On Mon, 5 Jan 2026 04:22:37 -0500 "Michael S. Tsirkin" wrote:
> On Mon, Jan 05, 2026 at 12:42:22AM -0800, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit: 349bd28a86f2 Merge tag 'vfio-v6.19-rc4' of https://github...
> > git tree: upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=13ccf29a580000
> > kernel config: https://syzkaller.appspot.com/x/.config?x=a94030c847137a18
> > dashboard link: https://syzkaller.appspot.com/bug?extid=a9528028ab4ca83e8bac
> > 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=13a67222580000
> >
> > Downloadable assets:
> > disk image:
> > https://storage.googleapis.com/syzbot-assets/47d669d196ca/disk-349bd28a.raw.xz
> > vmlinux:
> > https://storage.googleapis.com/syzbot-assets/f856a256a5eb/vmlinux-349bd28a.xz
> > kernel image:
> > https://storage.googleapis.com/syzbot-assets/0f8e3de3614b/bzImage-349bd28a.xz
> > mounted in repro #1:
> > https://storage.googleapis.com/syzbot-assets/3eec5fbffba2/mount_0.gz
> > mounted in repro #2:
> > https://storage.googleapis.com/syzbot-assets/c9eff1484b09/mount_6.gz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: [email protected]
> >
> > INFO: task vhost-7617:7618 blocked for more than 143 seconds.
> > Not tainted syzkaller #0
> > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > task:vhost-7617 state:D stack:29600 pid:7618 tgid:7593 ppid:5977
> > task_flags:0x404440 flags:0x00080000
> > Call Trace:
> > <TASK>
> > context_switch kernel/sched/core.c:5256 [inline]
> > __schedule+0x149b/0x4fd0 kernel/sched/core.c:6863
> > __schedule_loop kernel/sched/core.c:6945 [inline]
> > schedule+0x165/0x360 kernel/sched/core.c:6960
> > schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7017
> > __mutex_lock_common kernel/locking/mutex.c:692 [inline]
> > __mutex_lock+0x7e6/0x1350 kernel/locking/mutex.c:776
> > vhost_worker_killed+0x12b/0x390 drivers/vhost/vhost.c:476
> > vhost_task_fn+0x3d1/0x430 kernel/vhost_task.c:62
> > 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>
> >
> > Showing all locks held in the system:
> > 1 lock held by khungtaskd/32:
> > #0: ffffffff8df41aa0 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire
> > include/linux/rcupdate.h:331 [inline]
> > #0: ffffffff8df41aa0 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock
> > include/linux/rcupdate.h:867 [inline]
> > #0: ffffffff8df41aa0 (rcu_read_lock){....}-{1:3}, at:
> > debug_show_all_locks+0x2e/0x180 kernel/locking/lockdep.c:6775
> > 2 locks held by getty/5579:
> > #0: ffff88814e3cb0a0 (&tty->ldisc_sem){++++}-{0:0}, at:
> > tty_ldisc_ref_wait+0x25/0x70 drivers/tty/tty_ldisc.c:243
> > #1: ffffc9000332b2f0 (&ldata->atomic_read_lock){+.+.}-{4:4}, at:
> > n_tty_read+0x449/0x1460 drivers/tty/n_tty.c:2211
> > 1 lock held by syz-executor/5978:
> > #0: ffffffff8df475f8 (rcu_state.exp_mutex){+.+.}-{4:4}, at:
> > exp_funnel_lock kernel/rcu/tree_exp.h:311 [inline]
> > #0: ffffffff8df475f8 (rcu_state.exp_mutex){+.+.}-{4:4}, at:
> > synchronize_rcu_expedited+0x2b1/0x6e0 kernel/rcu/tree_exp.h:956
> > 2 locks held by syz.5.259/7601:
> > 3 locks held by vhost-7617/7618:
> > #0: ffff888054cc68e8 (&vtsk->exit_mutex){+.+.}-{4:4}, at:
> > vhost_task_fn+0x322/0x430 kernel/vhost_task.c:54
> > #1: ffff888024646a80 (&worker->mutex){+.+.}-{4:4}, at:
> > vhost_worker_killed+0x57/0x390 drivers/vhost/vhost.c:470
> > #2: ffff8880550c0258 (&vq->mutex){+.+.}-{4:4}, at:
> > vhost_worker_killed+0x12b/0x390 drivers/vhost/vhost.c:476
