Re: [RFC] workqueue: remove manual lockdep uses to detect deadlocks

[Byungchul Park]

On Mon, Aug 28, 2017 at 08:55:53AM +0200, Peter Zijlstra wrote:
> On Fri, Aug 25, 2017 at 05:41:03PM +0900, Byungchul Park wrote:
> > Hello all,
> > 
> > This is _RFC_.
> > 
> > I want to request for comments about if it's reasonable conceptually. If
> > yes, I want to resend after working it more carefully.
> > 
> > Could you let me know your opinions about this?
> > 
> > ----->8-----
> > From 448360c343477fff63df766544eec4620657a59e Mon Sep 17 00:00:00 2001
> > From: Byungchul Park <byungchul.park@xxxxxxx>
> > Date: Fri, 25 Aug 2017 17:35:07 +0900
> > Subject: [RFC] workqueue: remove manual lockdep uses to detect deadlocks
> > 
> > We introduced the following commit to detect deadlocks caused by
> > wait_for_completion() in flush_{workqueue, work}() and other locks. But
> > now LOCKDEP_COMPLETIONS is introduced, such works are automatically done
> > by LOCKDEP_COMPLETIONS. So it doesn't have to be done manually anymore.
> > Removed it.
> > 
> 
> No.. the existing annotation is strictly better because it will _always_
> warn. It doesn't need to first observe things just right.

In addition, the existing annotation is never good, but just able to
detect deadlocks aggresively. However, it's inevitable to create false
dependencies. I mean some dependencies between work/wq and any locks
inside of each work might be false ones sometimes.