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

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Message-ID: <Y8H0twLVEnM6jkgq@boqun-archlinux>
Date:   Fri, 13 Jan 2023 16:17:59 -0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 2/3] rcu: Equip sleepable RCU with lockdep dependency
 graph checks

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@...il.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@...il.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? If your example, A, B or E which one is srcu?

Regards,
Boqun