On Mon, Jun 20, 2022 at 02:42:24PM +0800, Zqiang wrote:
> 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>
Good catch -- that comment is quite misleading.
Is the wordsmithed version shown below OK?
Thanx, Paul
------------------------------------------------------------------------
commit 5ddf65777c8d0f60ff9137eef3b23dd4c77e6108
Author: Zqiang <qiang1.zhang@xxxxxxxxx>
Date: Mon Jun 20 14:42:24 2022 +0800
rcu: Update rcu_preempt_deferred_qs() comments for !PREEMPT kernels
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 <qiang1.zhang@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
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);
