Re: Confusing lockdep splat

[Waiman Long]

On 9/24/21 5:02 PM, Paul E. McKenney wrote:
> Hello!
>
> I got the lockdep splat below from an SRCU-T rcutorture run, which uses
> a !SMP !PREEMPT kernel.  This is a random event, and about half the time
> it happens within an hour or two.  My reproducer (on current -rcu "dev"
> branch for a 16-CPU system) is:
>
> 	tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 16 --configs "16*SRCU-T" --duration 7200
>
> My points of confusion are as follows:
>
> 1.	The locks involved in this deadlock cycle are irq-disabled
> 	raw spinlocks.	The claimed deadlock cycle uses two CPUs.
> 	There is only one CPU.	There is no possibility of preemption
> 	or interrupts.	So how can this deadlock actually happen?
>
> 2.	If there was more than one CPU, then yes, there would be
> 	a deadlock.  The PI lock is acquired by the wakeup code after
> 	acquiring the workqueue lock, and rcutorture tests the new ability
> 	of the scheduler to hold the PI lock across rcu_read_unlock(),
> 	and while it is at it, across the rest of the unlock primitives.
>
> 	But if there was more than one CPU, Tree SRCU would be used
> 	instead of Tiny SRCU, and there would be no wakeup invoked from
> 	srcu_read_unlock().
>
> 	Given only one CPU, there is no way to complete the deadlock
> 	cycle.
>
> For now, I am working around this by preventing rcutorture from holding
> the PI lock across Tiny srcu_read_unlock().
>
> Am I missing something subtle here?

I would say that the lockdep code just doesn't have enough intelligence 
to identify that deadlock is not possible in this special case. There 
are certainly false positives, and it can be hard to get rid of them.

Cheers,
Longman