Hello Paul and RCU folks,
I am afraid I correctly understand and fix it. But I really wonder why
sync_rcu_exp_handler() reports the quiescent state even in the case that
current task is within a RCU read-side section. Do I miss something?
If I correctly understand it and you agree with it, I can add more logic
which make it more expedited by boosting current or making it urgent
when we fail to report the quiescent state on the IPI.
----->8-----
>From 0b0191f506c19ce331a1fdb7c2c5a00fb23fbcf2 Mon Sep 17 00:00:00 2001
From: Byungchul Park <byungchul.p...@lge.com>
Date: Tue, 6 Mar 2018 13:54:41 +0900
Subject: [RFC] rcu: Prevent expedite reporting within RCU read-side section
We report the quiescent state for this cpu if it's out of RCU read-side
section at the moment IPI was just fired during the expedite process.
However, current code reports the quiescent state even in the case:
1) the current task is still within a RCU read-side section
2) the current task has been blocked within the RCU read-side section
Since we don't get to the quiescent state yet in the case, we shouldn't
report it but check it another time.
Signed-off-by: Byungchul Park <byungchul.p...@lge.com>
---
kernel/rcu/tree_exp.h | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 73e1d3d..cc69d14 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -731,13 +731,13 @@ static void sync_rcu_exp_handler(void *info)
/*
* We are either exiting an RCU read-side critical section (negative
* values of t->rcu_read_lock_nesting) or are not in one at all
- * (zero value of t->rcu_read_lock_nesting). Or we are in an RCU
- * read-side critical section that blocked before this expedited
- * grace period started. Either way, we can immediately report
- * the quiescent state.
+ * (zero value of t->rcu_read_lock_nesting). We can immediately
+ * report the quiescent state.
*/
- rdp = this_cpu_ptr(rsp->rda);
- rcu_report_exp_rdp(rsp, rdp, true);
+ if (t->rcu_read_lock_nesting <= 0) {
+ rdp = this_cpu_ptr(rsp->rda);
+ rcu_report_exp_rdp(rsp, rdp, true);
+ }
}
/**
--
1.9.1
