rcu: Point to documentation of ordering guarantees

Add comments to synchronize_rcu() and friends that point to
Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/srcutree.c

@@ -1000,6 +1000,9 @@ EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
  * synchronize_srcu(), srcu_read_lock(), and srcu_read_unlock() are
  * passed the same srcu_struct structure.
  *
+ * Implementation of these memory-ordering guarantees is similar to
+ * that of synchronize_rcu().
+ *
  * If SRCU is likely idle, expedite the first request.  This semantic
  * was provided by Classic SRCU, and is relied upon by its users, so TREE
  * SRCU must also provide it.  Note that detecting idleness is heuristic

kernel/rcu/tree.c

@@ -3084,6 +3084,9 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
  * between the call to call_rcu() and the invocation of "func()" -- even
  * if CPU A and CPU B are the same CPU (but again only if the system has
  * more than one CPU).
+ *
+ * Implementation of these memory-ordering guarantees is described here:
+ * Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst.
  */
 void call_rcu(struct rcu_head *head, rcu_callback_t func)
 {
@@ -3751,6 +3754,9 @@ static int rcu_blocking_is_gp(void)
  * to have executed a full memory barrier during the execution of
  * synchronize_rcu() -- even if CPU A and CPU B are the same CPU (but
  * again only if the system has more than one CPU).
+ *
+ * Implementation of these memory-ordering guarantees is described here:
+ * Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst.
  */
 void synchronize_rcu(void)
 {
@@ -3821,7 +3827,7 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu);
 /**
  * poll_state_synchronize_rcu - Conditionally wait for an RCU grace period
  *
- * @oldstate: return from call to get_state_synchronize_rcu() or start_poll_synchronize_rcu()
+ * @oldstate: value from get_state_synchronize_rcu() or start_poll_synchronize_rcu()
  *
  * If a full RCU grace period has elapsed since the earlier call from
  * which oldstate was obtained, return @true, otherwise return @false.
@@ -3837,6 +3843,11 @@ EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu);
  * (many hours even on 32-bit systems) should check them occasionally
  * and either refresh them or set a flag indicating that the grace period
  * has completed.
+ *
+ * This function provides the same memory-ordering guarantees that
+ * would be provided by a synchronize_rcu() that was invoked at the call
+ * to the function that provided @oldstate, and that returned at the end
+ * of this function.
  */
 bool poll_state_synchronize_rcu(unsigned long oldstate)
 {
@@ -3851,7 +3862,7 @@ EXPORT_SYMBOL_GPL(poll_state_synchronize_rcu);
 /**
  * cond_synchronize_rcu - Conditionally wait for an RCU grace period
  *
- * @oldstate: return value from earlier call to get_state_synchronize_rcu()
+ * @oldstate: value from get_state_synchronize_rcu() or start_poll_synchronize_rcu()
  *
  * If a full RCU grace period has elapsed since the earlier call to
  * get_state_synchronize_rcu() or start_poll_synchronize_rcu(), just return.
@@ -3861,6 +3872,11 @@ EXPORT_SYMBOL_GPL(poll_state_synchronize_rcu);
  * counter wrap is harmless.  If the counter wraps, we have waited for
  * more than 2 billion grace periods (and way more on a 64-bit system!),
  * so waiting for one additional grace period should be just fine.
+ *
+ * This function provides the same memory-ordering guarantees that
+ * would be provided by a synchronize_rcu() that was invoked at the call
+ * to the function that provided @oldstate, and that returned at the end
+ * of this function.
  */
 void cond_synchronize_rcu(unsigned long oldstate)
 {