=============================
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.0.534/7924:
 #0: ffffffff8c31eaa0 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313
 #1: ffff88801abc6148 (dev->qdisc_running_key ?: &qdisc_running_key){+...}-{0:0}, at: net_tx_action+0x6d9/0x880 net/core/dev.c:5154
 #2: ffff88801abc6108 (&sch->q.lock){+.-.}-{2:2}, at: spin_lock include/linux/spinlock.h:364 [inline]
 #2: ffff88801abc6108 (&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: 7924 Comm: syz.0.534 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:is_idle_task include/linux/sched.h:1868 [inline]
RIP: 0010:___might_sleep+0x199/0x610 kernel/sched/core.c:9627
Code: 8f 84 24 a0 00 00 00 f7 84 24 a0 00 00 00 00 02 00 00 42 c6 44 23 0c f8 74 43 4c 8d 7a 2c 4c 89 f8 48 c1 e8 03 42 0f b6 04 20 <84> c0 0f 85 88 03 00 00 41 f6 07 02 75 25 4c 8d ba 80 0a 00 00 4c
RSP: 0018:ffffc90003a8fb40 EFLAGS: 00000a07
RAX: 0000000000000000 RBX: 1ffff92000751f70 RCX: e56b5f9686e9c400
RDX: ffff888028301dc0 RSI: ffffffff8a2b3a20 RDI: ffffffff8a79f980
RBP: ffffc90003a8fc60 R08: ffffc90003a8f6e7 R09: 1ffff92000751edc
R10: dffffc0000000000 R11: fffff52000751edd R12: dffffc0000000000
R13: ffff8880283021fc R14: 1ffff1100506043f R15: ffff888028301dec
 __might_fault+0x6b/0x110 mm/memory.c:5354
 do_recvmmsg+0x3ab/0x850 net/socket.c:2806
 __sys_recvmmsg net/socket.c:2879 [inline]
 __do_sys_recvmmsg net/socket.c:2902 [inline]
 __se_sys_recvmmsg net/socket.c:2895 [inline]
 __x64_sys_recvmmsg+0x195/0x250 net/socket.c:2895
 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: 0033:0x7f54f5588819
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:00007f54f37c1028 EFLAGS: 00000246 ORIG_RAX: 000000000000012b
RAX: ffffffffffffffda RBX: 00007f54f5802090 RCX: 00007f54f5588819
RDX: 0000000000010106 RSI: 00002000000000c0 RDI: 0000000000000004
RBP: 00007f54f561ec91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000002 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f54f5802128 R14: 00007f54f5802090 R15: 00007ffd5abd0c78
 </TASK>
----------------
Code disassembly (best guess):
   0:	8f 84 24 a0 00 00 00 	pop    0xa0(%rsp)
   7:	f7 84 24 a0 00 00 00 	testl  $0x200,0xa0(%rsp)
   e:	00 02 00 00
  12:	42 c6 44 23 0c f8    	movb   $0xf8,0xc(%rbx,%r12,1)
  18:	74 43                	je     0x5d
  1a:	4c 8d 7a 2c          	lea    0x2c(%rdx),%r15
  1e:	4c 89 f8             	mov    %r15,%rax
  21:	48 c1 e8 03          	shr    $0x3,%rax
  25:	42 0f b6 04 20       	movzbl (%rax,%r12,1),%eax
* 2a:	84 c0                	test   %al,%al <-- trapping instruction
  2c:	0f 85 88 03 00 00    	jne    0x3ba
  32:	41 f6 07 02          	testb  $0x2,(%r15)
  36:	75 25                	jne    0x5d
  38:	4c 8d ba 80 0a 00 00 	lea    0xa80(%rdx),%r15
  3f:	4c                   	rex.WR