Re: [syzbot] possible deadlock in worker_thread

[Tetsuo Handa]

On 2022/02/13 1:37, Bart Van Assche wrote:
> On 2/11/22 21:31, Tetsuo Handa wrote:
>> But this report might be suggesting us that we should consider
>> deprecating (and eventually getting rid of) system-wide workqueues
>> (declared in include/linux/workqueue.h), for since flush_workqueue()
>> synchronously waits for completion, sharing system-wide workqueues
>> among multiple modules can generate unexpected locking dependency
>> chain (like this report).
> 
> I do not agree with deprecating system-wide workqueues. I think that
> all flush_workqueue(system_long_wq) calls should be reviewed since
> these are deadlock-prone.
> 
> Thanks,
> 
> Bart.

The loop module is not using flush_workqueue(system_long_wq) call; it only
scheduled a work via system_long_wq which will call flush_workqueue() (of
a local workqueue) from drain_workqueue() from destroy_workqueue() from
work function.

How can reviewing all flush_workqueue(system_long_wq) calls help?