workqueue: use rcu_read_lock_sched() instead for accessing pwq in RCU

rcu_read_lock_sched() is better than preempt_disable() if the code is
protected by RCU_SCHED.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -3962,7 +3962,7 @@ bool workqueue_congested(int cpu, struct workqueue_struct *wq)
 	struct pool_workqueue *pwq;
 	bool ret;
 
-	preempt_disable();
+	rcu_read_lock_sched();
 
 	if (!(wq->flags & WQ_UNBOUND))
 		pwq = per_cpu_ptr(wq->cpu_pwqs, cpu);
@@ -3970,7 +3970,7 @@ bool workqueue_congested(int cpu, struct workqueue_struct *wq)
 		pwq = first_pwq(wq);
 
 	ret = !list_empty(&pwq->delayed_works);
-	preempt_enable();
+	rcu_read_unlock_sched();
 
 	return ret;
 }
@@ -4354,16 +4354,16 @@ bool freeze_workqueues_busy(void)
 		 * nr_active is monotonically decreasing.  It's safe
 		 * to peek without lock.
 		 */
-		preempt_disable();
+		rcu_read_lock_sched();
 		for_each_pwq(pwq, wq) {
 			WARN_ON_ONCE(pwq->nr_active < 0);
 			if (pwq->nr_active) {
 				busy = true;
-				preempt_enable();
+				rcu_read_unlock_sched();
 				goto out_unlock;
 			}
 		}
-		preempt_enable();
+		rcu_read_unlock_sched();
 	}
 out_unlock:
 	mutex_unlock(&wq_mutex);