[syzbot] [nfs?] INFO: task hung in nfsd_nrthreads

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 502d45774af0 Merge tag 'clk-for-linus' of git://git.kernel..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11d58d49580000
kernel config: https://syzkaller.appspot.com/x/.config?x=c2d3e2a7997c1d5c
dashboard link: https://syzkaller.appspot.com/bug?extid=86a9d43d89b86def6d59
compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/13c40240edf7/disk-502d4577.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/ccf7e87c89cc/vmlinux-502d4577.xz
kernel image: https://storage.googleapis.com/syzbot-assets/9feb3a41684b/bzImage-502d4577.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+86a9d4...@syzkaller.appspotmail.com

INFO: task syz.5.1942:10848 blocked for more than 143 seconds.
Tainted: G L syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz.5.1942 state:D stack:27656 pid:10848 tgid:10847 ppid:8546 task_flags:0x400140 flags:0x00080002
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5520 [inline]
__schedule+0x1259/0x6920 kernel/sched/core.c:7270
__schedule_loop kernel/sched/core.c:7347 [inline]
schedule+0xdd/0x2c0 kernel/sched/core.c:7362
schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7419
__mutex_lock_common kernel/locking/mutex.c:726 [inline]
__mutex_lock+0xcbf/0x1bc0 kernel/locking/mutex.c:821
nfsd_nrthreads+0x57/0xd0 fs/nfsd/nfssvc.c:252
write_threads+0x34a/0x600 fs/nfsd/nfsctl.c:445
nfsctl_transaction_write+0x106/0x1a0 fs/nfsd/nfsctl.c:114
vfs_write+0x2aa/0x1050 fs/read_write.c:685
ksys_write+0x12a/0x250 fs/read_write.c:739
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x123/0x790 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f27b1d9e0d9
RSP: 002b:00007f27b2bd1028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f27b2025fa0 RCX: 00007f27b1d9e0d9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 00007f27b1e35024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f27b2026038 R14: 00007f27b2025fa0 R15: 00007fffbfec4448
</TASK>
INFO: task syz.5.1942:10849 blocked for more than 143 seconds.
Tainted: G L syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz.5.1942 state:D stack:28728 pid:10849 tgid:10847 ppid:8546 task_flags:0x400040 flags:0x00080002
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5520 [inline]
__schedule+0x1259/0x6920 kernel/sched/core.c:7270
__schedule_loop kernel/sched/core.c:7347 [inline]
schedule+0xdd/0x2c0 kernel/sched/core.c:7362
schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7419
__mutex_lock_common kernel/locking/mutex.c:726 [inline]
__mutex_lock+0xcbf/0x1bc0 kernel/locking/mutex.c:821
fdget_pos+0x2aa/0x380 fs/file.c:1259
class_fd_pos_constructor include/linux/file.h:85 [inline]
ksys_read+0x71/0x250 fs/read_write.c:707
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x123/0x790 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f27b1d9e0d9
RSP: 002b:00007f27b2bb0028 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 00007f27b2026090 RCX: 00007f27b1d9e0d9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 00007f27b1e35024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f27b2026128 R14: 00007f27b2026090 R15: 00007fffbfec4448
</TASK>

