Re: [syzbot] INFO: task hung in freeze_super (3)

[Hillf Danton <hdanton@sina.com>]

On 8 Jan 2023 20:11:55 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> On 2023/01/04 13:01, Hillf Danton wrote:
> > 
> > 	syz-executor330/3631			kworker/0:1H/120
> > 	---					---
> > 	down_write_nested(&s->s_umount);
> > 						glock_work_func()
> > 						down_write(&sb->s_umount);
> > 	flush_workqueue(glock_workqueue);
> > 
> > 
> > No deadlock is reported yet. Whyyy?
> > See if extending the aperture of wq->lockdep_map can catch it.
> 
> As far as I tested, this patch did not change likeliness of hitting lockdep warning.
> I feel that it was just by chance that this test request was able to hit lockdep warning.

	cpu0			cpu1
	---			---
	lock A
				work B runs
				a) lock_map_acquire(&wq->lockdep_map);

	__flush_workqueue
  	lock_map_acquire(&wq->lockdep_map);
	lock_map_release(&wq->lockdep_map);

				b) lock_map_acquire(&wq->lockdep_map);
				lock A

Currently report of deadlock escapes in mode b) because worker takes workqueue
lockdep_map after flusher releases it.

After this change, mode b) could be caught with no exception.

	cpu0			cpu1
	---			---
	lock A
				work B runs
				a) lock_map_acquire(&wq->lockdep_map);

	__flush_workqueue
  	lock_map_acquire(&wq->lockdep_map);

				b) lock_map_acquire(&wq->lockdep_map);
				lock A