Date: Fri, 1 Sep 2017 11:05:12 +0900
From: Byungchul Park <byungchul.park@lge.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, tj@kernel.org, boqun.feng@gmail.com,
        david@fromorbit.com, johannes@sipsolutions.net, oleg@redhat.com,
        linux-kernel@vger.kernel.org, kernel-team@lge.com
Subject: Re: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation
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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.

Remind we are talking about dependencies wrt cross-lock, not between
_holding_ locks. If you invalidate xhlock whenever work->fn(), we cannot
build dependencies like C->A and C->B every time. Right?

> All of that is contained in kernel/kthread and kernel/workqueue and can
> be audited if needed. Its a very limited amount of code.

I mean, doing it automatically w/o additional overhead is better than
considering the limited amount of code manually every time changing
kernel code. Do as you please.