RE: [PATCH v4 15/15] lockdep: Crossrelease feature documentation

[byungchul.park]

> -----Original Message-----
> From: byungchul.park [mailto:byungchul.park@xxxxxxx]
> Sent: Wednesday, January 18, 2017 9:15 PM
> To: 'Peter Zijlstra'
> Cc: 'Boqun Feng'; 'mingo@xxxxxxxxxx'; 'tglx@xxxxxxxxxxxxx';
> 'walken@xxxxxxxxxx'; 'kirill@xxxxxxxxxxxxx'; 'linux-
> kernel@xxxxxxxxxxxxxxx'; 'linux-mm@xxxxxxxxx'; 'iamjoonsoo.kim@xxxxxxx';
> 'akpm@xxxxxxxxxxxxxxxxxxxx'; 'npiggin@xxxxxxxxx'
> Subject: RE: [PATCH v4 15/15] lockdep: Crossrelease feature documentation
> 
> > -----Original Message-----
> > From: Peter Zijlstra [mailto:peterz@xxxxxxxxxxxxx]
> > Sent: Wednesday, January 18, 2017 9:08 PM
> > To: Byungchul Park
> > Cc: Boqun Feng; mingo@xxxxxxxxxx; tglx@xxxxxxxxxxxxx; walken@xxxxxxxxxx;
> > kirill@xxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx;
> > iamjoonsoo.kim@xxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx; npiggin@xxxxxxxxx
> > 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.
> 
> I am sorry for not understanding what you are talking about.
> 
> Do you mean that I should remove all statements below mutex_lock A in X?
> 
> Or should I move mutex_unlock as Boqun said? What will change?

Anyway, I will change it as he said even though I don't understand what is
different between them. :/ But I am just curious. It would be appreciated
if you answer my question.