rcu: Rename rcu_process_callbacks() to rcu_core() for Tree RCU

Although the name rcu_process_callbacks() still makes sense for Tiny
RCU, where most of what it does is invoke callbacks, it no longer makes
much sense for Tree RCU, especially given that the actually callback
invocation is relegated to rcu_do_batch(), or, for no-CBs CPUs, to the
rcuo kthreads.  Especially in the latter case, rcu_process_callbacks()
has very little to do with actual callbacks.  A better description of
this function is that it performs RCU's core processing.

This commit therefore changes the name of Tree RCU's rcu_process_callbacks()
function to rcu_core(), which also has the virtue of being consistent with
the existing invoke_rcu_core() function.

While in the area, the header comment is reworked.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html

@@ -491,7 +491,7 @@ <h4><a name="Self-Reported Quiescent States">
 
 <p>The next time an <tt>RCU_SOFTIRQ</tt> handler executes on
 this CPU (for example, after the next scheduler-clock
-interrupt), <tt>__rcu_process_callbacks()</tt> will invoke
+interrupt), <tt>rcu_core()</tt> will invoke
 <tt>rcu_check_quiescent_state()</tt>, which will notice the
 recorded quiescent state, and invoke
 <tt>rcu_report_qs_rdp()</tt>.

kernel/rcu/tree.c

@@ -2742,12 +2742,8 @@ void rcu_fwd_progress_check(unsigned long j)
 }
 EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
 
-/*
- * This does the RCU core processing work for the specified rcu_data
- * structures.  This may be called only from the CPU to whom the rdp
- * belongs.
- */
-static __latent_entropy void rcu_process_callbacks(struct softirq_action *unused)
+/* Perform RCU core processing work for the current CPU.  */
+static __latent_entropy void rcu_core(struct softirq_action *unused)
 {
 	unsigned long flags;
 	struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
@@ -3856,7 +3852,7 @@ void __init rcu_init(void)
 	rcu_init_one();
 	if (dump_tree)
 		rcu_dump_rcu_node_tree();
-	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
+	open_softirq(RCU_SOFTIRQ, rcu_core);
 
 	/*
 	 * We don't need protection against CPU-hotplug here because