linux-kernel - Re: [syzbot] possible deadlock in worker

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Date:   Sun, 13 Feb 2022 02:14:09 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Bart Van Assche <bvanassche@....org>,
        syzbot <syzbot+831661966588c802aae9@...kaller.appspotmail.com>,
        jgg@...pe.ca, linux-kernel@...r.kernel.org,
        linux-rdma@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        Tejun Heo <tj@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [syzbot] possible deadlock in worker_thread

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?