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_Y_QsvbGTy08SjsKpSFuB_zgxFNgzsP2 On 1/6/26 1:57 AM, Michael S. Tsirkin wrote: > On Tue, Jan 06, 2026 at 09:46:30AM +0800, Hillf Danton wrote: >>> 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. > > Well I think it's pretty clear but the issue is that just another lock > is not enough, we have bigger problems with this mutex. > > It's held around userspace accesses so if the vhost thread gets into > uninterruptible sleep holding that, a userspace thread trying to take it > with mutex_lock will be uninterruptible. > > So it propagates the uninterruptible status between vhost and a > userspace thread. > > It's not a new issue but the new(ish) thread management APIs make > it more visible. > > Here it's the kill handler that got hung but it's not really limited > to that, any ioctl can do that, and I do not want to add another > lock on data path. > Above, are you saying that the kill handler and a ioctl are trying to take the virtqueue->mutex in this bug? I've been trying to replicate this for a while, but I just can't hit what I'm seeing in the lockdep info from the initial email. We only see the kill handler trying to take the virtqueue->mutex. Is the theory that the locking info being reported is not correct? A userspace thread is doing an ioctl that took the mutex but it's not reported below? Originally I was using the vhost_dev->mutex for the locking in vhost_worker_killed but I saw we could take that during ioctls which did a flush, so I added the vhost_worker->mutex for some of the locking. If the virtqueue->mutex is also an issue I can do a patch. 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: