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

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Date:   Thu, 12 May 2022 20:32:10 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v3 (repost)] workqueue: Warn flushing of kernel-global
 workqueues

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 .

>          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.

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. 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.

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