linux-kernel - Re: [PATCH] workqueue: don't skip lockdep wq dependency in cancel_work

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Message-ID: <97cbf8a9-d5e1-376f-6a49-3474871ea6b4@I-love.SAKURA.ne.jp>
Date:   Fri, 29 Jul 2022 10:49:01 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Johannes Berg <johannes.berg@...el.com>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        Hillf Danton <hdanton@...a.com>,
        LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] workqueue: don't skip lockdep wq dependency in
 cancel_work_sync()

Johannes, why did you think that flagging it as if cancel_work_sync()
was flush_work() is a problem?

Unconditionally recording

  "struct mutex" mutex->lockdep_map => "struct work_struct" work1->lockdep_map
  "struct mutex" mutex->lockdep_map => "struct work_struct" work2->lockdep_map

chains has zero problem.

Unconditionally recording

  "struct mutex" mutex->lockdep_map => "struct workqueue_struct" ordered_wq->lockdep_map

chain when ordered_wq can process only one work item at a time
in order to indicate that the ordered_wq is currently unable to process
other works has zero problem.

The example shown in commit d6e89786bed977f3 ("workqueue: skip lockdep wq
dependency in cancel_work_sync()") is nothing but violation of a rule that
"Do not hold a lock from a work callback function (do not record

  "struct work_struct" work1->lockdep_map => "struct mutex" mutex->lockdep_map
  "struct workqueue_struct" ordered_wq->lockdep_map => "struct mutex" mutex->lockdep_map

chain) if somebody might wait for completion of that callback function with
that lock held (might record

  "struct mutex" mutex->lockdep_map => "struct work_struct" work1->lockdep_map
  "struct mutex" mutex->lockdep_map => "struct workqueue_struct" ordered_wq->lockdep_map

chain)."

Which in-tree ordered workqueue instance is hitting this problem?