[syzbot] [net?] INFO: rcu detected stall in kernfs_fop_write_iter (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: f8f97927abf7 Add linux-next specific files for 20260105
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=171a9e9a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=a0672dd8d69c3235
dashboard link: https://syzkaller.appspot.com/bug?extid=10b515098afdcfc455c7
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14315efc580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/1837bbc8e23e/disk-f8f97927.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/07390717f7e4/vmlinux-f8f97927.xz
kernel image: https://storage.googleapis.com/syzbot-assets/8f4a72ec80dc/bzImage-f8f97927.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+10b515...@syzkaller.appspotmail.com

rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
rcu: Tasks blocked on level-0 rcu_node (CPUs 0-1): P5196/1:b..l
rcu: (detected by 1, t=10503 jiffies, g=30937, q=540 ncpus=2)
task:udevd state:R running task stack:25016 pid:5196 tgid:5196 ppid:1 task_flags:0x400140 flags:0x00080000
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5258 [inline]
__schedule+0x14ed/0x5040 kernel/sched/core.c:6866
preempt_schedule_irq+0x4d/0xa0 kernel/sched/core.c:7193
irqentry_exit+0x5e3/0x670 kernel/entry/common.c:216
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
RIP: 0010:__orc_find arch/x86/kernel/unwind_orc.c:103 [inline]
RIP: 0010:orc_find arch/x86/kernel/unwind_orc.c:238 [inline]
RIP: 0010:unwind_next_frame+0x530/0x23d0 arch/x86/kernel/unwind_orc.c:510
Code: b9 00 00 00 00 00 fc ff df 0f b6 04 08 84 c0 75 27 49 63 07 4c 01 f8 49 8d 4f 04 4c 39 e0 48 0f 46 e9 49 8d 47 fc 48 0f 47 d8 <4d> 0f 46 ef 48 39 dd 76 a2 e9 76 fd ff ff 44 89 f9 80 e1 07 80 c1
RSP: 0018:ffffc90002fd7398 EFLAGS: 00000202
RAX: ffffffff8fe5d87c RBX: ffffffff8fe5d87c RCX: ffffffff8fe5d884
RDX: ffffffff8fe5d87c RSI: ffffffff9067beb4 RDI: ffffffff8be25080
RBP: ffffffff8fe5d87c R08: 0000000000000003 R09: ffffffff8e33f2e0
R10: ffffc90002fd74b8 R11: ffffffff81aceae0 R12: ffffffff82390804
R13: ffffffff8fe5d87c R14: ffffc90002fd7468 R15: ffffffff8fe5d880
arch_stack_walk+0x11c/0x150 arch/x86/kernel/stacktrace.c:25
stack_trace_save+0x9c/0xe0 kernel/stacktrace.c:122
kasan_save_stack mm/kasan/common.c:57 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
unpoison_slab_object mm/kasan/common.c:340 [inline]
__kasan_slab_alloc+0x6c/0x80 mm/kasan/common.c:366
kasan_slab_alloc include/linux/kasan.h:253 [inline]
slab_post_alloc_hook mm/slub.c:4953 [inline]
slab_alloc_node mm/slub.c:5267 [inline]
kmem_cache_alloc_noprof+0x37d/0x710 mm/slub.c:5274
skb_clone+0x212/0x3a0 net/core/skbuff.c:2087
do_one_broadcast net/netlink/af_netlink.c:1455 [inline]
netlink_broadcast_filtered+0x6ae/0x1000 net/netlink/af_netlink.c:1533
netlink_broadcast+0x37/0x50 net/netlink/af_netlink.c:1557
uevent_net_broadcast_untagged lib/kobject_uevent.c:331 [inline]
kobject_uevent_net_broadcast+0x378/0x560 lib/kobject_uevent.c:410
kobject_uevent_env+0x55c/0x9f0 lib/kobject_uevent.c:608
kobject_synth_uevent+0x527/0xb00 lib/kobject_uevent.c:207
uevent_store+0x26/0x70 drivers/base/core.c:2773
kernfs_fop_write_iter+0x3af/0x540 fs/kernfs/file.c:352
new_sync_write fs/read_write.c:593 [inline]
vfs_write+0x5c9/0xb30 fs/read_write.c:686
ksys_write+0x145/0x250 fs/read_write.c:738