> > 1 lock held by syz-executor/7850:
> > #0: ffffffff8df475f8 (rcu_state.exp_mutex){+.+.}-{4:4}, at:
> > exp_funnel_lock kernel/rcu/tree_exp.h:343 [inline]
> > #0: ffffffff8df475f8 (rcu_state.exp_mutex){+.+.}-{4:4}, at:
> > synchronize_rcu_expedited+0x36e/0x6e0 kernel/rcu/tree_exp.h:956
> > 1 lock held by syz.2.640/9940:
> > 4 locks held by syz.3.641/9946:
> > 3 locks held by syz.1.642/9954:
> >
> > =============================================
> >
> > NMI backtrace for cpu 0
> > CPU: 0 UID: 0 PID: 32 Comm: khungtaskd Not tainted syzkaller #0
> > PREEMPT(full)
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > Google 10/25/2025
> > Call Trace:
> > <TASK>
> > dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
> > nmi_cpu_backtrace+0x274/0x2d0 lib/nmi_backtrace.c:113
> > nmi_trigger_cpumask_backtrace+0x17a/0x300 lib/nmi_backtrace.c:62
> > trigger_all_cpu_backtrace include/linux/nmi.h:160 [inline]
> > __sys_info lib/sys_info.c:157 [inline]
> > sys_info+0x135/0x170 lib/sys_info.c:165
> > check_hung_uninterruptible_tasks kernel/hung_task.c:346 [inline]
> > watchdog+0xf95/0xfe0 kernel/hung_task.c:515
> > kthread+0x711/0x8a0 kernel/kthread.c:463
> > 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>
> > Sending NMI from CPU 0 to CPUs 1:
> > NMI backtrace for cpu 1
> > CPU: 1 UID: 0 PID: 5961 Comm: syz-executor Not tainted syzkaller #0
> > PREEMPT(full)
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > Google 10/25/2025
> > RIP: 0010:wq_watchdog_touch+0xb4/0x160 kernel/workqueue.c:7654
> > Code: d8 48 c1 e8 03 42 80 3c 38 00 74 08 48 89 df e8 e2 ba 9b 00 49 c7 c6
> > 78 52 80 92 4c 03 33 4c 89 f0 48 c1 e8 03 42 80 3c 38 00 <74> 08 4c 89 f7
> > e8 b2 bb 9b 00 49 89 2e eb 18 e8 88 9a 35 00 48 8d
> > RSP: 0018:ffffc90003157480 EFLAGS: 00000046
> > RAX: 1ffff110170e484f RBX: ffffffff8d9aedd8 RCX: ffff88807d8b9e80
> > RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000008
> > RBP: 0000000100002ff4 R08: ffff888022760237 R09: 1ffff110044ec046
> > R10: dffffc0000000000 R11: ffffffff8b583250 R12: 00000000000036b0
> > R13: 0000000100002326 R14: ffff8880b8724278 R15: dffffc0000000000
> > FS: 0000555582bd2500(0000) GS:ffff888125f1f000(0000) knlGS:0000000000000000
> > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 0000000000000000 CR3: 0000000077fb2000 CR4: 0000000000350ef0
> > Call Trace:
> > <TASK>
> > touch_nmi_watchdog include/linux/nmi.h:149 [inline]
> > wait_for_lsr+0x16a/0x2f0 drivers/tty/serial/8250/8250_port.c:1968
> > fifo_wait_for_lsr drivers/tty/serial/8250/8250_port.c:3234 [inline]
> > serial8250_console_fifo_write drivers/tty/serial/8250/8250_port.c:3257
> > [inline]
> > serial8250_console_write+0x1301/0x1b60
> > drivers/tty/serial/8250/8250_port.c:3342
> > console_emit_next_record kernel/printk/printk.c:3129 [inline]
> > console_flush_one_record kernel/printk/printk.c:3215 [inline]
> > console_flush_all+0x713/0xb00 kernel/printk/printk.c:3289
> > __console_flush_and_unlock kernel/printk/printk.c:3319 [inline]
> > console_unlock+0xbb/0x190 kernel/printk/printk.c:3359
> > vprintk_emit+0x47b/0x550 kernel/printk/printk.c:2426
> > _printk+0xcf/0x120 kernel/printk/printk.c:2451
> > __nilfs_msg+0x349/0x410 fs/nilfs2/super.c:78
> > nilfs_segctor_destroy fs/nilfs2/segment.c:2798 [inline]
> > nilfs_detach_log_writer+0x697/0xa30 fs/nilfs2/segment.c:2882
> > nilfs_put_super+0x4d/0x150 fs/nilfs2/super.c:509
