Re: [PATCH v3 1/3] lockdep: Make LOCKDEP_CROSSRELEASE configs all part of PROVE_LOCKING

[Peter Zijlstra]

On Tue, Aug 22, 2017 at 02:14:38PM +0900, Byungchul Park wrote:
> On Mon, Aug 21, 2017 at 05:46:00PM +0200, Peter Zijlstra wrote:
> > Now given the above observance rule and the fact that the below report
> > is from the complete, the thing that happened appears to be:
> > 
> > 
> > 	lockdep_map_acquire(&work->lockdep_map)
> > 	down_write(&A)
> > 
> > 			down_write(&A)
> > 			wait_for_completion(&C)
> > 
> > 					lockdep_map_acquire(&work_lockdep_map);
> > 					complete(&C)
> > 
> > Which lockdep then puked over because both sides saw the same work
> > class.
> > 
> > Byungchul; should we not exclude the work class itself, it appears to me
> > the workqueue code is explicitly parallel, or am I confused again?
> 
> Do you mean the lockdep_map_acquire(&work->lockdep_map) used manuallly?
> 
> That was introduced by Johannes:
> 
> commit 4e6045f134784f4b158b3c0f7a282b04bd816887
> "workqueue: debug flushing deadlocks with lockdep"
> 
> I am not sure but, for that purpose, IMHO, we can use a
> lockdep_map_acquire_read() instead, in process_one_work(), can't we?

That wouldn't work. That annotation is to help find deadlocks like:


	mutex_lock(&A)
				<work>
				mutex_lock(&A)

	flush_work(&work)


The 'fake' lock connects the lock chain inside the work to the
lock-chain at flush_work() and thus detects these. That's perfectly
fine.

It just seems to confuse the completions stuff... Let me go read Dave's
email and see if I can come up with something.