[02/10] rcu: Update rcu_preempt_deferred_qs() comments for !PREEMPT kernels

Commit Message

Paul E. McKenney Aug. 31, 2022, 6:07 p.m. UTC

From: Zqiang <[e-mail]>

In non-premptible kernels, tasks never do context switches within
RCU read-side critical sections.  Therefore, in such kernels, each
leaf rcu_node structure's ->blkd_tasks list will always be empty.
The comment on the non-preemptible version of rcu_preempt_deferred_qs()
confuses this point, so this commit therefore fixes it.

Signed-off-by: Zqiang <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>
---
 kernel/rcu/tree_plugin.h | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

Patch

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 86772c95ed0ae..4152816dd29f6 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -932,10 +932,13 @@  static notrace 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 in
+// non-preemptible kernels, there can be no context switches within RCU
+// read-side critical sections, which in turn means that the leaf rcu_node
+// structure's blocked-tasks list is always empty.  is therefore no need to
+// actually check it.  Instead, a quiescent state from this CPU suffices,
+// and this function is only called from such a quiescent state.
 notrace void rcu_preempt_deferred_qs(struct task_struct *t)
 {
 	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);