[PATCH] rcu: Update rcu_preempt_deferred_qs() comments for no preemptible RCU

[Zqiang]

For no preemptible RCU, the rcu_preempt_deferred_qs() only report
expedited QS state of current CPU, since preemption does not occur
in RCU critical sections, there are no tasks insert to leaf rnp
blocked-tasks list, that is to say the tasks structure's ->rcu_blocked_node
and the leaf rnp structure's  ->blkd_tasks are always empty.

Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
---
 kernel/rcu/tree_plugin.h | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index dc78726b993f..99424c2da5db 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -932,10 +932,12 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
 	return false;
 }
 
-// Except that we do need to respond to a request by an expedited grace
-// period for a quiescent state from this CPU.  Note that requests from
-// tasks are handled when removing the task from the blocked-tasks list
-// below.
+/*
+ * Except that we do need to respond to a request by an expedited grace
+ * period for a quiescent state from this CPU.  Note that for no preemptible
+ * RCU, since preemption does not occur in RCU critical sections, so the
+ * leaf rnp's blocked-tasks list is always empty.
+ */
 void rcu_preempt_deferred_qs(struct task_struct *t)
 {
 	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
-- 
2.25.1