[RFC PATCH 2/8] rcu/nocb: Prepare to change nocb cpumask from CPU-hotplug protected cpuset caller

[Waiman Long]

From: Frederic Weisbecker <frederic@xxxxxxxxxx>

cpusets is going to use the NOCB (de-)offloading interface while
holding hotplug lock. Therefore pull out the responsibility of protecting
against concurrent CPU-hotplug changes to the callers of
rcu_nocb_cpumask_update().

Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Zefan Li <lizefan.x@xxxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Phil Auld <pauld@xxxxxxxxxx>
Cc: Nicolas Saenz Julienne <nsaenz@xxxxxxxxxx>
Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
Cc: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
 kernel/rcu/rcutorture.c | 2 ++
 kernel/rcu/tree_nocb.h  | 4 ++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 228a5488eb5e..e935152346ff 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -2139,6 +2139,7 @@ static int rcu_nocb_toggle(void *arg)
 	do {
 		r = torture_random(&rand);
 		cpu = (r >> 1) % (maxcpu + 1);
+		cpus_read_lock();
 		if (r & 0x1) {
 			rcu_nocb_cpumask_update(cpumask_of(cpu), true);
 			atomic_long_inc(&n_nocb_offload);
@@ -2146,6 +2147,7 @@ static int rcu_nocb_toggle(void *arg)
 			rcu_nocb_cpumask_update(cpumask_of(cpu), false);
 			atomic_long_inc(&n_nocb_deoffload);
 		}
+		cpus_read_unlock();
 		toggle_delay = torture_random(&rand) % toggle_fuzz + toggle_interval;
 		set_current_state(TASK_INTERRUPTIBLE);
 		schedule_hrtimeout(&toggle_delay, HRTIMER_MODE_REL);
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 60b0a15ed6e2..bbcf6f4152a3 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1301,12 +1301,13 @@ int rcu_nocb_cpumask_update(const struct cpumask *cpumask, bool offload)
 	int err_cpu;
 	cpumask_var_t saved_nocb_mask;
 
+	lockdep_assert_cpus_held();
+
 	if (!alloc_cpumask_var(&saved_nocb_mask, GFP_KERNEL))
 		return -ENOMEM;
 
 	cpumask_copy(saved_nocb_mask, rcu_nocb_mask);
 
-	cpus_read_lock();
 	mutex_lock(&rcu_state.barrier_mutex);
 	for_each_cpu(cpu, cpumask) {
 		if (offload) {
@@ -1340,7 +1341,6 @@ int rcu_nocb_cpumask_update(const struct cpumask *cpumask, bool offload)
 	}
 
 	mutex_unlock(&rcu_state.barrier_mutex);
-	cpus_read_unlock();
 
 	free_cpumask_var(saved_nocb_mask);
 
-- 
2.39.3