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

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Message-ID: <20170901094747.iv6s532ccuuzpry2@hirez.programming.kicks-ass.net>
Date:   Fri, 1 Sep 2017 11:47:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Byungchul Park <byungchul.park@....com>
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 11:05:12AM +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
first few works will ever see them