Re: [RFC] workqueue: remove manual lockdep uses to detect deadlocks

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Aug 26, 2017 at 12:49:26AM +0900, Byungchul Park wrote:
> > However, how would it distinguish things like flushing another work
> 
> I think it must be distinguished with what it actually waits for, e.i.
> completion
> variables instead of work or wq. I will make it next week and let you know.

So no. The existing annotations are strictly better than relying on
cross-release.

As you know the problem with cross-release is that it is timing
dependent. You need to actually observe the problematic sequence before
it can warn, and only the whole instance->class mapping saves us from
actually hitting the deadlock.

Cross-release can result in deadlocks without warnings. If you were to
run:

	mutex_lock(A);
					mutex_lock(A);
					complete(C);
	wait_for_completion(C);

You'd deadlock without issue. Only if we observe this:

	mutex_lock(A);
	wait_for_completion(C);
					mutex_lock(A);
					complete(C);

Where we acquire A after wait_for_completion() but before complete()
will we observe the deadlock.

The same would be true for using cross-release for workqueues as well,
something like:

					W:
					mutex_lock(A)

	mutex_lock(A)
	flush_work(W)

would go unreported whereas the current workqueue annotation will
generate a splat.


This does not mean cross-release isn't worth it, its better than nothing,
but its strictly weaker than traditional annotations.

So where a traditional annotation is possible, we should use them.