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xec/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f133aca7407
RSP: 002b:00007ffe24984120 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f133b437880 RCX: 00007f133aca7407
RDX: 0000000000000007 RSI: 0000564480d619e0 RDI: 000000000000000c
RBP: 0000564480d619e0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000007
R13: 0000564480c80730 R14: 00007f133adefea0 R15: 00007ffe24984470
</TASK>
rcu: rcu_preempt kthread starved for 10524 jiffies! g30937 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:28248 pid:16 tgid:16 ppid:2 task_flags:0x208040 flags:0x00080000
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5258 [inline]
__schedule+0x14ed/0x5040 kernel/sched/core.c:6866
__schedule_loop kernel/sched/core.c:6948 [inline]
schedule+0x165/0x360 kernel/sched/core.c:6963
schedule_timeout+0x12b/0x270 kernel/time/sleep_timeout.c:99
rcu_gp_fqs_loop+0x301/0x11b0 kernel/rcu/tree.c:2095
rcu_gp_kthread+0x9b/0x2d0 kernel/rcu/tree.c:2297
kthread+0x389/0x480 kernel/kthread.c:467
ret_from_fork+0x510/0xa50 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246
</TASK>
rcu: Stack dump where RCU GP kthread last ran:
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0
CPU: 0 UID: 0 PID: 11808 Comm: syz.3.2836 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:cpu_max_bits_warn include/linux/cpumask.h:138 [inline]
RIP: 0010:cpumask_check include/linux/cpumask.h:145 [inline]
RIP: 0010:cpumask_test_cpu include/linux/cpumask.h:649 [inline]
RIP: 0010:cpu_online include/linux/cpumask.h:1231 [inline]
RIP: 0010:trace_x86_fpu_regs_activated arch/x86/include/asm/trace/fpu.h:47 [inline]
RIP: 0010:fpregs_activate+0x33/0x1c0 arch/x86/kernel/fpu/context.h:50
Code: e8 62 22 57 00 48 c7 c7 00 a0 85 8b e8 66 a8 fc 09 65 48 89 1d ee 5f 5c 11 0f 1f 44 00 00 e8 44 22 57 00 65 8b 1d f1 2a 5c 11 <bf> 07 00 00 00 89 de e8 71 26 57 00 83 fb 07 77 5d e8 27 22 57 00
RSP: 0018:ffffc900043bfa90 EFLAGS: 00000293
RAX: ffffffff8169d53c RBX: 0000000000000000 RCX: ffff888024fadac0
RDX: 0000000000000000 RSI: ffffffff8b85a000 RDI: ffffffff8be250e0
RBP: 0000000000000000 R08: ffff888024fadac7 R09: 1ffff110049f5b58
R10: dffffc0000000000 R11: ffffed10049f5b59 R12: ffffffff816a0cdb
R13: dffffc0000000000 R14: ffff888024faf440 R15: dffffc0000000000
FS: 00007fe31cefa6c0(0000) GS:ffff8881259c0000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000000058 CR3: 0000000076344000 CR4: 00000000003526f0
Call Trace:
<TASK>
fpregs_mark_activate+0x64/0xe0 arch/x86/kernel/fpu/core.c:903
restore_fpregs_from_user arch/x86/kernel/fpu/signal.c:317 [inline]
__fpu_restore_sig arch/x86/kernel/fpu/signal.c:346 [inline]
fpu__restore_sig+0x95b/0x10d0 arch/x86/kernel/fpu/signal.c:480
restore_sigcontext arch/x86/kernel/signal_64.c:95 [inline]
__ia32_sys_rt_sigreturn+0x704/0x860 arch/x86/kernel/signal_64.c:268
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xec/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fe31bf8f747
Code: ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89
RSP: 002b:00007fe31cefa0e8 EFLAGS: 00000246
RAX: 00000000000000ca RBX: 00007fe31c1e5fa8 RCX: 00007fe31bf8f749
RDX: 0000000000000000 RSI: 0000000000000080 RDI: 00007fe31c1e5fa8
RBP: 00007fe31c1e5fa0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fe31c1e6038 R14: 00007fffd135ad30 R15: 00007fffd135ae18
</TASK>


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