rcu: add comment stating that list_empty() applies to RCU-protected lists

Because list_empty() does not dereference any RCU-protected pointers, and
further does not pass such pointers to the caller (so that the caller
does not dereference them either), it is safe to use list_empty() on
RCU-protected lists.  There is no need for a list_empty_rcu().  This
commit adds a comment stating this explicitly.
Requested-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/rculist.h

@@ -9,6 +9,15 @@
 #include <linux/list.h>
 #include <linux/rcupdate.h>
 
+/*
+ * Why is there no list_empty_rcu()?  Because list_empty() serves this
+ * purpose.  The list_empty() function fetches the RCU-protected pointer
+ * and compares it to the address of the list head, but neither dereferences
+ * this pointer itself nor provides this pointer to the caller.  Therefore,
+ * it is not necessary to use rcu_dereference(), so that list_empty() can
+ * be used anywhere you would want to use a list_empty_rcu().
+ */
+
 /*
  * return the ->next pointer of a list_head in an rcu safe
  * way, we must not access it directly