Subject: Re: [syzbot] INFO: task hung in vhost_worker_killed (2)

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 5 Jan 2026 04:22:37 -0500

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-masked]
> 
> 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>
> 
> 
> ---
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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.