Showing all locks held in the system:
locks held by khungtaskd/31: 1, last CPU#0:
#0: ffffffff8edec060 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:309 [inline]
#0: ffffffff8edec060 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline]
#0: ffffffff8edec060 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x3d/0x184 kernel/locking/lockdep.c:6793
locks held by getty/5352: 2, on CPU#0:
#0: ffff8880338b20a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x24/0x80 drivers/tty/tty_ldisc.c:243
#1: ffffc9000321e2e8 (&ldata->atomic_read_lock){+.+.}-{4:4}, at: n_tty_read+0x419/0x14e0 drivers/tty/n_tty.c:2211
locks held by syz.4.1396/9282: 3, on CPU#1:
#0: ffff888020b775f0 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x2aa/0x380 fs/file.c:1259
#1: ffff88807db82460 (sb_writers#22){.+.+}-{0:0}, at: ksys_write+0x12a/0x250 fs/read_write.c:739
#2: ffffffff8f284440 (nfsd_mutex){+.+.}-{4:4}, at: write_ports+0xa5/0xcc0 fs/nfsd/nfsctl.c:867
locks held by syz.5.1942/10848: 3, on CPU#1:
#0: ffff88807562deb0 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x2aa/0x380 fs/file.c:1259
#1: ffff888040656460 (sb_writers#22){.+.+}-{0:0}, at: ksys_write+0x12a/0x250 fs/read_write.c:739
#2: ffffffff8f284440 (nfsd_mutex){+.+.}-{4:4}, at: nfsd_nrthreads+0x57/0xd0 fs/nfsd/nfssvc.c:252
locks held by syz.5.1942/10849: 1, on CPU#0:
#0: ffff88807562deb0 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x2aa/0x380 fs/file.c:1259
locks held by kworker/u8:14/13269: 4, last CPU#0:
#0: ffff88801f3d2940 ((wq_completion)iou_exit){+.+.}-{0:0}, at: process_one_work+0x1466/0x1b10 kernel/workqueue.c:3362
#1: ffffc9000611fd08 ((work_completion)(&ctx->exit_work)){+.+.}-{0:0}, at: process_one_work+0xa2c/0x1b10 kernel/workqueue.c:3363
#2: ffff88807f4d40a0 (&ctx->uring_lock){+.+.}-{4:4}, at: class_mutex_constructor include/linux/mutex.h:253 [inline]
#2: ffff88807f4d40a0 (&ctx->uring_lock){+.+.}-{4:4}, at: io_req_caches_free io_uring/io_uring.c:2150 [inline]
#2: ffff88807f4d40a0 (&ctx->uring_lock){+.+.}-{4:4}, at: io_ring_exit_work+0x422/0xd59 io_uring/io_uring.c:2354
#3: ffffffff8edec060 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:309 [inline]
#3: ffffffff8edec060 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline]
#3: ffffffff8edec060 (rcu_read_lock){....}-{1:3}, at: class_rcu_constructor include/linux/rcupdate.h:1216 [inline]
#3: ffffffff8edec060 (rcu_read_lock){....}-{1:3}, at: unwind_next_frame+0xbd/0x2090 arch/x86/kernel/unwind_orc.c:495
locks held by kworker/u8:22/14261: 2, on CPU#1:
#0: ffff88801c094140 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work+0x1466/0x1b10 kernel/workqueue.c:3362
#1: ffffc90006c9fd08 ((reaper_work).work){+.+.}-{0:0}, at: process_one_work+0xa2c/0x1b10 kernel/workqueue.c:3363
locks held by syz.0.3240/15025: 3, on CPU#1:
#0: ffff8880756840f0 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x2aa/0x380 fs/file.c:1259
#1: ffff888037bb8460 (sb_writers#10){.+.+}-{0:0}, at: ksys_write+0x12a/0x250 fs/read_write.c:739
#2: ffff888056933080 (&of->mutex){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x2c2/0x5f0 fs/kernfs/file.c:336
locks held by syz.0.3240/15030: 1, on CPU#1:
#0: ffff8880756840f0 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x2aa/0x380 fs/file.c:1259
locks held by syz-executor/15160: 4, last CPU#0:
#0: ffffffff8ef3cab0 (dup_mmap_sem){++++}-{0:0}, at: dup_mm kernel/fork.c:1542 [inline]
#0: ffffffff8ef3cab0 (dup_mmap_sem){++++}-{0:0}, at: copy_mm kernel/fork.c:1595 [inline]
#0: ffffffff8ef3cab0 (dup_mmap_sem){++++}-{0:0}, at: copy_process+0x4815/0x8370 kernel/fork.c:2307
#1: ffff8880504e1d38 (&mm->mmap_lock){++++}-{4:4}, at: mmap_write_lock_killable include/linux/mmap_lock.h:562 [inline]
#1: ffff8880504e1d38 (&mm->mmap_lock){++++}-{4:4}, at: dup_mmap+0x127/0x2370 mm/mmap.c:1716
#2: ffff8880339829f8 (&mm->mmap_lock/1){+.+.}-{4:4}, at: mmap_write_lock_nested include/linux/mmap_lock.h:552 [inline]
#2: ffff8880339829f8 (&mm->mmap_lock/1){+.+.}-{4:4}, at: dup_mmap+0x1c9/0x2370 mm/mmap.c:1723
#3: ffffffff8edec060 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:309 [inline]
#3: ffffffff8edec060 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline]
#3: ffffffff8edec060 (rcu_read_lock){....}-{1:3}, at: __pte_offset_map+0x2f/0x310 mm/pgtable-generic.c:290
locks held by syz.4.3499/15896: 1, last CPU#1:
#0: ffff88803306aac8 (&pcm->open_mutex){+.+.}-{4:4}, at: snd_pcm_oss_release+0xfb/0x300 sound/core/oss/pcm_oss.c:2589

