linux-kernel - Re: [PATCH] checkpatch: warn about flushing system-wide workqueues

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Date:   Fri, 6 May 2022 08:29:07 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Tejun Heo <tj@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Joe Perches <joe@...ches.com>,
        Andy Whitcroft <apw@...onical.com>,
        Dwaipayan Ray <dwaipayanray1@...il.com>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>
Subject: Re: [PATCH] checkpatch: warn about flushing system-wide workqueues

On 2022/05/06 2:32, Tejun Heo wrote:
> Given that we'll need runtime check anyway, why not resurrect the original
> runtime warning but exempt flush_schedule_work() if that's the only thing
> remaining right now (using a special flag or whatever)?

Yes, we will also need runtime check for robustness, for we can't catch usage
like

	struct workqueue_struct *my_wq = alloc_workqueue();
	if (!my_wq)
		my_wq = system_long_wq;
	flush_workqueue(my_wq);

using compile time checks.

I found that it is not easy to trigger flush_workqueue() paths. For example,
several modules are using flush_workqueue() only upon module unloading.
Therefore, I'm trying to catch obvious flush_workqueue() paths at compile
time when possible.

>                                                         If we're sure that
> we aren't triggering it spuriously, we can ask Andrew to take the warning
> patch into -mm so that it floats on top of everything else and gets pulled
> into the trunk during the coming merge window.

OK, the coming merge window means 5.19.

The original runtime checking will be used anyway. Is "workqueue: Wrap
flush_workqueue() using a macro" OK for you as a compile time check?