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

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Date:   Mon, 16 May 2022 10:56:58 +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 22:13, Dmitry Torokhov wrote:
> Also as far as I can see the patch was rejected.

Not rejected. I sent
https://lkml.kernel.org/r/7b2fecdb-59ae-2c54-5a5b-774ef7054d1b@I-love.SAKURA.ne.jp
for 5.19.

>> 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?

It is GFP_KERNEL context when module's __exit function is called. But whether
flush_workqueue() is called from restricted context depends on what locks does
the module's __exit function hold.

If request_module() is called from some work function using one of system-wide WQs,
and flush_workqueue() is called on that WQ from module's __exit function, the kernel
might deadlock on module_mutex lock. Making sure that flush_workqueue() is not called
on system-wide WQs is the safer choice.