linux-kernel - [PATCH 2/3] rcu/nocb: Consolidate rcu_nocb_cpu

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Message-Id: <20260103221117.521087-3-joelagnelf@nvidia.com>
Date: Sat,  3 Jan 2026 17:11:16 -0500
From: Joel Fernandes <joelagnelf@...dia.com>
To: linux-kernel@...r.kernel.org
Cc: "Paul E . McKenney" <paulmck@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	rcu@...r.kernel.org,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang@...ux.dev>,
	Joel Fernandes <joelagnelf@...dia.com>
Subject: [PATCH 2/3] rcu/nocb: Consolidate rcu_nocb_cpu_offload/deoffload functions

The rcu_nocb_cpu_offload() and rcu_nocb_cpu_deoffload() functions are
nearly duplicates.

Therefore, extract the common logic into rcu_nocb_cpu_toggle_offload()
which takes an 'offload' boolean, and make both exported functions
simple wrappers.

This eliminates a bunch of duplicate code at the call sites, namely
mutex locking, CPU hotplug locking and CPU online checks.

Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
---
 kernel/rcu/tree_nocb.h | 73 ++++++++++++++++++++++--------------------
 1 file changed, 38 insertions(+), 35 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index d9687317a2a3..946795ddf7d1 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1108,30 +1108,6 @@ static int rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
 	return 0;
 }
 
-int rcu_nocb_cpu_deoffload(int cpu)
-{
-	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
-	int ret = 0;
-
-	cpus_read_lock();
-	mutex_lock(&rcu_state.nocb_mutex);
-	if (rcu_rdp_is_offloaded(rdp)) {
-		if (!cpu_online(cpu)) {
-			ret = rcu_nocb_rdp_deoffload(rdp);
-			if (!ret)
-				cpumask_clear_cpu(cpu, rcu_nocb_mask);
-		} else {
-			pr_info("NOCB: Cannot CB-deoffload online CPU %d\n", rdp->cpu);
-			ret = -EINVAL;
-		}
-	}
-	mutex_unlock(&rcu_state.nocb_mutex);
-	cpus_read_unlock();
-
-	return ret;
-}
-EXPORT_SYMBOL_GPL(rcu_nocb_cpu_deoffload);
-
 static bool rcu_nocb_rdp_offload_wait_cond(struct rcu_data *rdp)
 {
 	unsigned long flags;
@@ -1176,28 +1152,55 @@ static int rcu_nocb_rdp_offload(struct rcu_data *rdp)
 	return 0;
 }
 
-int rcu_nocb_cpu_offload(int cpu)
+/* Common helper for CPU offload/deoffload operations. */
+static int rcu_nocb_cpu_toggle_offload(int cpu, bool offload)
 {
 	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+	bool currently_offloaded;
 	int ret = 0;
 
 	cpus_read_lock();
 	mutex_lock(&rcu_state.nocb_mutex);
-	if (!rcu_rdp_is_offloaded(rdp)) {
-		if (!cpu_online(cpu)) {
-			ret = rcu_nocb_rdp_offload(rdp);
-			if (!ret)
-				cpumask_set_cpu(cpu, rcu_nocb_mask);
-		} else {
-			pr_info("NOCB: Cannot CB-offload online CPU %d\n", rdp->cpu);
-			ret = -EINVAL;
-		}
+
+	currently_offloaded = rcu_rdp_is_offloaded(rdp);
+
+	/* Already in desired state, nothing to do. */
+	if (currently_offloaded == offload)
+		goto out_unlock;
+
+	if (cpu_online(cpu)) {
+		pr_info("NOCB: Cannot CB-%soffload online CPU %d\n",
+			offload ? "" : "de", rdp->cpu);
+		ret = -EINVAL;
+		goto out_unlock;
 	}
+
+	if (offload) {
+		ret = rcu_nocb_rdp_offload(rdp);
+		if (!ret)
+			cpumask_set_cpu(cpu, rcu_nocb_mask);
+	} else {
+		ret = rcu_nocb_rdp_deoffload(rdp);
+		if (!ret)
+			cpumask_clear_cpu(cpu, rcu_nocb_mask);
+	}
+
+out_unlock:
 	mutex_unlock(&rcu_state.nocb_mutex);
 	cpus_read_unlock();
-
 	return ret;
 }
+
+int rcu_nocb_cpu_deoffload(int cpu)
+{
+	return rcu_nocb_cpu_toggle_offload(cpu, false /* de-offload */);
+}
+EXPORT_SYMBOL_GPL(rcu_nocb_cpu_deoffload);
+
+int rcu_nocb_cpu_offload(int cpu)
+{
+	return rcu_nocb_cpu_toggle_offload(cpu, true /* offload */);
+}
 EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
 
 #ifdef CONFIG_RCU_LAZY
-- 
2.34.1