linux-kernel - [PATCH v4 2/2] rcu: Invert the locking dependency order between rcu_state.barrier

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Message-Id: <20220322131753.1680329-3-qiang1.zhang@intel.com>
Date:   Tue, 22 Mar 2022 21:17:53 +0800
From:   Zqiang <qiang1.zhang@...el.com>
To:     paulmck@...nel.org, frederic@...nel.org
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v4 2/2] rcu: Invert the locking dependency order between rcu_state.barrier_mutex and hotplug lock

When call rcutree_prepare_cpu(), the cpus write lock has been held,
just lock the barrier_mutex before calling rcu_nocb_rdp_deoffload()
from failure path. therefore, invert the locking dependency order.

Signed-off-by: Zqiang <qiang1.zhang@...el.com>
---
 kernel/rcu/tree_nocb.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 154934f3daa9..e3d1bd26d6eb 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1074,8 +1074,8 @@ int rcu_nocb_cpu_deoffload(int cpu)
 	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 	int ret = 0;
 
-	mutex_lock(&rcu_state.barrier_mutex);
 	cpus_read_lock();
+	mutex_lock(&rcu_state.barrier_mutex);
 	if (rcu_rdp_is_offloaded(rdp)) {
 		if (cpu_online(cpu)) {
 			ret = work_on_cpu(cpu, rcu_nocb_rdp_deoffload, rdp);
@@ -1086,8 +1086,8 @@ int rcu_nocb_cpu_deoffload(int cpu)
 			ret = -EINVAL;
 		}
 	}
-	cpus_read_unlock();
 	mutex_unlock(&rcu_state.barrier_mutex);
+	cpus_read_unlock();
 
 	return ret;
 }
@@ -1169,8 +1169,8 @@ int rcu_nocb_cpu_offload(int cpu)
 	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 	int ret = 0;
 
-	mutex_lock(&rcu_state.barrier_mutex);
 	cpus_read_lock();
+	mutex_lock(&rcu_state.barrier_mutex);
 	if (!rcu_rdp_is_offloaded(rdp)) {
 		if (cpu_online(cpu)) {
 			ret = work_on_cpu(cpu, rcu_nocb_rdp_offload, rdp);
@@ -1181,8 +1181,8 @@ int rcu_nocb_cpu_offload(int cpu)
 			ret = -EINVAL;
 		}
 	}
-	cpus_read_unlock();
 	mutex_unlock(&rcu_state.barrier_mutex);
+	cpus_read_unlock();
 
 	return ret;
 }
-- 
2.25.1