=============================
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
4 locks held by syz.7.380/6798:
 #0: ffffffff8c31eaa0 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313
 #1: ffff8880254e1948 (dev->qdisc_running_key ?: &qdisc_running_key){+...}-{0:0}, at: net_tx_action+0x6d9/0x880 net/core/dev.c:5154
 #2: ffff8880254e1908 (&sch->q.lock){+.-.}-{2:2}, at: spin_lock include/linux/spinlock.h:364 [inline]
 #2: ffff8880254e1908 (&sch->q.lock){+.-.}-{2:2}, at: sch_direct_xmit+0x311/0x4b0 net/sched/sch_generic.c:357
 #3: ffffffff8c31eaa0 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x5/0x30 include/linux/rcupdate.h:312

stack backtrace:
CPU: 1 PID: 6798 Comm: syz.7.380 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
Call Trace:
 <IRQ>
 dump_stack_lvl+0x188/0x250 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:783
 cake_dequeue+0x1bd6/0x4b10 net/sched/sch_cake.c:2205
 qdisc_peek_dequeued+0x6e/0x1f0 include/net/sch_generic.h:1115
 tbf_dequeue+0x7d/0xce0 net/sched/sch_tbf.c:265
 dequeue_skb net/sched/sch_generic.c:295 [inline]
 qdisc_restart net/sched/sch_generic.c:400 [inline]
 __qdisc_run+0x236/0x1490 net/sched/sch_generic.c:418
 qdisc_run+0x103/0x2f0 include/net/pkt_sched.h:131
 net_tx_action+0x6d9/0x880 net/core/dev.c:5154
 handle_softirqs+0x339/0x830 kernel/softirq.c:576
 __do_softirq kernel/softirq.c:610 [inline]
 invoke_softirq kernel/softirq.c:450 [inline]
 __irq_exit_rcu+0x13b/0x230 kernel/softirq.c:659
 irq_exit_rcu+0x5/0x20 kernel/softirq.c:671
 instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1108 [inline]
 sysvec_apic_timer_interrupt+0xa0/0xc0 arch/x86/kernel/apic/apic.c:1108
 </IRQ>
 <TASK>
 asm_sysvec_apic_timer_interrupt+0x16/0x20 arch/x86/include/asm/idtentry.h:676
RIP: 0010:preempt_latency_start kernel/sched/core.c:5492 [inline]
RIP: 0010:preempt_count_add+0xbf/0x190 kernel/sched/core.c:5515
Code: e8 56 d0 0b 00 65 4c 8b 35 8e 0e b0 7e 49 81 c6 e8 14 00 00 4c 89 f0 48 c1 e8 03 42 80 3c 38 00 74 08 4c 89 f7 e8 a1 cb 6b 00 <49> 89 1e 5b 41 5e 41 5f c3 89 fb e8 11 54 8e 02 89 df 85 c0 74 9b
RSP: 0018:ffffc9000114f750 EFLAGS: 00000246
RAX: 1ffff1100f021655 RBX: 0000000000000000 RCX: ffffffff962daf00
RDX: 0000000000000002 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc9000114f930 R08: ffffffff8d89e36f R09: 1ffffffff1b13c6d
R10: dffffc0000000000 R11: fffffbfff1b13c6e R12: 1ffff92000229f0c
R13: ffff88807dc46e78 R14: ffff88807810b2a8 R15: dffffc0000000000
 __raw_spin_lock include/linux/spinlock_api_smp.h:141 [inline]
 _raw_spin_lock+0xe/0x40 kernel/locking/spinlock.c:154
 spin_lock include/linux/spinlock.h:364 [inline]
 unix_dgram_sendmsg+0xb1e/0x18a0 net/unix/af_unix.c:1847
 sock_sendmsg_nosec net/socket.c:706 [inline]
 __sock_sendmsg net/socket.c:718 [inline]
 ____sys_sendmsg+0x5b7/0x8f0 net/socket.c:2445
 ___sys_sendmsg+0x236/0x2e0 net/socket.c:2499
 __sys_sendmmsg+0x2ba/0x500 net/socket.c:2585
 __do_sys_sendmmsg net/socket.c:2614 [inline]
 __se_sys_sendmmsg net/socket.c:2611 [inline]
 __x64_sys_sendmmsg+0x9c/0xb0 net/socket.c:2611
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x66/0xd0
RIP: 0033:0x7f4273b00819
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f4271d5a028 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
RAX: ffffffffffffffda RBX: 00007f4273d79fa0 RCX: 00007f4273b00819
RDX: 0000000000000651 RSI: 0000200000000000 RDI: 0000000000000004
RBP: 00007f4273b96c91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f4273d7a038 R14: 00007f4273d79fa0 R15: 00007fff93da3458
 </TASK>
----------------
Code disassembly (best guess):
   0:	e8 56 d0 0b 00       	call   0xbd05b
   5:	65 4c 8b 35 8e 0e b0 	mov    %gs:0x7eb00e8e(%rip),%r14        # 0x7eb00e9b
   c:	7e
   d:	49 81 c6 e8 14 00 00 	add    $0x14e8,%r14
  14:	4c 89 f0             	mov    %r14,%rax
  17:	48 c1 e8 03          	shr    $0x3,%rax
  1b:	42 80 3c 38 00       	cmpb   $0x0,(%rax,%r15,1)
  20:	74 08                	je     0x2a
  22:	4c 89 f7             	mov    %r14,%rdi
  25:	e8 a1 cb 6b 00       	call   0x6bcbcb
* 2a:	49 89 1e             	mov    %rbx,(%r14) <-- trapping instruction
  2d:	5b                   	pop    %rbx
  2e:	41 5e                	pop    %r14
  30:	41 5f                	pop    %r15
  32:	c3                   	ret
  33:	89 fb                	mov    %edi,%ebx
  35:	e8 11 54 8e 02       	call   0x28e544b
  3a:	89 df                	mov    %ebx,%edi
  3c:	85 c0                	test   %eax,%eax
  3e:	74 9b                	je     0xffffffdb