[v6.1] INFO: rcu detected stall in sys_sendmmsg (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: f6e38ae624cf Linux 6.1.158
git tree: linux-6.1.y
console output: https://syzkaller.appspot.com/x/log.txt?x=16277e12580000
kernel config: https://syzkaller.appspot.com/x/.config?x=7eb38bd5021fec61
dashboard link: https://syzkaller.appspot.com/bug?extid=cedf6a23be74bf99204c
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/2b7003e2661d/disk-f6e38ae6.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/0696cea8d016/vmlinux-f6e38ae6.xz
kernel image: https://storage.googleapis.com/syzbot-assets/6b7a89e636d7/bzImage-f6e38ae6.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+cedf6a...@syzkaller.appspotmail.com

rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu: Tasks blocked on level-0 rcu_node (CPUs 0-1): P9866/3:b..l
(detected by 1, t=10502 jiffies, g=52997, q=275 ncpus=2)
task:syz.9.1175 state:R running task stack:23264 pid:9866 ppid:8004 flags:0x00004004
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5244 [inline]
__schedule+0x10ec/0x40b0 kernel/sched/core.c:6561
preempt_schedule_irq+0xb1/0x150 kernel/sched/core.c:6873
irqentry_exit+0x63/0x70 kernel/entry/common.c:439
asm_sysvec_apic_timer_interrupt+0x16/0x20 arch/x86/include/asm/idtentry.h:691
RIP: 0010:skb_set_nfct include/linux/skbuff.h:4635 [inline]
RIP: 0010:nf_ct_set include/net/netfilter/nf_conntrack.h:374 [inline]
RIP: 0010:resolve_normal_ct net/netfilter/nf_conntrack_core.c:1901 [inline]
RIP: 0010:nf_conntrack_in+0x8d8/0x1770 net/netfilter/nf_conntrack_core.c:2031
Code: 6c 24 48 0f 85 d3 05 00 00 41 c1 ef 13 0f b6 44 24 60 44 08 f8 0f b6 c0 c1 e0 13 41 81 e4 ff ff f7 ff 41 09 c4 48 8b 44 24 40 <44> 89 20 48 8b 84 24 80 00 00 00 80 3c 18 00 49 89 dc 4c 8b 74 24
RSP: 0018:ffffc9000c2e7340 EFLAGS: 00000206
RAX: ffff888059686bc2 RBX: dffffc0000000000 RCX: 0000000000000002
RDX: ffff888027549dc0 RSI: 0000000000080000 RDI: 0000000000000000
RBP: ffffc9000c2e7600 R08: dffffc0000000000 R09: ffffed100fa1b419
R10: ffffed100fa1b419 R11: 1ffff1100fa1b418 R12: 0000000000080000
R13: ffff888059686ba8 R14: 1ffff1100b2d0d78 R15: 0000000000000001
nf_hook_entry_hookfn include/linux/netfilter.h:142 [inline]
nf_hook_slow+0xb9/0x200 net/netfilter/core.c:614
nf_hook+0x211/0x370 include/linux/netfilter.h:257
__ip_local_out+0x49c/0x5d0 net/ipv4/ip_output.c:115
ip_local_out net/ipv4/ip_output.c:124 [inline]
__ip_queue_xmit+0xf66/0x19b0 net/ipv4/ip_output.c:532
l2tp_ip_sendmsg+0x613/0x1580 net/l2tp/l2tp_ip.c:500
sock_sendmsg_nosec net/socket.c:718 [inline]
__sock_sendmsg net/socket.c:730 [inline]
____sys_sendmsg+0x59b/0x970 net/socket.c:2519
___sys_sendmsg+0x21c/0x290 net/socket.c:2573
__sys_sendmmsg+0x272/0x490 net/socket.c:2659
__do_sys_sendmmsg net/socket.c:2688 [inline]
__se_sys_sendmmsg net/socket.c:2685 [inline]
__x64_sys_sendmmsg+0x9c/0xb0 net/socket.c:2685
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:81
entry_SYSCALL_64_after_hwframe+0x68/0xd2
RIP: 0033:0x7fd5b518f749
RSP: 002b:00007fd5b60d5038 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
RAX: ffffffffffffffda RBX: 00007fd5b53e5fa0 RCX: 00007fd5b518f749
RDX: 00000000040000cf RSI: 0000200000000900 RDI: 0000000000000003
RBP: 00007fd5b5213f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fd5b53e6038 R14: 00007fd5b53e5fa0 R15: 00007ffedc84b048
</TASK>
rcu: rcu_preempt kthread starved for 9741 jiffies! g52997 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=0
rcu: Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
rcu: RCU grace-period kthread stack dump:
task:rcu_preempt state:R running task stack:27200 pid:16 ppid:2 flags:0x00004000
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5244 [inline]
__schedule+0x10ec/0x40b0 kernel/sched/core.c:6561
schedule+0xb9/0x180 kernel/sched/core.c:6637
schedule_timeout+0x15c/0x280 kernel/time/timer.c:2167
rcu_gp_fqs_loop+0x2f2/0x1310 kernel/rcu/tree.c:1706
rcu_gp_kthread+0x95/0x380 kernel/rcu/tree.c:1905
kthread+0x29d/0x330 kernel/kthread.c:376
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
</TASK>
rcu: Stack dump where RCU GP kthread last ran:
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0 skipped: idling at native_safe_halt arch/x86/include/asm/irqflags.h:51 [inline]
NMI backtrace for cpu 0 skipped: idling at arch_safe_halt arch/x86/include/asm/irqflags.h:89 [inline]
NMI backtrace for cpu 0 skipped: idling at default_idle+0xb/0x10 arch/x86/kernel/process.c:741


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