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

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Date:   Wed, 30 Aug 2017 20:25:46 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     'Peter Zijlstra' <peterz@...radead.org>
Cc:     mingo@...nel.org, tj@...nel.org, boqun.feng@...il.com,
        david@...morbit.com, johannes@...solutions.net, oleg@...hat.com,
        linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation

On Wed, Aug 30, 2017 at 06:24:39PM +0900, Byungchul Park wrote:
> > -----Original Message-----
> > From: Peter Zijlstra [mailto:peterz@...radead.org]
> > Sent: Wednesday, August 30, 2017 6:12 PM
> > To: Byungchul Park
> > Cc: mingo@...nel.org; tj@...nel.org; boqun.feng@...il.com;
> > david@...morbit.com; johannes@...solutions.net; oleg@...hat.com; linux-
> > kernel@...r.kernel.org; kernel-team@....com
> > Subject: Re: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation
> > 
> > On Wed, Aug 30, 2017 at 06:01:59PM +0900, Byungchul Park wrote:
> > > My point is that we inevitably lose valuable dependencies by yours.
> > That's
> > > why I've endlessly asked you 'do you have any reason you try those
> > patches?'
> > > a ton of times. And you have never answered it.
> > 
> > The only dependencies that are lost are those between the first work and
> > the setup of the workqueue thread.
> > 
> > And there obviously _should_ not be any dependencies between those. A
> 
> 100% right. Since there obviously should not be any, it would be better
> to check them. So I've endlessly asked you 'do you have any reason removing
> the opportunity for that check?'. Overhead? Logical problem? Or want to
> believe workqueue setup code perfect forever? I mean, is it a problem if we
> check them?
> 
> > work should not depend on the setup of the thread.
> 
> 100% right.

For example - I'm giving you the same example repeatedly:

context X                 context Y
---------                 ---------
                          wait_for_completion(C)
acquire(A)
process_one_work()
   acquire(B)
   work->fn()
      complete(C)

Please check C->A and C->B.