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

[Hillf Danton <hdanton@sina.com>]

On Fri, 13 Jan 2023 23:32:01 -0800 Boqun Feng <boqun.feng@gmail.com>
> On Sat, Jan 14, 2023 at 03:18:32PM +0800, Hillf Danton wrote:
> >
> >  	task X			task Y
> >  	---			---
> >  	mutex_lock(A);
> >  				srcu_read_lock(B);
> > 				srcu_lock_acquire(&B->dep_map);
> > 				a) lock_map_acquire_read(&B->dep_map);
> >  	synchronze_srcu(B);
> > 	__synchronize_srcu(B);
> > 	srcu_lock_sync(&B->dep_map);
> > 	lock_map_sync(&B->dep_map);
> > 	lock_sync(&B->dep_map);
> > 	__lock_acquire(&B->dep_map);
> 
> At this time, lockdep add dependency A -> B in the dependency graph.
> 
> > 				b) lock_map_acquire_read(&B->dep_map);
> > 	__lock_release(&B->dep_map);
> > 				c) lock_map_acquire_read(&B->dep_map);
> >  				mutex_lock(A);
> 
> and here, lockdep will try to add dependency B -> A into the dependency
> graph, and find that A -> B -> A will form a circle (with strong
> dependency), therefore lockdep knows it's a deadlock. 

Is the strong dependency applying to mode c)?
If yes then deadlock should be also detected in the following locking
pattern that has no deadlock.

	cpu0			cpu1
	---			---
	mutex_lock A
	mutex_lock B
	mutex_unlock B
				mutex_lock B
				mutex_lock A
> 
> >  
> > No deadlock could be detected if taskY takes mutexA after taskX releases B,
> 
> The timing that taskX releases B doesn't master, since lockdep uses
> graph to detect deadlocks rather than after-fact detection.
> 
> > and how taskY acquires B does not matter as per the a), b) and c) modes in
> > the above chart, again because releasing lock can break deadlock in general.
> 
> I have test cases showing the above deadlock can be detected, so if you
> think there is a deadlock that may dodge from my change, feel free to
> add a test case in lib/locking-selftest.c ;-)
> 
> Regards,
> Boqun