linux-kernel - Re: [RFC] workqueue: remove manual lockdep uses to detect deadlocks

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Date:   Tue, 29 Aug 2017 09:23:33 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Tejun Heo <tj@...nel.org>
Cc:     johannes.berg@...el.com, peterz@...radead.org, mingo@...nel.org,
        tglx@...utronix.de, linux-kernel@...r.kernel.org,
        kernel-team@....com
Subject: Re: [RFC] workqueue: remove manual lockdep uses to detect deadlocks

On Fri, Aug 25, 2017 at 06:34:43AM -0700, Tejun Heo wrote:
> On Fri, Aug 25, 2017 at 05:41:03PM +0900, Byungchul Park wrote:
> > Hello all,
> > 
> > This is _RFC_.
> > 
> > I want to request for comments about if it's reasonable conceptually. If
> > yes, I want to resend after working it more carefully.
> > 
> > Could you let me know your opinions about this?
> > 
> > ----->8-----
> > From 448360c343477fff63df766544eec4620657a59e Mon Sep 17 00:00:00 2001
> > From: Byungchul Park <byungchul.park@....com>
> > Date: Fri, 25 Aug 2017 17:35:07 +0900
> > Subject: [RFC] workqueue: remove manual lockdep uses to detect deadlocks
> > 
> > We introduced the following commit to detect deadlocks caused by
> > wait_for_completion() in flush_{workqueue, work}() and other locks. But
> > now LOCKDEP_COMPLETIONS is introduced, such works are automatically done
> > by LOCKDEP_COMPLETIONS. So it doesn't have to be done manually anymore.
> > Removed it.
> 
> I'm not following lockdep development, so can't really comment but if
> you're saying that wq can retain the same level of protection while
> not having explicit annotations, conceptually, it's of course great.
> However, how would it distinguish things like flushing another work
> item on a workqueue w/ max_active of 1?

Do you mean the following?

process_one_work()
   acquire(W1) <---------+- distinguishable?
   work->fn()            |
      flush_work(W2)     |
         acquire(W2) <---+
         release(W2)
   release(W1)