On Mon, 9 May 2022 09:16:37 +0900
Byungchul Park [off-list ref] 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
detect that regardless of L. A nested lock associates the the nesting with
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.

-- Steve

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