=============================
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
6 locks held by syz.3.64/4491:
 #0: ffff88802bd41d28 (&mm->mmap_lock){++++}-{3:3}, at: mmap_read_trylock include/linux/mmap_lock.h:136 [inline]
 #0: ffff88802bd41d28 (&mm->mmap_lock){++++}-{3:3}, at: do_user_addr_fault+0x2b9/0xc80 arch/x86/mm/fault.c:1296
 #1: ffffffff8c31f320 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x5/0x30 include/linux/rcupdate.h:312
 #2: ffffffff8c31f320 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x9/0x30 include/linux/rcupdate.h:313
 #3: ffff88807e031948 (dev->qdisc_running_key ?: &qdisc_running_key){+...}-{0:0}, at: net_tx_action+0x6d9/0x880 net/core/dev.c:5154
 #4: ffff88807e031908 (&sch->q.lock){+.-.}-{2:2}, at: spin_lock include/linux/spinlock.h:364 [inline]
 #4: ffff88807e031908 (&sch->q.lock){+.-.}-{2:2}, at: sch_direct_xmit+0x311/0x4b0 net/sched/sch_generic.c:357
 #5: ffffffff8c31f320 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x5/0x30 include/linux/rcupdate.h:312

stack backtrace:
CPU: 1 PID: 4491 Comm: syz.3.64 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
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:1125
 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:debug_lockdep_rcu_enabled+0x2/0x30 kernel/rcu/update.c:280
Code: 03 00 75 e8 48 c7 c7 a0 2f 2b 8a 48 c7 c6 20 54 2b 8a e8 11 0a f5 ff 0f 0b eb d1 00 00 cc cc 00 00 cc cc 00 00 cc cc 00 31 c0 <83> 3d db 19 cd 03 00 74 1d 83 3d 52 4c cd 03 00 74 14 65 48 8b 0d
RSP: 0000:ffffc90005e47bf8 EFLAGS: 00000246
RAX: 0000000000000000 RBX: 0000000000000014 RCX: 3eb77037a4828900
RDX: 0000000000000000 RSI: ffffffff8a2b43a0 RDI: ffffffff8a7a0700
RBP: ffffc90005e47cb0 R08: dffffc0000000000 R09: 1ffffffff203ac18
R10: dffffc0000000000 R11: fffffbfff203ac19 R12: dffffc0000000000
R13: 1ffff92000bc8f80 R14: ffff88802bd41c00 R15: 00007fa4a4783fc0
 rcu_read_lock include/linux/rcupdate.h:741 [inline]
 count_memcg_event_mm+0x87/0x370 include/linux/memcontrol.h:1078
 handle_mm_fault+0x134/0x4410 mm/memory.c:4863
 do_user_addr_fault+0x489/0xc80 arch/x86/mm/fault.c:1355
 handle_page_fault arch/x86/mm/fault.c:1443 [inline]
 exc_page_fault+0x60/0x100 arch/x86/mm/fault.c:1496
 asm_exc_page_fault+0x22/0x30 arch/x86/include/asm/idtentry.h:606
RIP: 0033:0x7fa4a4783fc0
Code: Unable to access opcode bytes at RIP 0x7fa4a4783f96.
RSP: 002b:00007fa4a29f3bd8 EFLAGS: 00010212
RAX: 00002000000001c0 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 00000000000003ff RSI: 00002000000001c0 RDI: 00007fa4a29f3c00
RBP: 00007fa4a29f3c00 R08: 0000000000000000 R09: 0000000000000000
R10: 00002000000001c0 R11: 0000000000000000 R12: cccccccccccccccd
R13: 00007fa4a4a14038 R14: 00007fa4a4a13fa0 R15: 00007ffc069e8f18
 </TASK>