Re: [syzbot] [usb?] INFO: task hung in unbind_store

[Michal Pecio <michal.pecio@gmail.com>]

On Sun, 23 Aug 2026 13:46:43 +0200, Greg KH wrote:
> On Sun, Aug 23, 2026 at 04:40:33AM -0700, syzbot wrote:
> > INFO: task syz.4.23:6285 blocked for more than 143 seconds.
> >       Not tainted syzkaller #0
> > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > task:syz.4.23        state:D stack:27592 pid:6285  tgid:6285  ppid:6213   task_flags:0x400140 flags:0x00080002
> > Call Trace:
> >  <TASK>
> >  context_switch kernel/sched/core.c:5510 [inline]
> >  __schedule+0x17d4/0x5630 kernel/sched/core.c:7239
> >  __schedule_loop kernel/sched/core.c:7316 [inline]
> >  schedule+0x164/0x2b0 kernel/sched/core.c:7331
> >  schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7388
> >  __mutex_lock_common kernel/locking/mutex.c:726 [inline]
> >  __mutex_lock+0x7c1/0x1550 kernel/locking/mutex.c:821
> >  device_lock include/linux/device.h:1104 [inline]
> >  __device_driver_lock drivers/base/dd.c:1170 [inline]
> >  device_release_driver_internal+0x93/0x880 drivers/base/dd.c:1369
> >  unbind_store+0x1a1/0x1d0 drivers/base/bus.c:244  
> 
> Ok, I'm going to add a new TAINT flag for when unbind is written to as
> that is obviously not a normal operation and is only for debugging
> things by kernel developers.  Adding loads of work-arounds in the kernel
> for this not-real-workload-path is just not required.
> 
> If syzbot could stop hitting this path, that would be great, as it's a
> root-only thing for debugging and not something "real".

The root cause is probe() call of one USB driver (sisusbvga) taking
an eternity to complete on nonresponsive hardware, which is actually
a pretty real and not so uncommon annoying behavior.

And something is also wrong with those timeouts, because 20 times 5s
should still be less than 143 s. I asked Syzbot to try 500ms instead,
which ended up being over 3s in practice:

[  566.662036][ T6636] usb 4-1: sisusb_send_bulk_msg()
[  569.717341][ T6636] usb 4-1: sisusb_send_bulk_msg()
[  572.777761][ T6636] usb 4-1: sisusb_send_bulk_msg()
[  575.845365][ T6636] usb 4-1: sisusb_send_bulk_msg()

Same on my system, but patch below reduces the timeout to 500ms.

I have no idea what's happening here. Looks like a bug?
Is it known that jiffies are totally unreliable like that?

Regards,
Michal

--- a/drivers/usb/misc/sisusbvga/sisusbvga.c
+++ b/drivers/usb/misc/sisusbvga/sisusbvga.c
@@ -225,9 +225,13 @@ static int sisusb_bulkout_msg(struct sisusb_usb_data *sisusb, int index,
 
 	/* If OK, and if timeout > 0, wait for completion */
 	if ((retval == 0) && timeout) {
+		u64 time = ktime_get_ns();
+
 		wait_event_timeout(sisusb->wait_q,
 				(!(sisusb->urbstatus[index] & SU_URB_BUSY)),
 				timeout);
+		dev_info(&sisusb->sisusb_dev->dev, "sisusb_bulkout_msg() waited %lld\n", ktime_get_ns() - time);
+
 		if (sisusb->urbstatus[index] & SU_URB_BUSY) {
 			/* URB timed out... kill it and report error */
 			usb_kill_urb(urb);