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Date: Thu, 24 Aug 2017 17:11:01 +0900
From: Byungchul Park <byungchul.park@lge.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>, mingo@kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@lge.com,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Dave Chinner <david@fromorbit.com>, Tejun Heo <tj@kernel.org>,
        johannes@sipsolutions.net
Subject: Re: [PATCH v3 1/3] lockdep: Make LOCKDEP_CROSSRELEASE configs all
 part of PROVE_LOCKING
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On Thu, Aug 24, 2017 at 04:37:13PM +0900, Byungchul Park wrote:
> On Thu, Aug 24, 2017 at 03:11:53PM +0900, Byungchul Park wrote:
> > On Wed, Aug 23, 2017 at 07:47:14PM +0200, Peter Zijlstra wrote:
> > > Those are fine and are indeed the flush_work() vs work inversion.
> > > 
> > > The two straight forward annotations are:
> > > 
> > > flush_work(work)	process_one_work(wq, work)
> > >   A(work)		  A(work)
> > >   R(work)		  work->func(work);
> > > 			  R(work)
> > > 
> > > Which catches:
> > > 
> > > Task-1:			work:
> > > 
> > >   mutex_lock(&A);	mutex_lock(&A);
> > >   flush_work(work);
> > 
> > I'm not sure but, with LOCKDEP_COMPLETE enabled, this issue would
> > automatically be covered w/o additional A(work)/R(work). Right?
> > 
> > A(work)/R(work) seem to be used for preventing wait_for_completion()
> > in flush_work() from waiting for the completion forever because of the
> > work using mutex_lock(&A). Am I understanding correctly?
> > 
> > If yes, we can use just LOCKDEP_COMPLETE for that purpose.
> 
> I'm not familiar with workqueue but, the manual lockdep_map_acquire() in
> workqueue code seems to be introduced to do what LOCKDEP_COMPLETE does
> for wait_for_completion() and complete().
> 
> Wrong?

As I understand how workqueue code works more, thanks to Peterz, I get
convinced. What they want to detect with acquire(w/wq) is a deadlock
caused by wait_for_completion() mixed with typical locks.

We have to detect it with _variables_ which it actually waits for, i.e.
completion variable, neither _work_ nor _workqueue_ which we are
currently using.