[PATCH rcu 2/7] rcu/nocb: Invert rcu_state.barrier_mutex VS hotplug lock locking order

[Paul E. McKenney]

From: Zqiang <qiang1.zhang@xxxxxxxxx>

In case of failure to spawn either rcuog or rcuo[p] kthreads for a given
rdp, rcu_nocb_rdp_deoffload() needs to be called with the hotplug
lock and the barrier_mutex held. However cpus write lock is already held
while calling rcutree_prepare_cpu(). It's not possible to call
rcu_nocb_rdp_deoffload() from there with just locking the barrier_mutex
or this would result in a locking inversion against
rcu_nocb_cpu_deoffload() which holds both locks in the reverse order.

Simply solve this with inverting the locking order inside
rcu_nocb_cpu_[de]offload(). This will be a pre-requisite to toggle NOCB
states toward cpusets anyway.

Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Uladzislau Rezki <uladzislau.rezki@xxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 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 dac74952e1d1b..f2f2cab6285a1 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1055,8 +1055,8 @@ 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);
@@ -1067,8 +1067,8 @@ int rcu_nocb_cpu_deoffload(int cpu)
 			ret = -EINVAL;
 		}
 	}
-	cpus_read_unlock();
 	mutex_unlock(&rcu_state.barrier_mutex);
+	cpus_read_unlock();
 
 	return ret;
 }
@@ -1134,8 +1134,8 @@ 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);
@@ -1146,8 +1146,8 @@ int rcu_nocb_cpu_offload(int cpu)
 			ret = -EINVAL;
 		}
 	}
-	cpus_read_unlock();
 	mutex_unlock(&rcu_state.barrier_mutex);
+	cpus_read_unlock();
 
 	return ret;
 }
-- 
2.31.1.189.g2e36527f23