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

[Hillf Danton <hdanton@sina.com>]

On Fri, 13 Jan 2023 16:17:59 -0800 Boqun Feng <boqun.feng@gmail.com>
> On Sat, Jan 14, 2023 at 07:58:09AM +0800, Hillf Danton wrote:
> > On 13 Jan 2023 09:58:10 -0800 Boqun Feng <boqun.feng@gmail.com>
> > > On Fri, Jan 13, 2023 at 09:03:30PM +0800, Hillf Danton wrote:
> > > > On 12 Jan 2023 22:59:54 -0800 Boqun Feng <boqun.feng@gmail.com>
> > > > > --- a/kernel/rcu/srcutree.c
> > > > > +++ b/kernel/rcu/srcutree.c
> > > > > @@ -1267,6 +1267,8 @@ static void __synchronize_srcu(struct srcu_struct *ssp, bool do_norm)
> > > > >  {
> > > > >  	struct rcu_synchronize rcu;
> > > > >  
> > > > > +	srcu_lock_sync(&ssp->dep_map);
> > > > > +
> > > > >  	RCU_LOCKDEP_WARN(lockdep_is_held(ssp) ||
> > > > >  			 lock_is_held(&rcu_bh_lock_map) ||
> > > > >  			 lock_is_held(&rcu_lock_map) ||
> > > > > -- 
> > > > > 2.38.1
> > > > 
> > > > The following deadlock is able to escape srcu_lock_sync() because the
> > > > __lock_release folded in sync leaves one lock on the sync side.
> > > > 
> > > > 	cpu9		cpu0
> > > > 	---		---
> > > > 	lock A		srcu_lock_acquire(&ssp->dep_map);
> > > > 	srcu_lock_sync(&ssp->dep_map);
> > > > 			lock A
> > > 
> > > But isn't it just the srcu_mutex_ABBA test case in patch #3, and my run
> > > of lockdep selftest shows we can catch it. Anything subtle I'm missing?
> > 
> > I am leaning to not call it ABBA deadlock, because B is unlocked.
> > 
> > 	task X		task Y
> > 	---		---
> > 	lock A
> > 	lock B
> > 			lock B
> > 	unlock B
> > 	wait_for_completion E
> > 
> > 			lock A
> > 			complete E
> > 
> > And no deadlock should be detected/caught after B goes home.
> 
> Your example makes me more confused.. given the case:
> 
> 	task X		task Y
> 	---		---
> 	mutex_lock(A);
> 			srcu_read_lock(B);
> 	synchronze_srcu(B);
> 			mutex_lock(A);
> 
> isn't it a deadlock?

Yes and nope, see below.

> If your example, A, B or E which one is srcu?

A and B are mutex, and E is completion in my example to show the failure
of catching deadlock in case of non-fake lock. Now see srcu after your change.

 	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);
				b) lock_map_acquire_read(&B->dep_map);
	__lock_release(&B->dep_map);
				c) lock_map_acquire_read(&B->dep_map);
 				mutex_lock(A);
 
No deadlock could be detected if taskY takes mutexA after taskX releases B,
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.