linux-kernel - [PATCH tip/core/rcu 15/18] rcu: Fix force_qs

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Date:   Tue, 26 Mar 2019 16:13:11 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        Zhouyi Zhou <zhouzhouyi@...il.com>,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH tip/core/rcu 15/18] rcu: Fix force_qs_rnp() header comment

From: Zhouyi Zhou <zhouzhouyi@...il.com>

Previously, threads blocked on offlining CPUS were migrated to the
root rcu_node structure, thus requiring RCU priority boosting on this
structure.  However, since commit d19fb8d1f3f6 ("rcu: Don't migrate
blocked tasks even if all corresponding CPUs offline"), RCU does not
migrate blocked tasks.  Consequently, RCU no longer does RCU priority
boosting on the root rcu_node structure as of commit 1be0085b515e ("rcu:
Don't initiate RCU priority boosting on root rcu_node").

This commit therefore brings comments for the force_qs_rnp() function's
header comment in line with this new no-root-boosting reality.

Signed-off-by: Zhouyi Zhou <zhouzhouyi@...il.com>
[ paulmck: Also remove obsolete comment on suppressing new grace periods. ]
Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
---
 kernel/rcu/tree.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e117732bcd5d..abc8512ceb5f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2548,11 +2548,11 @@ void rcu_sched_clock_irq(int user)
 }
 
 /*
- * Scan the leaf rcu_node structures, processing dyntick state for any that
- * have not yet encountered a quiescent state, using the function specified.
- * Also initiate boosting for any threads blocked on the root rcu_node.
- *
- * The caller must have suppressed start of new grace periods.
+ * Scan the leaf rcu_node structures.  For each structure on which all
+ * CPUs have reported a quiescent state and on which there are tasks
+ * blocking the current grace period, initiate RCU priority boosting.
+ * Otherwise, invoke the specified function to check dyntick state for
+ * each CPU that has not yet reported a quiescent state.
  */
 static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
 {
-- 
2.17.1