Re: Confusing lockdep splat

[Boqun Feng]

On Fri, Sep 24, 2021 at 03:43:37PM -0700, Paul E. McKenney wrote:
> On Fri, Sep 24, 2021 at 05:41:17PM -0400, Waiman Long wrote:
> > 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.
> 
> Would it make sense for lockdep to filter out reports involving more
> than one CPU unless there is at least one sleeplock in the cycle?
> 

I think SRCU is special here, because it has different implementations
in SMP and UP. For other code, if the implemenation in SMP and UP is the
same, we want lockdep to detect the deadlock even if it's not in UP.

We can provide an annotation similar to data_race() for SRCU to mark
UP-only code

	#define LOCKDEP_UP_ONLY(expr) ({			\
		BUILD_BUG_ON(IS_ENABLED(CONFIG_SMP));		\

		<disable lockdep>
		<...> v = expr;
		<enable lockdep>
		v
	})

and in __srcu_read_unlock():

	LOCKDEP_UP_ONLY(swake_up_one(...));

Thoughts?

Regards,
Boqun

> Of course, it gets more complicated when interrupts are involved...
> 
> 							Thanx, Paul