rcu: Update tree_exp.h function-header comments

The function-header comments in kernel/rcu/tree_exp.h have gotten a bit
out of date, so this commit updates a number of them.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree_exp.h

@@ -21,7 +21,7 @@ static void rcu_exp_gp_seq_start(void)
 }
 
 /*
- * Return then value that expedited-grace-period counter will have
+ * Return the value that the expedited-grace-period counter will have
  * at the end of the current grace period.
  */
 static __maybe_unused unsigned long rcu_exp_gp_seq_endval(void)
@@ -39,7 +39,9 @@ static void rcu_exp_gp_seq_end(void)
 }
 
 /*
- * Take a snapshot of the expedited-grace-period counter.
+ * Take a snapshot of the expedited-grace-period counter, which is the
+ * earliest value that will indicate that a full grace period has
+ * elapsed since the current time.
  */
 static unsigned long rcu_exp_gp_seq_snap(void)
 {
@@ -143,22 +145,18 @@ static void __maybe_unused sync_exp_reset_tree(void)
  * Return non-zero if there is no RCU expedited grace period in progress
  * for the specified rcu_node structure, in other words, if all CPUs and
  * tasks covered by the specified rcu_node structure have done their bit
- * for the current expedited grace period.  Works only for preemptible
- * RCU -- other RCU implementation use other means.
- *
- * Caller must hold the specificed rcu_node structure's ->lock
+ * for the current expedited grace period.
  */
 static bool sync_rcu_exp_done(struct rcu_node *rnp)
 {
 	raw_lockdep_assert_held_rcu_node(rnp);
-
 	return rnp->exp_tasks == NULL &&
 	       READ_ONCE(rnp->expmask) == 0;
 }
 
 /*
- * Like sync_rcu_exp_done(), but this function assumes the caller doesn't
- * hold the rcu_node's ->lock, and will acquire and release the lock itself
+ * Like sync_rcu_exp_done(), but where the caller does not hold the
+ * rcu_node's ->lock.
  */
 static bool sync_rcu_exp_done_unlocked(struct rcu_node *rnp)
 {
@@ -180,8 +178,6 @@ static bool sync_rcu_exp_done_unlocked(struct rcu_node *rnp)
  * which the task was queued or to one of that rcu_node structure's ancestors,
  * recursively up the tree.  (Calm down, calm down, we do the recursion
  * iteratively!)
- *
- * Caller must hold the specified rcu_node structure's ->lock.
  */
 static void __rcu_report_exp_rnp(struct rcu_node *rnp,
 				 bool wake, unsigned long flags)
@@ -189,6 +185,7 @@ static void __rcu_report_exp_rnp(struct rcu_node *rnp,
 {
 	unsigned long mask;
 
+	raw_lockdep_assert_held_rcu_node(rnp);
 	for (;;) {
 		if (!sync_rcu_exp_done(rnp)) {
 			if (!rnp->expmask)
@@ -452,6 +449,10 @@ static void sync_rcu_exp_select_cpus(void)
 			flush_work(&rnp->rew.rew_work);
 }
 
+/*
+ * Wait for the expedited grace period to elapse, issuing any needed
+ * RCU CPU stall warnings along the way.
+ */
 static void synchronize_sched_expedited_wait(void)
 {
 	int cpu;
@@ -781,7 +782,7 @@ static int rcu_print_task_exp_stall(struct rcu_node *rnp)
  * implementations, it is still unfriendly to real-time workloads, so is
  * thus not recommended for any sort of common-case code.  In fact, if
  * you are using synchronize_rcu_expedited() in a loop, please restructure
- * your code to batch your updates, and then Use a single synchronize_rcu()
+ * your code to batch your updates, and then use a single synchronize_rcu()
  * instead.
  *
  * This has the same semantics as (but is more brutal than) synchronize_rcu().