=============================================

NMI backtrace for cpu 0
CPU: 0 UID: 0 PID: 31 Comm: khungtaskd Tainted: G L syzkaller #0 PREEMPT(full)
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/05/2026
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
nmi_cpu_backtrace.cold+0x12d/0x151 lib/nmi_backtrace.c:123
nmi_trigger_cpumask_backtrace+0x21f/0x2a0 lib/nmi_backtrace.c:66
trigger_all_cpu_backtrace include/linux/nmi.h:164 [inline]
__sys_info lib/sys_info.c:157 [inline]
sys_info+0x141/0x190 lib/sys_info.c:165
check_hung_uninterruptible_tasks kernel/hung_task.c:353 [inline]
watchdog+0xcb1/0x1030 kernel/hung_task.c:561
kthread+0x373/0x450 kernel/kthread.c:436
ret_from_fork+0x730/0xd60 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 UID: 0 PID: 15832 Comm: syz.1.3471 Tainted: G L syzkaller #0 PREEMPT(full)
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/05/2026
RIP: 0010:__sanitizer_cov_trace_pc+0x6b/0x70 kernel/kcov.c:231
Code: 82 08 17 00 00 83 f8 02 75 20 48 8b 8a 10 17 00 00 8b 92 0c 17 00 00 48 8b 01 48 83 c0 01 48 39 d0 73 07 48 89 01 48 89 34 c1 <e9> 40 e1 c4 09 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3
RSP: 0018:ffffc90004e378f0 EFLAGS: 00000046
RAX: 0000000000080000 RBX: ffff88802d38a2bc RCX: ffffc9001b780000
RDX: 0000000000080000 RSI: ffffffff820ec036 RDI: ffff88805a58cb00
RBP: 0000000000000001 R08: 0000000000000007 R09: 0000000000000000
R10: 00000000000002ac R11: 0000000000000001 R12: 0000000000000000
R13: ffffc90004e379a8 R14: 0000000000000000 R15: ffff88801d2bc000
FS: 00007fb805db56c0(0000) GS:ffff888123c6d000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b34422ff8 CR3: 00000000297f8000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 000000000000000a DR2: 0000000000000000
DR3: 00000000c0f00480 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
rb_reserve_next_event kernel/trace/ring_buffer.c:4988 [inline]
ring_buffer_lock_reserve+0x4b6/0x1220 kernel/trace/ring_buffer.c:5039
__trace_buffer_lock_reserve kernel/trace/trace.h:1676 [inline]
trace_event_buffer_lock_reserve+0x123/0x6f0 kernel/trace/trace.c:1924
trace_event_buffer_reserve+0x1f9/0x350 kernel/trace/trace_events.c:715
do_trace_event_raw_event_msr_trace_class arch/x86/include/asm/msr-trace.h:21 [inline]
trace_event_raw_event_msr_trace_class+0xe8/0x270 arch/x86/include/asm/msr-trace.h:21
__do_trace_write_msr arch/x86/include/asm/msr-trace.h:45 [inline]
trace_write_msr+0x160/0x1f0 arch/x86/include/asm/msr-trace.h:45
apic_write arch/x86/include/asm/apic.h:405 [inline]
lapic_next_event+0x10/0x20 arch/x86/kernel/apic/apic.c:418
clockevents_program_event+0x375/0x820 kernel/time/clockevents.c:372
tick_program_event+0xa9/0x140 kernel/time/tick-oneshot.c:48
hrtimer_rearm_event kernel/time/hrtimer.c:710 [inline]
hrtimer_rearm kernel/time/hrtimer.c:2169 [inline]
__hrtimer_rearm_deferred+0x234/0x7b0 kernel/time/hrtimer.c:2189
hrtimer_rearm_deferred_tif include/linux/hrtimer_rearm.h:58 [inline]
hrtimer_rearm_deferred include/linux/hrtimer_rearm.h:67 [inline]
irqentry_exit_to_kernel_mode_after_preempt include/linux/irq-entry-common.h:505 [inline]
irqentry_exit_to_kernel_mode include/linux/irq-entry-common.h:542 [inline]
irqentry_exit+0x5f3/0xa10 kernel/entry/common.c:167
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:674
RIP: 0010:__raw_spin_unlock_irq include/linux/spinlock_api_smp.h:220 [inline]
RIP: 0010:_raw_spin_unlock_irq+0x29/0x50 kernel/locking/spinlock.c:229
Code: 90 f3 0f 1e fa 53 48 8b 74 24 08 48 89 fb 48 83 c7 18 e8 ca a7 16 f6 48 89 df e8 e2 fd 16 f6 e8 cd b7 43 f6 fb bf 01 00 00 00 <e8> e2 07 06 f6 65 48 83 3d 71 c6 ba 08 00 74 06 5b c3 cc cc cc cc
RSP: 0018:ffffc90004e37e40 EFLAGS: 00000206
RAX: 0000000000050741 RBX: ffff88802da61bc0 RCX: 0000000000000000
RDX: 0000000000000001 RSI: ffffffff8e3eb0b9 RDI: 0000000000000001
RBP: ffff88805a58d498 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: ffff88805a58cb00
R13: 0000000100000000 R14: 0000000000000007 R15: 0000000000000000
spin_unlock_irq include/linux/spinlock.h:413 [inline]
__set_current_blocked kernel/signal.c:3227 [inline]
set_current_blocked+0xdd/0x120 kernel/signal.c:3211
__do_sys_rt_sigreturn+0x1a2/0x2c0 arch/x86/kernel/signal_64.c:263
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x123/0x790 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fb804f3f459
Code: 11 06 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 90 66 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 c7 c0 0f 00 00 00 0f 05 <0f> 1f 80 00 00 00 00 48 81 ec 48 01 00 00 49 89 d0 64 48 8b 0c 25
RSP: 002b:00007fb805db4340 EFLAGS: 00000206 ORIG_RAX: 000000000000000f
RAX: ffffffffffffffda RBX: 00007fb805225fa0 RCX: 00007fb804f3f459
RDX: 00007fb805db4340 RSI: 00007fb805db4470 RDI: 0000000000000021
RBP: 00007fb805035024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000206 R12: 0000000000000000
R13: 00007fb805226038 R14: 00007fb805225fa0 R15: 00007ffee7eee2b8
</TASK>


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