Re: [PATCH RFC v6 00/21] DEPT(Dependency Tracker)

[Byungchul Park]

On Sat, May 07, 2022 at 04:20:50PM +0900, Hyeonggon Yoo wrote:
> On Fri, May 06, 2022 at 09:11:35AM +0900, Byungchul Park wrote:
> > Linus wrote:
> > >
> > > On Wed, May 4, 2022 at 1:19 AM Byungchul Park <byungchul.park@xxxxxxx> wrote:
> > > >
> > > > Hi Linus and folks,
> > > >
> > > > I've been developing a tool for detecting deadlock possibilities by
> > > > tracking wait/event rather than lock(?) acquisition order to try to
> > > > cover all synchonization machanisms.
> > > 
> > > So what is the actual status of reports these days?
> > > 
> > > Last time I looked at some reports, it gave a lot of false positives
> > > due to mis-understanding prepare_to_sleep().
> > 
> > Yes, it was. I handled the case in the following way:
> > 
> > 1. Stage the wait at prepare_to_sleep(), which might be used at commit.
> >    Which has yet to be an actual wait that Dept considers.
> > 2. If the condition for sleep is true, the wait will be committed at
> >    __schedule(). The wait becomes an actual one that Dept considers.
> > 3. If the condition is false and the task gets back to TASK_RUNNING,
> >    clean(=reset) the staged wait.
> > 
> > That way, Dept only works with what actually hits to __schedule() for
> > the waits through sleep.
> > 
> > > For this all to make sense, it would need to not have false positives
> > > (or at least a very small number of them together with a way to sanely
> > 
> > Yes. I agree with you. I got rid of them that way I described above.
> >
> 
> IMHO DEPT should not report what lockdep allows (Not talking about

No.

> wait events). I mean lockdep allows some kind of nested locks but
> DEPT reports them.

You have already asked exactly same question in another thread of
LKML. That time I answered to it but let me explain it again.

---

CASE 1.

   lock L with depth n
   lock_nested L' with depth n + 1
   ...
   unlock L'
   unlock L

This case is allowed by Lockdep.
This case is allowed by DEPT cuz it's not a deadlock.

CASE 2.

   lock L with depth n
   lock A
   lock_nested L' with depth n + 1
   ...
   unlock L'
   unlock A
   unlock L

This case is allowed by Lockdep.
This case is *NOT* allowed by DEPT cuz it's a *DEADLOCK*.

---

The following scenario would explain why CASE 2 is problematic.

   THREAD X			THREAD Y

   lock L with depth n
				lock L' with depth n
   lock A
				lock A
   lock_nested L' with depth n + 1
				lock_nested L'' with depth n + 1
   ...				...
   unlock L'			unlock L''
   unlock A			unlock A
   unlock L			unlock L'

Yes. I need to check if the report you shared with me is a true one, but
it's not because DEPT doesn't work with *_nested() APIs.

	Byungchul