linux-kernel - Re: [PATCH v3 1/3] lockdep: Make LOCKDEP_CROSSRELEASE configs all part of PROVE

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Message-ID: <20170825011407.GB3858@X58A-UD3R>
Date:   Fri, 25 Aug 2017 10:14:07 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Oleg Nesterov <oleg@...hat.com>, mingo@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@....com,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Dave Chinner <david@...morbit.com>, Tejun Heo <tj@...nel.org>,
        johannes@...solutions.net
Subject: Re: [PATCH v3 1/3] lockdep: Make LOCKDEP_CROSSRELEASE configs all
 part of PROVE_LOCKING

On Thu, Aug 24, 2017 at 05:11:01PM +0900, Byungchul Park wrote:
> 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.

Please read this more _carefully_.

I would be sorry if wrong,

_BUT_,

it could be a key to solve the issue of workqueue in right way, IIUC.