[PATCH RT v2 1/3] rcu: Acquire RCU lock when disabling BHs

[Scott Wood <swood@redhat.com>]

A plain local_bh_disable() is documented as creating an RCU critical
section, and (at least) rcutorture expects this to be the case.  However,
in_softirq() doesn't block a grace period on PREEMPT_RT, since RCU checks
preempt_count() directly.  Even if RCU were changed to check
in_softirq(), that wouldn't allow blocked BH disablers to be boosted.

Fix this by calling rcu_read_lock() from local_bh_disable(), and update
rcu_read_lock_bh_held() accordingly.

Signed-off-by: Scott Wood <swood@redhat.com>
---
Another question is whether non-raw spinlocks are intended to create an
RCU read-side critical section due to implicit preempt disable.  If they
are, then we'd need to add rcu_read_lock() there as well since RT doesn't
disable preemption (and rcutorture should explicitly test with a
spinlock).  If not, the documentation should make that clear.

 include/linux/rcupdate.h |  4 ++++
 kernel/rcu/update.c      |  4 ++++
 kernel/softirq.c         | 12 +++++++++---
 3 files changed, 17 insertions(+), 3 deletions(-)

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
 }
 
 /*
@@ -628,10 +630,12 @@ static inline void rcu_read_lock_bh(void)
  */
 static inline void rcu_read_unlock_bh(void)
 {
+#ifndef CONFIG_PREEMPT_RT_FULL
 	RCU_LOCKDEP_WARN(!rcu_is_watching(),
 			 "rcu_read_unlock_bh() used illegally while idle");
 	rcu_lock_release(&rcu_bh_lock_map);
 	__release(RCU_BH);
+#endif
 	local_bh_enable();
 }
 
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 016c66a98292..a9cdf3d562bc 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -296,7 +296,11 @@ int rcu_read_lock_bh_held(void)
 		return 0;
 	if (!rcu_lockdep_current_cpu_online())
 		return 0;
+#ifdef CONFIG_PREEMPT_RT_FULL
+	return lock_is_held(&rcu_lock_map) || irqs_disabled();
+#else
 	return in_softirq() || irqs_disabled();
+#endif
 }
 EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
 
diff --git a/kernel/softirq.c b/kernel/softirq.c
index d16d080a74f7..6080c9328df1 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -115,8 +115,10 @@ void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
 	long soft_cnt;
 
 	WARN_ON_ONCE(in_irq());
-	if (!in_atomic())
+	if (!in_atomic()) {
 		local_lock(bh_lock);
+		rcu_read_lock();
+	}
 	soft_cnt = this_cpu_inc_return(softirq_counter);
 	WARN_ON_ONCE(soft_cnt == 0);
 	current->softirq_count += SOFTIRQ_DISABLE_OFFSET;
@@ -151,8 +153,10 @@ void _local_bh_enable(void)
 #endif
 
 	current->softirq_count -= SOFTIRQ_DISABLE_OFFSET;
-	if (!in_atomic())
+	if (!in_atomic()) {
+		rcu_read_unlock();
 		local_unlock(bh_lock);
+	}
 }
 
 void _local_bh_enable_rt(void)
@@ -185,8 +189,10 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
 	WARN_ON_ONCE(count < 0);
 	local_irq_enable();
 
-	if (!in_atomic())
+	if (!in_atomic()) {
+		rcu_read_unlock();
 		local_unlock(bh_lock);
+	}
 
 	current->softirq_count -= SOFTIRQ_DISABLE_OFFSET;
 	preempt_check_resched();
-- 
1.8.3.1