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

[Catalin Marinas]

On Wed, May 11, 2022 at 07:04:51PM +0900, Hyeonggon Yoo wrote:
> On Wed, May 11, 2022 at 08:39:29AM +0900, Byungchul Park wrote:
> > On Tue, May 10, 2022 at 08:18:12PM +0900, Hyeonggon Yoo wrote:
> > > On Mon, May 09, 2022 at 09:16:37AM +0900, Byungchul Park wrote:
> > > > 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.

The above kmemleak sequence shouldn't deadlock since those locks, even
if taken in a different order, are serialised by scan_mutex. For various
reasons, trying to reduce the latency, I ended up with some
fine-grained, per-object locking.

For object allocation (rbtree modification) and tree search, we use
kmemleak_lock. During scanning (which can take minutes under
scan_mutex), we want to prevent (a) long latencies and (b) freeing the
object being scanned. We release the locks regularly for (a) and hold
the object->lock for (b).

In another thread Byungchul mentioned:

|    context X			context Y
| 
|    lock mutex A		lock mutex A
|    lock B			lock C
|    lock C			lock B
|    unlock C			unlock B
|    unlock B			unlock C
|    unlock mutex A		unlock mutex A
| 
| In my opinion, lock B and lock C are unnecessary if they are always
| along with lock mutex A. Or we should keep correct lock order across all
| the code.

If these are the only two places, yes, locks B and C would be
unnecessary. But we have those locks acquired (not nested) on the
allocation path (kmemleak_lock) and freeing path (object->lock). We
don't want to block those paths while scan_mutex is held.

That said, we may be able to use a single kmemleak_lock for everything.
The object freeing path may be affected slightly during scanning but the
code does release it every MAX_SCAN_SIZE bytes. It may even get slightly
faster as we'd hammer a single lock (I'll do some benchmarks).

But from a correctness perspective, I think the DEPT tool should be
improved a bit to detect when such out of order locking is serialised by
an enclosing lock/mutex.

-- 
Catalin