: ffff888024c63908 (&sch->q.lock){+.-.}-{2:2}, at: __dev_xmit_skb net/core/dev.c:3911 [inline]
 #6: ffff888024c63908 (&sch->q.lock){+.-.}-{2:2}, at: __dev_queue_xmit+0xb8a/0x2ed0 net/core/dev.c:4253
 #7: ffffffff8c11c760 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x5/0x30 include/linux/rcupdate.h:312

stack backtrace:
CPU: 0 PID: 14 Comm: ksoftirqd/0 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0x168/0x230 lib/dump_stack.c:106
 qdisc_lookup+0xa6/0x650 net/sched/sch_api.c:304
 qdisc_tree_reduce_backlog+0x190/0x430 net/sched/sch_api.c:793
 pfifo_tail_enqueue+0x372/0x4d0 net/sched/sch_fifo.c:54
 qdisc_enqueue include/net/sch_generic.h:832 [inline]
 tbf_segment net/sched/sch_tbf.c:209 [inline]
 tbf_enqueue+0x430/0x6e0 net/sched/sch_tbf.c:240
 dev_qdisc_enqueue+0x48/0x210 net/core/dev.c:3852
 __dev_xmit_skb net/core/dev.c:3936 [inline]
 __dev_queue_xmit+0xd7e/0x2ed0 net/core/dev.c:4253
 neigh_hh_output include/net/neighbour.h:493 [inline]
 neigh_output include/net/neighbour.h:507 [inline]
 ip_finish_output2+0xc69/0x1080 net/ipv4/ip_output.c:228
 dst_output include/net/dst.h:452 [inline]
 ip_local_out net/ipv4/ip_output.c:126 [inline]
 __ip_queue_xmit+0x11c3/0x1c00 net/ipv4/ip_output.c:532
 __tcp_transmit_skb+0x1ced/0x31c0 net/ipv4/tcp_output.c:1404
 tcp_transmit_skb net/ipv4/tcp_output.c:1422 [inline]
 tcp_write_xmit+0x18d3/0x6580 net/ipv4/tcp_output.c:2718
 __tcp_push_pending_frames+0x92/0x250 net/ipv4/tcp_output.c:2903
 tcp_push_pending_frames include/net/tcp.h:1916 [inline]
 tcp_data_snd_check net/ipv4/tcp_input.c:5572 [inline]
 tcp_rcv_established+0xb44/0x1cb0 net/ipv4/tcp_input.c:6068
 tcp_v4_do_rcv+0x44b/0x9b0 net/ipv4/tcp_ipv4.c:1731
 tcp_v4_rcv+0x268f/0x2cb0 net/ipv4/tcp_ipv4.c:2143
 ip_protocol_deliver_rcu+0x3ad/0x770 net/ipv4/ip_input.c:204
 ip_local_deliver_finish+0x1d5/0x320 net/ipv4/ip_input.c:231
 NF_HOOK+0x2d6/0x360 include/linux/netfilter.h:302
 NF_HOOK+0x2d6/0x360 include/linux/netfilter.h:302
 __netif_receive_skb_one_core net/core/dev.c:5525 [inline]
 __netif_receive_skb+0xcc/0x290 net/core/dev.c:5639
 process_backlog+0x364/0x780 net/core/dev.c:6516
 __napi_poll+0xc0/0x430 net/core/dev.c:7075
 napi_poll net/core/dev.c:7142 [inline]
 net_rx_action+0x4a8/0x9c0 net/core/dev.c:7232
 handle_softirqs+0x328/0x820 kernel/softirq.c:576
 run_ksoftirqd+0x98/0xf0 kernel/softirq.c:943
 smpboot_thread_fn+0x4f6/0x970 kernel/smpboot.c:164
 kthread+0x436/0x520 kernel/kthread.c:334
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:287
 </TASK>