linux-kernel - Re: [PATCH v4 15/15] lockdep: Crossrelease feature documentation

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Date:   Wed, 18 Jan 2017 13:07:57 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     Boqun Feng <boqun.feng@...il.com>, mingo@...nel.org,
        tglx@...utronix.de, walken@...gle.com, kirill@...temov.name,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        npiggin@...il.com
Subject: Re: [PATCH v4 15/15] lockdep: Crossrelease feature documentation

On Wed, Jan 18, 2017 at 08:54:28PM +0900, Byungchul Park wrote:
> On Wed, Jan 18, 2017 at 12:03:17PM +0100, Peter Zijlstra wrote:
> > On Wed, Jan 18, 2017 at 07:53:47PM +0900, Byungchul Park wrote:
> > > On Wed, Jan 18, 2017 at 02:42:30PM +0800, Boqun Feng wrote:
> > > > On Fri, Dec 09, 2016 at 02:12:11PM +0900, Byungchul Park wrote:
> > > > [...]
> > > > > +Example 1:
> > > > > +
> > > > > +   CONTEXT X		   CONTEXT Y
> > > > > +   ---------		   ---------
> > > > > +   mutext_lock A
> > > > > +			   lock_page B
> > > > > +   lock_page B
> > > > > +			   mutext_lock A /* DEADLOCK */
> > > > 
> > > > s/mutext_lock/mutex_lock
> > > 
> > > Thank you.
> > > 
> > > > > +Example 3:
> > > > > +
> > > > > +   CONTEXT X		   CONTEXT Y
> > > > > +   ---------		   ---------
> > > > > +			   mutex_lock A
> > > > > +   mutex_lock A
> > > > > +   mutex_unlock A
> > > > > +			   wait_for_complete B /* DEADLOCK */
> > > > 
> > > > I think this part better be:
> > > > 
> > > >    CONTEXT X		   CONTEXT Y
> > > >    ---------		   ---------
> > > >    			   mutex_lock A
> > > >    mutex_lock A
> > > >    			   wait_for_complete B /* DEADLOCK */
> > > >    mutex_unlock A
> > > > 
> > > > , right? Because Y triggers DEADLOCK before X could run mutex_unlock().
> > > 
> > > There's no different between two examples.
> > 
> > There is..
> > 
> > > No matter which one is chosen, mutex_lock A in CONTEXT X cannot be passed.
> > 
> > But your version shows it does mutex_unlock() before CONTEXT Y does
> > wait_for_completion().
> > 
> > The thing about these diagrams is that both columns are assumed to have
> > the same timeline.
> 
> X cannot acquire mutex A because Y already acquired it.
> 
> In order words, all statements below mutex_lock A in X cannot run.

But your timeline shows it does, which is the error that Boqun pointed
out.