Re: [PATCH v5] workqueue: Wrap flush_workqueue() using a macro

[Tejun Heo]

On Wed, Jun 01, 2022 at 04:32:47PM +0900, Tetsuo Handa wrote:
> Since flush operation synchronously waits for completion, flushing
> system-wide WQs (e.g. system_wq) might introduce possibility of deadlock
> due to unexpected locking dependency. Tejun Heo commented at [1] that it
> makes no sense at all to call flush_workqueue() on the shared WQs as the
> caller has no idea what it's gonna end up waiting for.
> 
> Although there is flush_scheduled_work() which flushes system_wq WQ with
> "Think twice before calling this function! It's very easy to get into
> trouble if you don't take great care." warning message, syzbot found a
> circular locking dependency caused by flushing system_wq WQ [2].
> 
> Therefore, let's change the direction to that developers had better use
> their local WQs if flush_scheduled_work()/flush_workqueue(system_*_wq) is
> inevitable.
> 
> Steps for converting system-wide WQs into local WQs are explained at [3],
> and a conversion to stop flushing system-wide WQs is in progress. Now we
> want some mechanism for preventing developers who are not aware of this
> conversion from again start flushing system-wide WQs.
> 
> Since I found that WARN_ON() is complete but awkward approach for teaching
> developers about this problem, let's use __compiletime_warning() for
> incomplete but handy approach. For completeness, we will also insert
> WARN_ON() into __flush_workqueue() after all in-tree users stopped calling
> flush_scheduled_work().
> 
> Link: https://lore.kernel.org/all/YgnQGZWT%2Fn3VAITX@xxxxxxxxxxxxxxx/ [1]
> Link: https://syzkaller.appspot.com/bug?extid=bde0f89deacca7c765b8 [2]
> Link: https://lkml.kernel.org/r/49925af7-78a8-a3dd-bce6-cfc02e1a9236@xxxxxxxxxxxxxxxxxxx [3]
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>

Applied to wq/for-5.19-fixes.

Thanks.

-- 
tejun