rcu: Move __rcu_read_unlock()'s barrier() within if-statement

We only need to constrain the compiler if we are actually exiting
the top-level RCU read-side critical section.  This commit therefore
moves the first barrier() cal in __rcu_read_unlock() to inside the
"if" statement, thus avoiding needless register flushes for inner
rcu_read_unlock() calls.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

2 files changed, 3 insertions, 13 deletions

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index af186e260c43..2cf4226ade7e 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -134,16 +134,6 @@ extern void call_rcu_sched(struct rcu_head *head,
 
 extern void synchronize_sched(void);
 
-static inline void __rcu_read_lock_bh(void)
-{
-        local_bh_disable();
-}
-
-static inline void __rcu_read_unlock_bh(void)
-{
-        local_bh_enable();
-}
-
 #ifdef CONFIG_PREEMPT_RCU
 
 extern void __rcu_read_lock(void);
@@ -686,7 +676,7 @@ static inline void rcu_read_unlock(void)
  */
 static inline void rcu_read_lock_bh(void)
 {
-        __rcu_read_lock_bh();
+        local_bh_disable();
         __acquire(RCU_BH);
         rcu_read_acquire_bh();
 }
@@ -700,7 +690,7 @@ static inline void rcu_read_unlock_bh(void)
 {
         rcu_read_release_bh();
         __release(RCU_BH);
-        __rcu_read_unlock_bh();
+        local_bh_enable();
 }
 
 /**
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 4bac5a29fb69..ed70f6bf4c31 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -415,10 +415,10 @@ void __rcu_read_unlock(void)
 {
         struct task_struct *t = current;
 
-        barrier();  /* needed if we ever invoke rcu_read_unlock in rcutree.c */
         if (t->rcu_read_lock_nesting != 1)
                 --t->rcu_read_lock_nesting;
         else {
+                barrier();  /* critical section before exit code. */
                 t->rcu_read_lock_nesting = INT_MIN;
                 barrier();  /* assign before ->rcu_read_unlock_special load */
                 if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))