Subject: [syzbot ci] Re: net_sched: fix skb memory leak in deferred qdisc drops

From: syzbot ci <[email-masked]>
Date: Tue, 07 Apr 2026 23:45:29 -0700

syzbot ci has tested the following series

[v1] net_sched: fix skb memory leak in deferred qdisc drops
https://lore.kernel.org/all/[email-masked]
* [PATCH net] net_sched: fix skb memory leak in deferred qdisc drops

and found the following issue:
WARNING: suspicious RCU usage in cake_dequeue

Full report is available here:
https://ci.syzbot.org/series/90acbb62-3e93-4bb7-af7a-51d555f5956b

***

WARNING: suspicious RCU usage in cake_dequeue

tree:      net
URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/netdev/net.git
base:      a9b8b18364fffce4c451e6f6fd218fa4ab646705
arch:      amd64
compiler:  Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
config:    https://ci.syzbot.org/builds/a5e0fad0-c9c8-4102-b2af-b4d1c7947ee3/config
syz repro: https://ci.syzbot.org/findings/b1dd239a-f59f-414f-8bde-578258899e94/syz_repro

=============================
WARNING: suspicious RCU usage
syzkaller #0 Not tainted
-----------------------------
./include/net/sch_generic.h:581 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
2 locks held by syz.0.51/6016:
 #0: ffffffff8e75e640 (rcu_read_lock_bh){....}-{1:3}, at: local_bh_disable include/linux/bottom_half.h:20 [inline]
 #0: ffffffff8e75e640 (rcu_read_lock_bh){....}-{1:3}, at: rcu_read_lock_bh include/linux/rcupdate.h:903 [inline]
 #0: ffffffff8e75e640 (rcu_read_lock_bh){....}-{1:3}, at: __dev_queue_xmit+0x277/0x3890 net/core/dev.c:4778
 #1: ffff8881ad9e8170 (&sch->root_lock_key#31){+...}-{3:3}, at: spin_lock include/linux/spinlock.h:341 [inline]
 #1: ffff8881ad9e8170 (&sch->root_lock_key#31){+...}-{3:3}, at: __dev_xmit_skb net/core/dev.c:4241 [inline]
 #1: ffff8881ad9e8170 (&sch->root_lock_key#31){+...}-{3:3}, at: __dev_queue_xmit+0xbf2/0x3890 net/core/dev.c:4819

stack backtrace:
CPU: 1 UID: 0 PID: 6016 Comm: syz.0.51 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 lockdep_rcu_suspicious+0x13f/0x1d0 kernel/locking/lockdep.c:6876
 qdisc_root_sleeping include/net/sch_generic.h:581 [inline]
 qdisc_dequeue_drop include/net/sch_generic.h:1173 [inline]
 cake_dequeue+0x23a7/0x4e50 net/sched/sch_cake.c:2240
 dequeue_skb net/sched/sch_generic.c:297 [inline]
 qdisc_restart net/sched/sch_generic.c:402 [inline]
 __qdisc_run+0x26c/0x15d0 net/sched/sch_generic.c:420
 qdisc_run include/net/pkt_sched.h:120 [inline]
 __dev_xmit_skb net/core/dev.c:4286 [inline]
 __dev_queue_xmit+0x1bad/0x3890 net/core/dev.c:4819
 packet_snd net/packet/af_packet.c:3076 [inline]
 packet_sendmsg+0x3eb6/0x50f0 net/packet/af_packet.c:3108
 sock_sendmsg_nosec net/socket.c:727 [inline]
 __sock_sendmsg net/socket.c:742 [inline]
 __sys_sendto+0x672/0x710 net/socket.c:2206
 __do_sys_sendto net/socket.c:2213 [inline]
 __se_sys_sendto net/socket.c:2209 [inline]
 __x64_sys_sendto+0xde/0x100 net/socket.c:2209
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f077b39c819
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:00007f077c28c028 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007f077b615fa0 RCX: 00007f077b39c819
RDX: 000000000000fc13 RSI: 0000200000000800 RDI: 0000000000000003
RBP: 00007f077b432c91 R08: 0000000000000000 R09: fffffffffffffef0
R10: 0000000000000880 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f077b616038 R14: 00007f077b615fa0 R15: 00007fffceb20038
 </TASK>

***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
  Tested-by: [email-masked]

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