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

Byungchul Park
2017-01-18T11:54:47Z

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.