linux-kernel - [PATCH tip/core/rcu 20/24] rcu: Clarify comments about FQS loop reporting quiescent states

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Message-Id: <20200831180116.32690-20-paulmck@kernel.org>
Date:   Mon, 31 Aug 2020 11:01:12 -0700
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, 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,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 20/24] rcu: Clarify comments about FQS loop reporting quiescent states

From: "Joel Fernandes (Google)" <joel@...lfernandes.org>

Since at least v4.19, the FQS loop no longer reports quiescent states
for offline CPUs except in emergency situations.

This commit therefore fixes the comment in rcu_gp_init() to match the
current code.

Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 52108dd..2c7afe4 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1706,10 +1706,13 @@ static bool rcu_gp_init(void)
 	raw_spin_unlock_irq_rcu_node(rnp);
 
 	/*
-	 * Apply per-leaf buffered online and offline operations to the
-	 * rcu_node tree.  Note that this new grace period need not wait
-	 * for subsequent online CPUs, and that quiescent-state forcing
-	 * will handle subsequent offline CPUs.
+	 * Apply per-leaf buffered online and offline operations to
+	 * the rcu_node tree. Note that this new grace period need not
+	 * wait for subsequent online CPUs, and that RCU hooks in the CPU
+	 * offlining path, when combined with checks in this function,
+	 * will handle CPUs that are currently going offline or that will
+	 * go offline later.  Please also refer to "Hotplug CPU" section
+	 * of RCU's Requirements documentation.
 	 */
 	rcu_state.gp_state = RCU_GP_ONOFF;
 	rcu_for_each_leaf_node(rnp) {
-- 
2.9.5