linux-kernel - Re: [PATCH v3] rapidio/tsi721: Replace flush_scheduled_work() with flush

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Date:   Tue, 27 Sep 2022 07:13:59 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Tejun Heo <tj@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Matt Porter <mporter@...nel.crashing.org>,
        Alexandre Bounine <alex.bou9@...il.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3] rapidio/tsi721: Replace flush_scheduled_work() with
 flush_work().

On 2022/09/27 0:06, Alan Stern wrote:
>> Alan Stern suggested to use cancel_work_sync() in
>> commit eef6a7d5c2f38ada ("workqueue: warn about flush_scheduled_work()")
>> and Tejun Heo suggested to use flush_work() in
>> https://lkml.kernel.org/r/YjivtdkpY+reW0Gt@slm.duckdns.org .
>>
>> Is there some reason to prefer one over the other?
>> I think that user-visible results between flush_work() and cancel_work_sync()
>> are the same because both wait until work completes.
> 
> No, you haven't got it quite right.  flush_work() waits until the work 
> completes, but cancel_work_sync() first tries to cancel the work item.  
> It then waits until the work item is either cancelled or completed.

I know there is a difference if the cancellation was successful.
But unless cancel_work_sync() is called immediately after schedule_work(),
that work likely (e.g. 99%+) already started running or already completed.

> 
> If the cancellation is successful (i.e., it happens before the work item 
> starts to run) then the call will return at that time and the work item 
> will never run -- hence it will never complete.

A difficult to judge thing is whether the owner/maintainer of that code wants
that work completed or cancelled.
Unlike e.g. https://lkml.kernel.org/r/Yy3byxFrfAfQL9xK@intel.com ,
tsi721_remove() does not say whether pending works should run.