=============================
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
5 locks held by syz.0.140/4745:
 #0: ffffc90000007be0 ((&d->timer)){+.-.}-{0:0}, at: lockdep_copy_map include/linux/lockdep.h:45 [inline]
 #0: ffffc90000007be0 ((&d->timer)){+.-.}-{0:0}, at: call_timer_fn+0xbb/0x530 kernel/time/timer.c:1441
 #1: ffffffff8c11c720 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x5/0x30 include/linux/rcupdate.h:312
 #2: ffffffff8c11c780 (rcu_read_lock_bh){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313
 #3: ffff88807e9df908 (&sch->q.lock){+.-.}-{2:2}, at: spin_lock include/linux/spinlock.h:364 [inline]
 #3: ffff88807e9df908 (&sch->q.lock){+.-.}-{2:2}, at: __dev_xmit_skb net/core/dev.c:3911 [inline]
 #3: ffff88807e9df908 (&sch->q.lock){+.-.}-{2:2}, at: __dev_queue_xmit+0xb8a/0x2ed0 net/core/dev.c:4253
 #4: ffffffff8c11c720 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x5/0x30 include/linux/rcupdate.h:312

stack backtrace:
CPU: 0 PID: 4745 Comm: syz.0.140 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
Call Trace:
 <IRQ>
 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:783
 cake_drop net/sched/sch_cake.c:1611 [inline]
 cake_enqueue+0x3889/0x7bb0 net/sched/sch_cake.c:1948
 qdisc_enqueue include/net/sch_generic.h:832 [inline]
 tbf_enqueue+0x224/0x6e0 net/sched/sch_tbf.c:243
 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
 tipc_l2_send_msg+0x30a/0x3c0 net/tipc/bearer.c:518
 tipc_bearer_xmit_skb+0x292/0x3c0 net/tipc/bearer.c:577
 tipc_disc_timeout+0x568/0x6b0 net/tipc/discover.c:338
 call_timer_fn+0x16c/0x530 kernel/time/timer.c:1451
 expire_timers kernel/time/timer.c:1496 [inline]
 __run_timers+0x525/0x7c0 kernel/time/timer.c:1767
 run_timer_softirq+0x63/0xf0 kernel/time/timer.c:1780
 handle_softirqs+0x328/0x820 kernel/softirq.c:576
 __do_softirq kernel/softirq.c:610 [inline]
 invoke_softirq kernel/softirq.c:450 [inline]
 __irq_exit_rcu+0x12f/0x220 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:lock_is_held_type+0x4c/0x190 kernel/locking/lockdep.c:5659
Code: 3d 6c 7f c7 03 00 0f 84 03 01 00 00 65 8b 05 5b 8f 60 76 85 c0 0f 85 f4 00 00 00 65 48 8b 1d cb 85 60 76 83 bb ec 0a 00 00 00 <0f> 85 df 00 00 00 41 89 f7 49 89 fe 48 c7 04 24 00 00 00 00 9c 8f
RSP: 0018:ffffc900032df438 EFLAGS: 00000246
RAX: 0000000000000000 RBX: ffff888078875940 RCX: ffff888078875940
RDX: 0000000000000000 RSI: 00000000ffffffff RDI: ffffffff8c11c7e0
RBP: 00000000ffffffff R08: ffffffff88810661 R09: 0000000000000000
R10: fffff5200065bf04 R11: 1ffff9200065bf00 R12: dffffc0000000000
R13: 0000000000000000 R14: dffffc0000000000 R15: 0000000000000000
 lock_is_held include/linux/lockdep.h:287 [inline]
 ___might_sleep+0xf2/0x600 kernel/sched/core.c:9624
 __mutex_lock_common+0xc1/0x2390 kernel/locking/mutex.c:573
 __mutex_lock kernel/locking/mutex.c:729 [inline]
 mutex_lock_nested+0x17/0x20 kernel/locking/mutex.c:743
 __unix_dgram_recvmsg+0x1e1/0xd50 net/unix/af_unix.c:2312
 ____sys_recvmsg+0x291/0x580 net/socket.c:-1
 ___sys_recvmsg+0x1af/0x4f0 net/socket.c:2707
 do_recvmmsg+0x344/0x7a0 net/socket.c:2801
 __sys_recvmmsg net/socket.c:2880 [inline]
 __do_sys_recvmmsg net/socket.c:2903 [inline]
 __se_sys_recvmmsg net/socket.c:2896 [inline]
 __x64_sys_recvmmsg+0x18d/0x240 net/socket.c:2896
 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: 0033:0x7f6d75b56749
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f6d73d9c038 EFLAGS: 00000246 ORIG_RAX: 000000000000012b
RAX: ffffffffffffffda RBX: 00007f6d75dad090 RCX: 00007f6d75b56749
RDX: 0000000000010106 RSI: 00002000000000c0 RDI: 0000000000000005
RBP: 00007f6d75bdaf91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000002 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f6d75dad128 R14: 00007f6d75dad090 R15: 00007ffc1de87a38
 </TASK>
----------------
Code disassembly (best guess):
   0:	3d 6c 7f c7 03       	cmp    $0x3c77f6c,%eax
   5:	00 0f                	add    %cl,(%rdi)
   7:	84 03                	test   %al,(%rbx)
   9:	01 00                	add    %eax,(%rax)
   b:	00 65 8b             	add    %ah,-0x75(%rbp)
   e:	05 5b 8f 60 76       	add    $0x76608f5b,%eax
  13:	85 c0                	test   %eax,%eax
  15:	0f 85 f4 00 00 00    	jne    0x10f
  1b:	65 48 8b 1d cb 85 60 	mov    %gs:0x766085cb(%rip),%rbx        # 0x766085ee
  22:	76
  23:	83 bb ec 0a 00 00 00 	cmpl   $0x0,0xaec(%rbx)
* 2a:	0f 85 df 00 00 00    	jne    0x10f <-- trapping instruction
  30:	41 89 f7             	mov    %esi,%r15d
  33:	49 89 fe             	mov    %rdi,%r14
  36:	48 c7 04 24 00 00 00 	movq   $0x0,(%rsp)
  3d:	00
  3e:	9c                   	pushf
  3f:	8f                   	.byte 0x8f