[PATCH RT v3 1/5] rcu: Acquire RCU lock when disabling BHs

[Scott Wood]

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@xxxxxxxxxx>
---
v3: Remove change to rcu_read_lock_bh_held(), and move debug portions
of rcu_read_[un]lock_bh() to separate functions
---
 include/linux/rcupdate.h | 40 ++++++++++++++++++++++++++++++++--------
 kernel/softirq.c         | 12 +++++++++---
 2 files changed, 41 insertions(+), 11 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 388ace315f32..9ce7c5006a5e 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -600,6 +600,36 @@ static inline void rcu_read_unlock(void)
 	rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */
 }
 
+#ifdef CONFIG_PREEMPT_RT_FULL
+/*
+ * On RT, local_bh_disable() calls rcu_read_lock() -- no need to
+ * track it twice.
+ */
+static inline void rcu_bh_lock_acquire(void)
+{
+}
+
+static inline void rcu_bh_lock_release(void)
+{
+}
+#else
+static inline void rcu_bh_lock_acquire(void)
+{
+	__acquire(RCU_BH);
+	rcu_lock_acquire(&rcu_bh_lock_map);
+	RCU_LOCKDEP_WARN(!rcu_is_watching(),
+			 "rcu_read_lock_bh() used illegally while idle");
+}
+
+static inline void rcu_bh_lock_release(void)
+{
+	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
+
 /**
  * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
  *
@@ -615,10 +645,7 @@ static inline void rcu_read_unlock(void)
 static inline void rcu_read_lock_bh(void)
 {
 	local_bh_disable();
-	__acquire(RCU_BH);
-	rcu_lock_acquire(&rcu_bh_lock_map);
-	RCU_LOCKDEP_WARN(!rcu_is_watching(),
-			 "rcu_read_lock_bh() used illegally while idle");
+	rcu_bh_lock_acquire();
 }
 
 /*
@@ -628,10 +655,7 @@ static inline void rcu_read_lock_bh(void)
  */
 static inline void rcu_read_unlock_bh(void)
 {
-	RCU_LOCKDEP_WARN(!rcu_is_watching(),
-			 "rcu_read_unlock_bh() used illegally while idle");
-	rcu_lock_release(&rcu_bh_lock_map);
-	__release(RCU_BH);
+	rcu_bh_lock_release();
 	local_bh_enable();
 }
 
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