rcu: Clean up code based on review feedback from Josh Triplett, part 2

These issues identified during an old-fashioned face-to-face code
review extending over many hours.

o	Add comments for tricky parts of code, and correct comments
	that have passed their sell-by date.

o	Get rid of the vestiges of rcu_init_sched(), which is no
	longer needed now that PREEMPT_RCU is gone.

o	Move the #include of rcutree_plugin.h to the end of
	rcutree.c, which means that, rather than having a random
	collection of forward declarations, the new set of forward
	declarations document the set of plugins.  The new home for
	this #include also allows __rcu_init_preempt() to move into
	rcutree_plugin.h.

o	Fix rcu_preempt_check_callbacks() to be static.

Suggested-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: akpm@linux-foundation.org
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <12537246443924-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Peter Zijlstra <peterz@infradead.org>

include/linux/rcupdate.h

@@ -196,6 +196,8 @@ static inline void rcu_read_lock_sched(void)
 	__acquire(RCU_SCHED);
 	rcu_read_acquire();
 }
+
+/* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
 static inline notrace void rcu_read_lock_sched_notrace(void)
 {
 	preempt_disable_notrace();
@@ -213,6 +215,8 @@ static inline void rcu_read_unlock_sched(void)
 	__release(RCU_SCHED);
 	preempt_enable();
 }
+
+/* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
 static inline notrace void rcu_read_unlock_sched_notrace(void)
 {
 	__release(RCU_SCHED);

include/linux/rcutree.h

@@ -90,10 +90,6 @@ extern long rcu_batches_completed(void);
 extern long rcu_batches_completed_bh(void);
 extern long rcu_batches_completed_sched(void);
 
-static inline void rcu_init_sched(void)
-{
-}
-
 #ifdef CONFIG_NO_HZ
 void rcu_enter_nohz(void);
 void rcu_exit_nohz(void);
@@ -106,7 +102,7 @@ static inline void rcu_exit_nohz(void)
 }
 #endif /* CONFIG_NO_HZ */
 
-/* A context switch is a grace period for rcutree. */
+/* A context switch is a grace period for RCU-sched and RCU-bh. */
 static inline int rcu_blocking_is_gp(void)
 {
 	return num_online_cpus() == 1;