Subject: Re: [PATCH v1 1/6] rcu: Add support for consolidated-RCU reader checking

From: Joel Fernandes <[email-masked]>
Date: Fri, 12 Jul 2019 11:10:51 -0400

On Fri, Jul 12, 2019 at 01:11:25PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 11, 2019 at 07:43:56PM -0400, Joel Fernandes (Google) wrote:
> > +int rcu_read_lock_any_held(void)
> > +{
> > +	int lockdep_opinion = 0;
> > +
> > +	if (!debug_lockdep_rcu_enabled())
> > +		return 1;
> > +	if (!rcu_is_watching())
> > +		return 0;
> > +	if (!rcu_lockdep_current_cpu_online())
> > +		return 0;
> > +
> > +	/* Preemptible RCU flavor */
> > +	if (lock_is_held(&rcu_lock_map))
> 
> you forgot debug_locks here.

Actually, it turns out debug_locks checking is not even needed. If
debug_locks == 0, then debug_lockdep_rcu_enabled() returns 0 and we would not
get to this point.

> > +		return 1;
> > +
> > +	/* BH flavor */
> > +	if (in_softirq() || irqs_disabled())
> 
> I'm not sure I'd put irqs_disabled() under BH, also this entire
> condition is superfluous, see below.
> 
> > +		return 1;
> > +
> > +	/* Sched flavor */
> > +	if (debug_locks)
> > +		lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
> > +	return lockdep_opinion || !preemptible();
> 
> that !preemptible() turns into:
> 
>   !(preempt_count()==0 && !irqs_disabled())
> 
> which is:
> 
>   preempt_count() != 0 || irqs_disabled()
> 
> and already includes irqs_disabled() and in_softirq().
> 
> > +}
> 
> So maybe something lke:
> 
> 	if (debug_locks && (lock_is_held(&rcu_lock_map) ||
> 			    lock_is_held(&rcu_sched_lock_map)))
> 		return true;

Agreed, I will do it this way (without the debug_locks) like:

---8<-----------------------

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index ba861d1716d3..339aebc330db 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -296,27 +296,15 @@ EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
 
 int rcu_read_lock_any_held(void)
 {
-	int lockdep_opinion = 0;
-
 	if (!debug_lockdep_rcu_enabled())
 		return 1;
 	if (!rcu_is_watching())
 		return 0;
 	if (!rcu_lockdep_current_cpu_online())
 		return 0;
-
-	/* Preemptible RCU flavor */
-	if (lock_is_held(&rcu_lock_map))
-		return 1;
-
-	/* BH flavor */
-	if (in_softirq() || irqs_disabled())
-		return 1;
-
-	/* Sched flavor */
-	if (debug_locks)
-		lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
-	return lockdep_opinion || !preemptible();
+	if (lock_is_held(&rcu_lock_map) || lock_is_held(&rcu_sched_lock_map))
+		return 1;
+	return !preemptible();
 }
 EXPORT_SYMBOL_GPL(rcu_read_lock_any_held);