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

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Date:   Wed, 30 Aug 2017 16:41:17 +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 11:09:53AM +0900, Byungchul Park wrote:
> On Tue, Aug 29, 2017 at 10:59:39AM +0200, Peter Zijlstra wrote:
> > Subject: lockdep: Untangle xhlock history save/restore from task independence
> > 
> > Where XHLOCK_{SOFT,HARD} are save/restore points in the xhlocks[] to
> > ensure the temporal IRQ events don't interact with task state, the
> > XHLOCK_PROC is a fundament different beast that just happens to share
> > the interface.
> > 
> > The purpose of XHLOCK_PROC is to annotate independent execution inside
> > one task. For example workqueues, each work should appear to run in its
> > own 'pristine' 'task'.
> > 
> > Remove XHLOCK_PROC in favour of its own interface to avoid confusion.
> 
> Much better to me than the patch you did previously, but, see blow.
> 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > index c0331891dec1..ab3c0dc8c7ed 100644
> > --- a/kernel/workqueue.c
> > +++ b/kernel/workqueue.c
> > @@ -2107,14 +2107,14 @@ __acquires(&pool->lock)
> >  	 * Which would create W1->C->W1 dependencies, even though there is no
> >  	 * actual deadlock possible. There are two solutions, using a
> >  	 * read-recursive acquire on the work(queue) 'locks', but this will then
> > -	 * hit the lockdep limitation on recursive locks, or simly discard
> > +	 * hit the lockdep limitation on recursive locks, or simply discard
> >  	 * these locks.
> >  	 *
> >  	 * AFAICT there is no possible deadlock scenario between the
> >  	 * flush_work() and complete() primitives (except for single-threaded
> >  	 * workqueues), so hiding them isn't a problem.
> >  	 */
> > -	crossrelease_hist_start(XHLOCK_PROC, true);
> > +	lockdep_invariant_state(true);
> 
> This is what I am always curious about. It would be ok if you agree with
> removing this work-around after fixing acquire things in wq. But, you
> keep to say this is essencial.
> 
> You should focus on what dependencies actually are, than saparating
> contexts unnecessarily. Of course, we have to do it for each work, _BUT_
> not between outside of work and each work since there might be
> dependencies between them certainly.

You have never answered it. I'm curious about your answer. If you can't,
I think you have to revert all your patches. All yours are wrong.