linux-kernel - [PATCH rcu 07/14] rcu: Comment rcu_gp_init() code waiting for CPU-hotplug operations

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Message-Id: <20210915233343.3906738-7-paulmck@kernel.org>
Date:   Wed, 15 Sep 2021 16:33:36 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.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 rcu 07/14] rcu: Comment rcu_gp_init() code waiting for CPU-hotplug operations

Near the beginning of rcu_gp_init() is a per-rcu_node loop that waits
for CPU-hotplug operations that might have started before the new
grace period did.  This commit adds a comment explaining that this
wait does not exclude CPU-hotplug operations.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0bfebec94277..e6e1b9281530 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1775,6 +1775,8 @@ static noinline_for_stack bool rcu_gp_init(void)
 	 */
 	WRITE_ONCE(rcu_state.gp_state, RCU_GP_ONOFF);
 	rcu_for_each_leaf_node(rnp) {
+		// Wait for CPU-hotplug operations that might have
+		// started before this grace period did.
 		smp_mb(); // Pair with barriers used when updating ->ofl_seq to odd values.
 		firstseq = READ_ONCE(rnp->ofl_seq);
 		if (firstseq & 0x1)
-- 
2.31.1.189.g2e36527f23