linux-kernel - Re: [PATCH] workqueue: Use private WQ for schedule_on_each

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Message-ID: <16a33a65-3c67-ef66-ccc8-9c4fffb0ae5a@I-love.SAKURA.ne.jp>
Date:   Wed, 23 Feb 2022 09:57:27 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Tejun Heo <tj@...nel.org>
Cc:     0day robot <lkp@...el.com>, LKML <linux-kernel@...r.kernel.org>,
        lkp@...ts.01.org, kernel test robot <oliver.sang@...el.com>
Subject: Re: [PATCH] workqueue: Use private WQ for schedule_on_each_cpu() API

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.