rcu: Inline rcu_preempt_do_callback() into its sole caller

The rcu_preempt_do_callbacks() function was introduced in commit
09223371dea(rcu: Use softirq to address performance regression), where it
was necessary to handle kernel builds both containing and not containing
RCU-preempt.  Since then, various changes (most notably f8b7fc6b51
("rcu: use softirq instead of kthreads except when RCU_BOOST=y")) have
resulted in this function being invoked only from rcu_kthread_do_work(),
which is present only in kernels containing RCU-preempt, which in turn
means that the rcu_preempt_do_callbacks() function is no longer needed.

This commit therefore inlines rcu_preempt_do_callbacks() into its
sole remaining caller and also removes the rcu_state_p and rcu_data_p
indirection for added clarity.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
[ paulmck: Remove the rcu_state_p and rcu_data_p indirection. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>

kernel/rcu/tree.h

@@ -438,7 +438,6 @@ static void rcu_preempt_boost_start_gp(struct rcu_node *rnp);
 static void invoke_rcu_callbacks_kthread(void);
 static bool rcu_is_callbacks_kthread(void);
 #ifdef CONFIG_RCU_BOOST
-static void rcu_preempt_do_callbacks(void);
 static int rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
 						 struct rcu_node *rnp);
 #endif /* #ifdef CONFIG_RCU_BOOST */

kernel/rcu/tree_plugin.h

@@ -685,15 +685,6 @@ static void rcu_preempt_check_callbacks(void)
 		t->rcu_read_unlock_special.b.need_qs = true;
 }
 
-#ifdef CONFIG_RCU_BOOST
-
-static void rcu_preempt_do_callbacks(void)
-{
-	rcu_do_batch(rcu_state_p, this_cpu_ptr(rcu_data_p));
-}
-
-#endif /* #ifdef CONFIG_RCU_BOOST */
-
 /**
  * call_rcu() - Queue an RCU callback for invocation after a grace period.
  * @head: structure to be used for queueing the RCU updates.
@@ -1140,7 +1131,7 @@ static void rcu_kthread_do_work(void)
 {
 	rcu_do_batch(&rcu_sched_state, this_cpu_ptr(&rcu_sched_data));
 	rcu_do_batch(&rcu_bh_state, this_cpu_ptr(&rcu_bh_data));
-	rcu_preempt_do_callbacks();
+	rcu_do_batch(&rcu_preempt_state, this_cpu_ptr(&rcu_preempt_data));
 }
 
 static void rcu_cpu_kthread_setup(unsigned int cpu)