> > generic_shutdown_super+0x135/0x2c0 fs/super.c:643
> > kill_block_super+0x44/0x90 fs/super.c:1722
> > deactivate_locked_super+0xbc/0x130 fs/super.c:474
> > cleanup_mnt+0x425/0x4c0 fs/namespace.c:1318
> > task_work_run+0x1d4/0x260 kernel/task_work.c:233
> > resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
> > __exit_to_user_mode_loop kernel/entry/common.c:44 [inline]
> > exit_to_user_mode_loop+0xef/0x4e0 kernel/entry/common.c:75
> > __exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
> > syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:256
> > [inline]
> > syscall_exit_to_user_mode_work include/linux/entry-common.h:159 [inline]
> > syscall_exit_to_user_mode include/linux/entry-common.h:194 [inline]
> > do_syscall_64+0x2b7/0xf80 arch/x86/entry/syscall_64.c:100
> > entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > RIP: 0033:0x7efe38790a77
> > Code: a8 ff ff ff f7 d8 64 89 01 48 83 c8 ff c3 0f 1f 44 00 00 31 f6 e9 09
> > 00 00 00 66 0f 1f 84 00 00 00 00 00 b8 a6 00 00 00 0f 05 <48> 3d 00 f0 ff
> > ff 77 01 c3 48 c7 c2 a8 ff ff ff f7 d8 64 89 02 b8
> > RSP: 002b:00007ffc337cbd88 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
> > RAX: 0000000000000000 RBX: 00007efe38813d7d RCX: 00007efe38790a77
> > RDX: 0000000000000000 RSI: 0000000000000009 RDI: 00007ffc337cbe40
> > RBP: 00007ffc337cbe40 R08: 0000000000000000 R09: 0000000000000000
> > R10: 00000000ffffffff R11: 0000000000000246 R12: 00007ffc337cced0
> > R13: 00007efe38813d7d R14: 0000000000067345 R15: 00007ffc337ccf10
> > </TASK>
> >
> >
> > ---
> > This report is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at [email protected].
> >
> > syzbot will keep track of this issue. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> >
> > If the report is already addressed, let syzbot know by replying with:
> > #syz fix: exact-commit-title
> >
> > If you want syzbot to run the reproducer, reply with:
> > #syz test: git://repo/address.git branch-or-commit-hash
> > If you attach or paste a git patch, syzbot will apply it before testing.
> >
> > If you want to overwrite report's subsystems, reply with:
> > #syz set subsystems: new-subsystem
> > (See the list of subsystem names on the web dashboard)
> >
> > If the report is a duplicate of another one, reply with:
> > #syz dup: exact-subject-of-another-report
> >
> > If you want to undo deduplication, reply with:
> > #syz undup
>
>
> Yea well
>
> static void vhost_worker_killed(void *data)
> {
> struct vhost_worker *worker = data;
> struct vhost_dev *dev = worker->dev;
> struct vhost_virtqueue *vq;
> int i, attach_cnt = 0;
>
> mutex_lock(&worker->mutex);
> worker->killed = true;
>
> for (i = 0; i < dev->nvqs; i++) {
> vq = dev->vqs[i];
>
> mutex_lock(&vq->mutex);
> if (worker ==
> rcu_dereference_check(vq->worker,
> lockdep_is_held(&vq->mutex))) {
> rcu_assign_pointer(vq->worker, NULL);
> attach_cnt++;
> }
> mutex_unlock(&vq->mutex);
> }
>
> worker->attachment_cnt -= attach_cnt;
> if (attach_cnt)
> synchronize_rcu();
> /*
> * Finish vhost_worker_flush calls and any other works that snuck in
> * before the synchronize_rcu.
> */
> vhost_run_work_list(worker);
> mutex_unlock(&worker->mutex);
> }
>
>
> taking vq mutex in a kill handler is probably not wise.
> we should have a separate lock just for handling worker
> assignment.
>
Better not before showing us the root cause of the hung to
avoid adding a blind lock.
