[v6.6] INFO: rcu detected stall in sys_sendmmsg

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: d1cfde2d5d15 Linux 6.6.143
git tree: linux-6.6.y
console output: https://syzkaller.appspot.com/x/log.txt?x=17ec19fa580000
kernel config: https://syzkaller.appspot.com/x/.config?x=441765643cbfb8d
dashboard link: https://syzkaller.appspot.com/bug?extid=199dd69d4d7ea23a46c0
compiler: Debian clang version 22.1.6 (++20260514074242+fc4aad7b5db3-1~exp1~20260514074407.73), Debian LLD 22.1.6

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/4e37bcd98892/disk-d1cfde2d.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/64b2ed856057/vmlinux-d1cfde2d.xz
kernel image: https://storage.googleapis.com/syzbot-assets/0a6949479d66/bzImage-d1cfde2d.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+199dd6...@syzkaller.appspotmail.com

rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu: Tasks blocked on level-0 rcu_node (CPUs 0-1): P14285/1:b..l
rcu: (detected by 1, t=10502 jiffies, g=78557, q=401 ncpus=2)
task:syz.1.2571 state:R running task stack:24488 pid:14285 ppid:13916 flags:0x00004006
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5381 [inline]
__schedule+0x15ae/0x4660 kernel/sched/core.c:6700
preempt_schedule_irq+0xbf/0x150 kernel/sched/core.c:7010
irqentry_exit+0x67/0x70 kernel/entry/common.c:438
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:687
RIP: 0010:percpu_counter_add_batch+0x25c/0x2a0 lib/percpu_counter.c:103
Code: 4d 85 ff 75 16 e8 b4 31 2c fd eb 15 e8 ad 31 2c fd e8 98 24 29 06 4d 85 ff 74 ea e8 9e 31 2c fd fb 48 c7 44 24 40 0e 36 e0 45 <4b> c7 04 26 00 00 00 00 43 c7 44 26 08 00 00 00 00 65 48 8b 04 25
RSP: 0018:ffffc900050a7300 EFLAGS: 00000246
RAX: ffffffff845ab3b2 RBX: 0000000000000000 RCX: 0000000000080000
RDX: ffffc90013d61000 RSI: 000000000007ffff RDI: 0000000000080000
RBP: ffffc900050a73f0 R08: ffffffff8e8ae5ef R09: 1ffffffff1d15cbd
R10: dffffc0000000000 R11: fffffbfff1d15cbe R12: dffffc0000000000
R13: 000000000000000c R14: 1ffff92000a14e68 R15: 0000000000000200
dst_alloc+0x12e/0x170 net/core/dst.c:94
rt_dst_alloc net/ipv4/route.c:1662 [inline]
__mkroute_output net/ipv4/route.c:2626 [inline]
ip_route_output_key_hash_rcu+0x14a2/0x2350 net/ipv4/route.c:2848
ip_route_output_key_hash+0x1f3/0x330 net/ipv4/route.c:2677
__ip_route_output_key include/net/route.h:134 [inline]
ip_route_output_flow+0x2a/0x150 net/ipv4/route.c:2905
udp_sendmsg+0x160e/0x23d0 net/ipv4/udp.c:1236
sock_sendmsg_nosec net/socket.c:730 [inline]
__sock_sendmsg net/socket.c:745 [inline]
____sys_sendmsg+0x61b/0x980 net/socket.c:2595
___sys_sendmsg+0x2a6/0x360 net/socket.c:2649
__sys_sendmmsg+0x2ca/0x510 net/socket.c:2735
__do_sys_sendmmsg net/socket.c:2764 [inline]
__se_sys_sendmmsg net/socket.c:2761 [inline]
__x64_sys_sendmmsg+0xa0/0xb0 net/socket.c:2761
do_syscall_x64 arch/x86/entry/common.c:46 [inline]
do_syscall_64+0x55/0xb0 arch/x86/entry/common.c:76
entry_SYSCALL_64_after_hwframe+0x68/0xd2
RIP: 0033:0x7f9e8a99ce59
RSP: 002b:00007f9e8b834028 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
RAX: ffffffffffffffda RBX: 00007f9e8ac15fa0 RCX: 00007f9e8a99ce59
RDX: 000000000800001d RSI: 0000200000007fc0 RDI: 0000000000000003
RBP: 00007f9e8aa32e6f R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f9e8ac16038 R14: 00007f9e8ac15fa0 R15: 00007ffcbe2ae1d8
</TASK>
rcu: rcu_preempt kthread starved for 10543 jiffies! g78557 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=1
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:27304 pid:17 ppid:2 flags:0x00004000
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5381 [inline]
__schedule+0x15ae/0x4660 kernel/sched/core.c:6700
schedule+0xbd/0x170 kernel/sched/core.c:6774
schedule_timeout+0x188/0x2d0 kernel/time/timer.c:2168
rcu_gp_fqs_loop+0x30d/0x1590 kernel/rcu/tree.c:1667
rcu_gp_kthread+0x9d/0x3b0 kernel/rcu/tree.c:1866
kthread+0x2fa/0x390 kernel/kthread.c:388
ret_from_fork+0x48/0x80 arch/x86/kernel/process.c:152
ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:293
</TASK>
rcu: Stack dump where RCU GP kthread last ran:
CPU: 1 PID: 0 Comm: swapper/1 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
RIP: 0010:pv_native_safe_halt+0xf/0x10 arch/x86/kernel/paravirt.c:148
Code: dc 23 02 c3 cc cc cc cc cc cc cc f3 0f 1e fa 0f 0b 66 2e 0f 1f 84 00 00 00 00 00 f3 0f 1e fa 66 90 0f 00 2d 43 67 44 00 fb f4 <c3> 66 0f 1f 00 55 41 57 41 56 41 54 53 50 8b 2f eb 2c 89 cb 80 f9
RSP: 0018:ffffc90000187de0 EFLAGS: 000002c6
RAX: c8891e3ee90bf900 RBX: ffffffff81628b01 RCX: c8891e3ee90bf900
RDX: 0000000000000001 RSI: ffffffff8acacb60 RDI: ffffffff8b1c7be0
RBP: ffffc90000187f20 R08: ffff8880b8f36bab R09: 1ffff110171e6d75
R10: dffffc0000000000 R11: ffffed10171e6d76 R12: 1ffff92000030fc8
R13: dffffc0000000000 R14: 1ffff110036cd780 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff8880b8f00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000000058 CR3: 000000002d728000 CR4: 00000000003506e0
Call Trace:
<TASK>
arch_safe_halt arch/x86/include/asm/paravirt.h:108 [inline]
default_idle+0x13/0x20 arch/x86/kernel/process.c:753
default_idle_call+0x6c/0xa0 kernel/sched/idle.c:97
cpuidle_idle_call kernel/sched/idle.c:178 [inline]
do_idle+0x221/0x5c0 kernel/sched/idle.c:302
cpu_startup_entry+0x43/0x60 kernel/sched/idle.c:401
start_secondary+0xee/0xf0 arch/x86/kernel/smpboot.c:323
secondary_startup_64_no_verify+0x179/0x17b
</TASK>


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