linux-kernel - Re: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation

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Date:   Tue, 5 Sep 2017 17:30:23 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Byungchul Park <max.byungchul.park@...il.com>,
        Ingo Molnar <mingo@...nel.org>, Tejun Heo <tj@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>, david@...morbit.com,
        Johannes Berg <johannes@...solutions.net>, oleg@...hat.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel-team@....com
Subject: Re: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation

On Tue, Sep 05, 2017 at 09:08:25AM +0200, Peter Zijlstra wrote:
> > Your patches only do avoiding the wq issue now we focus on.
> > 
> > Look at:
> > 
> >  worker thread                         another context
> >  -------------                         ---------------
> >                                        wait_for_completion()
> >        |
> >        |      (1)
> >        v
> >   +---------+
> >   | Work  A | (2)
> >   +---------+
> >        |
> >        |      (3)
> >        v
> >   +---------+
> >   | Work  B | (4)
> >   +---------+
> >        |
> >        |      (5)
> >        v
> >   +---------+
> >   | Work  C | (6)
> >   +---------+
> >        |
> >        v
> > 
> > We have to consider whole context of the worker to build dependencies
> > with a crosslock e.g. wait_for_commplete().
> > 
> > Only thing we have to care here is to make all works e.g. (2), (4) and
> > (6) independent, because workqueue does _concurrency control_. As I said
> > last year at the very beginning, for works not applied the control e.g.
> > max_active == 1, we don't need that isolation. I said, it's a future work.
> > 
> > It would have been much easier to communicate with each other if you
> > *tried* to understand my examples like now or you *tried* to give me one
> > example at least. You didn't even *try*. Only thing I want to ask you
> > for is to *try* to understand my opinions on conflicts.
> > 
> > Now, understand what I intended? Still unsufficient?
> 
> So you worry about max_active==1 ? Or you worry about pool->lock or
> about the thread setup? I'm still not sure.

It's close to the letter. Precisely, I worry about (1), (3), (5) and so
on, since they certainly create dependencies with crosslocks e.g.
completion in my example.