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

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Date:   Fri, 1 Sep 2017 21:09:46 +0900
From:   박병철/선임연구원/SW Platform(연)AOT팀(byungchul.park@....com) 
        <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     "mingo@...nel.org" <mingo@...nel.org>,
        "tj@...nel.org" <tj@...nel.org>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        "david@...morbit.com" <david@...morbit.com>,
        "johannes@...solutions.net" <johannes@...solutions.net>,
        "oleg@...hat.com" <oleg@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel-team@....com" <kernel-team@....com>
Subject: RE: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation

> -----Original Message-----
> From: Byungchul Park [mailto:byungchul.park@....com]
> Sent: Friday, September 01, 2017 7:16 PM
> To: Peter Zijlstra
> 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 Fri, Sep 01, 2017 at 11:47:47AM +0200, Peter Zijlstra wrote:
> > On Fri, Sep 01, 2017 at 11:05:12AM +0900, Byungchul Park wrote:
> > > On Thu, Aug 31, 2017 at 10:34:53AM +0200, Peter Zijlstra wrote:
> > > > On Thu, Aug 31, 2017 at 05:15:01PM +0900, Byungchul Park wrote:
> > > > > It's not important. Ok, check the following, instead:
> > > > >
> > > > > context X                 context Y
> > > > > ---------                 ---------
> > > > >                           wait_for_completion(C)
> > > > > acquire(A)
> > > > > release(A)
> > > > > process_one_work()
> > > > >    acquire(B)
> > > > >    release(B)
> > > > >    work->fn()
> > > > >       complete(C)
> > > > >
> > > > > We don't need to lose C->A and C->B dependencies unnecessarily.
> > > >
> > > > I really can't be arsed about them. Its really only the first few
> works
> > > > that will retain that dependency anyway, even if you were to retain
> > > > them.
> > >
> > > Wrong.
> > >
> > > Every 'work' doing complete() for different classes of completion
> > > variable suffers from losing valuable dependencies, every time, not
> > > first few ones.
> >
> > The moment you overrun the history array its gone. So yes, only the
> 
> It would be gone _only_ at the time the history overrun, and then it
> will be built again. So, you are wrong.
> 
> Let me show you an example: (I hope you also show examples.)
> 
> context X                  context Y
> ---------                  ---------
>                            wait_for_completion(D)
> while (true)
>    acquire(A)
>    release(A)
>    process_one_work()
>       acquire(B)
>       release(B)
>       work->fn()
>          complete(C)
>    acquire(D)
>    release(D)
> 
> When happening an overrun in a 'work', 'A' and 'B' will be gone _only_
> at the time, and then 'D', 'A' and 'B' will be queued into the xhlock
> *again* from the next loop on, and they can be used to generate useful
> dependencies again.
> 
> You are being confused now. Acquisitions we are focusing now are not
> _stacked_ like hlocks, but _accumulated_ continuously onto the ring
> buffer e.i. xhlock array.

Agree?