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

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Message-ID: <afa1ac2c-a023-a91e-e596-60931b38247e@I-love.SAKURA.ne.jp>
Date:   Fri, 29 Jul 2022 11:28:40 +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()

Thinking more, I might be confused by difference between

  the lockdep "struct work_struct" dependency handling

and

  the lockdep "struct work" dependency handling

. In an example that

      work3_function() { /* nothing */ }
      work4_function() { mutex_lock(&mutex); ... }

      other_function() {
        queue_work(ordered_wq, &work3);
        queue_work(ordered_wq, &work4);
        mutex_lock(&mutex);
        cancel_work_sync(&work4);
      }

, possibility of deadlock must be reported by lockdep via
the lockdep "struct work" dependency handling.

Then, prior to commit d6e89786bed977f3, lockdep flagging

  the lockdep "struct work_struct" dependency handling

as if cancel_work_sync() was flush_work() is a problem, and
lockdep not flagging

  the lockdep "struct work" dependency handling

 from cancel_work_sync() as if flush_work() is also a problem.

Then, commit d6e89786bed977f3 might be OK, but commit 87915adc3f0acdf0
was wrong in that it preserved lockdep not flagging

  the lockdep "struct work" dependency handling

 from cancel_work_sync() as if flush_work().