rcu: Don't check irq nesting from rcu idle entry/exit

Because tasks do not nest, rcu_idle_enter() and rcu_idle_exit() do
not need to check for nesting.  This commit therefore moves nesting
checks from rcu_idle_enter_common() to rcu_irq_exit() and from
rcu_idle_exit_common() to rcu_irq_enter().

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcutree.c

@@ -350,10 +350,6 @@ static int rcu_implicit_offline_qs(struct rcu_data *rdp)
  */
 static void rcu_idle_enter_common(struct rcu_dynticks *rdtp, long long oldval)
 {
-	if (rdtp->dynticks_nesting) {
-		trace_rcu_dyntick("--=", oldval, rdtp->dynticks_nesting);
-		return;
-	}
 	trace_rcu_dyntick("Start", oldval, rdtp->dynticks_nesting);
 	if (!is_idle_task(current)) {
 		struct task_struct *idle = idle_task(smp_processor_id());
@@ -426,7 +422,10 @@ void rcu_irq_exit(void)
 	oldval = rdtp->dynticks_nesting;
 	rdtp->dynticks_nesting--;
 	WARN_ON_ONCE(rdtp->dynticks_nesting < 0);
-	rcu_idle_enter_common(rdtp, oldval);
+	if (rdtp->dynticks_nesting)
+		trace_rcu_dyntick("--=", oldval, rdtp->dynticks_nesting);
+	else
+		rcu_idle_enter_common(rdtp, oldval);
 	local_irq_restore(flags);
 }
 
@@ -439,10 +438,6 @@ void rcu_irq_exit(void)
  */
 static void rcu_idle_exit_common(struct rcu_dynticks *rdtp, long long oldval)
 {
-	if (oldval) {
-		trace_rcu_dyntick("++=", oldval, rdtp->dynticks_nesting);
-		return;
-	}
 	smp_mb__before_atomic_inc();  /* Force ordering w/previous sojourn. */
 	atomic_inc(&rdtp->dynticks);
 	/* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
@@ -518,7 +513,10 @@ void rcu_irq_enter(void)
 	oldval = rdtp->dynticks_nesting;
 	rdtp->dynticks_nesting++;
 	WARN_ON_ONCE(rdtp->dynticks_nesting == 0);
-	rcu_idle_exit_common(rdtp, oldval);
+	if (oldval)
+		trace_rcu_dyntick("++=", oldval, rdtp->dynticks_nesting);
+	else
+		rcu_idle_exit_common(rdtp, oldval);
 	local_irq_restore(flags);
 }