Messages in this thread | | | Date | Tue, 7 Jun 2022 07:08:42 -1000 | From | Tejun Heo <> | Subject | Re: [PATCH v5] workqueue: Wrap flush_workqueue() using a macro |
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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@slm.duckdns.org/ [1] > Link: https://syzkaller.appspot.com/bug?extid=bde0f89deacca7c765b8 [2] > Link: https://lkml.kernel.org/r/49925af7-78a8-a3dd-bce6-cfc02e1a9236@I-love.SAKURA.ne.jp [3] > Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Applied to wq/for-5.19-fixes.
Thanks.
-- tejun
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