linux-kernel - [PATCH 04/16] rcu/nocb: Only (re-)initialize segcblist when needed on CPU up

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Message-Id: <20210128171222.131380-5-frederic@kernel.org>
Date:   Thu, 28 Jan 2021 18:12:10 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Josh Triplett <josh@...htriplett.org>,
        Stable <stable@...r.kernel.org>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: [PATCH 04/16] rcu/nocb: Only (re-)initialize segcblist when needed on CPU up

Simply checking if the segcblist is enabled is enough to know if we
need to initialize it or not. It's safe to check within hotplug
machine.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
Cc: Boqun Feng <boqun.feng@...il.com>
---
 kernel/rcu/tree.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 4c5a1ac54fa6..f74a9ba62c12 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4066,14 +4066,13 @@ int rcutree_prepare_cpu(unsigned int cpu)
 	rdp->dynticks_nesting = 1;	/* CPU not up, no tearing. */
 	rcu_dynticks_eqs_online();
 	raw_spin_unlock_rcu_node(rnp);		/* irqs remain disabled. */
+
 	/*
-	 * Lock in case the CB/GP kthreads are still around handling
-	 * old callbacks.
+	 * Only non-NOCB CPUs that didn't have early-boot callbacks need to be
+	 * (re-)initialized.
 	 */
-	rcu_nocb_lock(rdp);
-	if (rcu_segcblist_empty(&rdp->cblist)) /* No early-boot CBs? */
+	if (!rcu_segcblist_is_enabled(&rdp->cblist))
 		rcu_segcblist_init(&rdp->cblist);  /* Re-enable callbacks. */
-	rcu_nocb_unlock(rdp);
 
 	/*
 	 * Add CPU to leaf rcu_node pending-online bitmask.  Any needed
-- 
2.25.1