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

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Message-ID: <20220509233838.GC6047@X58A-UD3R>
Date:   Tue, 10 May 2022 08:38:38 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Hyeonggon Yoo <42.hyeyoo@...il.com>, torvalds@...ux-foundation.org,
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Subject: Re: [PATCH RFC v6 00/21] DEPT(Dependency Tracker)

On Mon, May 09, 2022 at 04:47:12PM -0400, Steven Rostedt wrote:
> On Mon, 9 May 2022 09:16:37 +0900
> Byungchul Park <byungchul.park@....com> wrote:
> 
> > 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
> 
> I'm confused by what exactly you are saying is a deadlock above.
> 
> Are you saying that lock A and L' are inversed? If so, lockdep had better

Hi Steven,

Yes, I was talking about A and L'.

> detect that regardless of L. A nested lock associates the the nesting with

When I checked Lockdep code, L' with depth n + 1 and L' with depth n
have different classes in Lockdep.

That's why I said Lockdep cannot detect it. By any chance, has it
changed so as to consider this case? Or am I missing something?

> the same type of lock. That is, in lockdep nested tells lockdep not to
> trigger on the L and L' but it will not ignore that A was taken.

It will not ignore A but it would work like this:

   THREAD X			THREAD Y

   lock Ln
				lock Ln
   lock A
				lock A
   lock_nested Lm
				lock_nested Lm

So, Lockdep considers this case safe, actually not tho.

	Byungchul

> 
> -- Steve
> 
> 
> 
> > 				lock_nested L'' with depth n + 1
> >    ...				...
> >    unlock L'			unlock L''
> >    unlock A			unlock A
> >    unlock L			unlock L'