Re: [PATCH 2/3] rcu: Equip sleepable RCU with lockdep dependency graph checks

[Paolo Bonzini]

On 1/13/23 20:11, Paul E. McKenney wrote:
> On Fri, Jan 13, 2023 at 10:05:22AM -0800, Boqun Feng wrote:
> > On Fri, Jan 13, 2023 at 03:29:49AM -0800, Paul E. McKenney wrote:
> > I prefer that the first two patches go through your tree, because it
> > reduces the synchronization among locking, rcu and KVM trees to the
> > synchronization betwen rcu and KVM trees.
>
> Very well, I have queued and pushed these with the usual wordsmithing,
> thank you!

I'm worried about this case:

	CPU 0				CPU 1
	--------------------		------------------
	lock A				srcu lock B
	srcu lock B			lock A
	srcu unlock B			unlock A
	unlock A			srcu unlock B

While a bit unclean, there is nothing that downright forbids this; as 
long as synchronize_srcu does not happen inside lock A, no deadlock can 
occur.

However, if srcu is replaced with an rwlock then lockdep should and does 
report a deadlock.  Boqun, do you get a false positive or do your 
patches correctly suppress this?

Paolo