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

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Mon, May 09, 2022 at 09:16:37AM +0900, Byungchul Park wrote:
> 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@lge.com> 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*.
>

Yeah, in previous threads we discussed this [1]

And the case was:
	scan_mutex -> object_lock -> kmemleak_lock -> object_lock
And dept reported:
	object_lock -> kmemleak_lock, kmemleak_lock -> object_lock as
	deadlock.

But IIUC - What DEPT reported happens only under scan_mutex and
It is not simple just not to take them because the object can be removed from the
list and freed while scanning via kmemleak_free() without kmemleak_lock and object_lock.

Just I'm still not sure that someone will fix the warning in the future - even if the
locking rule is not good - if it will not cause a real 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.
>

Sorry, It was not right just to say DEPT doesn't work with _nested() APIs.

> 	Byungchul

[1] https://lore.kernel.org/lkml/20220304002809.GA6112@X58A-UD3R/

-- 
Thanks,
Hyeonggon