On Thu, Aug 22, 2019 at 09:36:21PM -0500, Scott Wood wrote:

On Wed, 2019-08-21 at 16:33 -0700, Paul E. McKenney wrote:

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On Wed, Aug 21, 2019 at 06:19:04PM -0500, Scott Wood wrote:

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diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 388ace315f32..d6e357378732 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h

@@ -615,10 +615,12 @@ static inline void rcu_read_unlock(void)
 static inline void rcu_read_lock_bh(void)
 {
 	local_bh_disable();
+#ifndef CONFIG_PREEMPT_RT_FULL
 	__acquire(RCU_BH);
 	rcu_lock_acquire(&rcu_bh_lock_map);
 	RCU_LOCKDEP_WARN(!rcu_is_watching(),
 			 "rcu_read_lock_bh() used illegally while idle");
+#endif

Any chance of this using "if (!IS_ENABLED(CONFIG_PREEMPT_RT_FULL))"?
We should be OK providing a do-nothing __maybe_unused rcu_bh_lock_map
for lockdep-enabled -rt kernels, right?

OK.

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@@ -185,8 +189,10 @@ void __local_bh_enable_ip(unsigned long ip,

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unsigned int cnt)

 	WARN_ON_ONCE(count < 0);
 	local_irq_enable();
 
-	if (!in_atomic())
+	if (!in_atomic()) {
+		rcu_read_unlock();
 		local_unlock(bh_lock);
+	}

The return from in_atomic() is guaranteed to be the same at
local_bh_enable() time as was at the call to the corresponding
local_bh_disable()?

That's an existing requirement on RT (which rcutorture currently violates)
due to bh_lock.

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I could have sworn that I ran afoul of this last year.  Might these
added rcu_read_lock() and rcu_read_unlock() calls need to check for
CONFIG_PREEMPT_RT_FULL?

This code is already under a PREEMPT_RT_FULL ifdef.

Good enough, then!

							Thanx, Paul