Re: [PATCH v3 (repost)] workqueue: Warn flushing of kernel-global workqueues

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Thu, May 12, 2022 at 08:32:10PM +0900, Tetsuo Handa wrote:
> On 2022/05/12 19:38, Dmitry Torokhov wrote:
> > Hi Tejun,
> > 
> > On Mon, Mar 21, 2022 at 07:02:45AM -1000, Tejun Heo wrote:
> >> I'm willing to bet that the majority of the use cases can be converted to
> >> use flush_work() and that'd be the preference. We need a separate workqueue
> >> iff the flush requrement is complex (e.g. there are multiple dynamic work
> >> items in flight which need to be flushed together) or the work items needs
> >> some special attributes (such as MEM_RECLAIM or HIGHPRI) which don't apply
> >> to the system_wq users in the first place.
> > 
> > This means that now the code has to keep track of all work items that it
> > allocated, instead of being able "fire and forget" works (when dealing
> > with extremely infrequent events) and rely on flush_workqueue() to
> > cleanup.
> 
> Yes. Moreover, a patch to catch and refuse at compile time was proposed at
> https://lkml.kernel.org/r/738afe71-2983-05d5-f0fc-d94efbdf7634@I-love.SAKURA.ne.jp .

My comment was not a wholesale endorsement of Tejun's statement, but
rather a note of the fact that it again adds complexity (at least as far
as driver writers are concerned) to the kernel code.

Also as far as I can see the patch was rejected.

> 
> >          That flush typically happens in module unload path, and I
> > wonder if the restriction on flush_workqueue() could be relaxed to allow
> > calling it on unload.
> 
> A patch for drivers/input/mouse/psmouse-smbus.c is waiting for your response at
> https://lkml.kernel.org/r/25e2b787-cb2c-fb0d-d62c-6577ad1cd9df@I-love.SAKURA.ne.jp .
> Like many modules, flush_workqueue() happens on only module unload in your case.

Yes, I saw that patch, and that is what prompted my response. I find it
adding complexity and I was wondering if it could be avoided. It also
unclear to me if there is an additional cost coming from allocating a
dedicated workqueue.

> 
> We currently don't have a flag to tell whether the caller is inside module unload
> path. And even inside module unload path, flushing the system-wide workqueue is
> problematic under e.g. GFP_NOFS/GFP_NOIO context.

Sorry, I do not follow here. Are there module unloading code that runs
as GFP_NOFS/GFP_NOIO?

> Therefore, I don't think that
> the caller is inside module unload path as a good exception.
> 
> Removing flush_scheduled_work() is for proactively avoiding new problems like
> https://lkml.kernel.org/r/385ce718-f965-4005-56b6-34922c4533b8@I-love.SAKURA.ne.jp
> and https://lkml.kernel.org/r/20220225112405.355599-10-Jerome.Pouiller@silabs.com .
> 
> Using local WQ also helps for documentation purpose.
> This change makes clear where the work's dependency is.
> Please grep the linux-next.git tree. Some have been already converted.

I understand that for some of them the change makes sense, but it would
be nice to continue using simple API under limited circumstances.

> 
> Any chance you have too many out-of-tree modules to convert?
> 

No, we are trying to get everything upstream.

Thanks.

-- 
Dmitry