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

From: Tetsuo Handa
Date: Tue Feb 22 2022 - 19:57:51 EST


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.