Re: [PATCH] workqueue: Use private WQ for schedule_on_each_cpu() API

[Tejun Heo <tj@kernel.org>]

On Wed, Feb 23, 2022 at 09:57:27AM +0900, Tetsuo Handa wrote:
> On 2022/02/23 2:29, Tejun Heo wrote:
> > On Mon, Feb 21, 2022 at 07:38:09PM +0900, Tetsuo Handa wrote:
> >> Since schedule_on_each_cpu() calls schedule_work_on() and flush_work(),
> >> we should avoid using system_wq in order to avoid unexpected locking
> >> dependency.
> > 
> > I don't get it. schedule_on_each_cpu() is flushing each work item and thus
> > shouldn't need its own flushing domain. What's this change for?
> 
> A kernel test robot tested "[PATCH v2] workqueue: Warn flush attempt using
> system-wide workqueues" on 5.16.0-06523-g29bd199e4e73 and hit a lockdep
> warning ( https://lkml.kernel.org/r/20220221083358.GC835@xsang-OptiPlex-9020 ).
> 
> Although the circular locking dependency itself needs to be handled by
> lockless console printing support, we won't be able to apply
> "[PATCH v2] workqueue: Warn flush attempt using system-wide workqueues"
> if schedule_on_each_cpu() continues using system-wide workqueues.

The patch seems pretty wrong. What's problematic is system workqueue flushes
(which flushes the entire workqueue), not work item flushes.

Thanks.

-- 
tejun