rcu-tasks: Mark RCU Tasks accesses to current->rcu_tasks_idle_cpu

The task_struct structure's ->rcu_tasks_idle_cpu can be concurrently
read and written from the RCU Tasks grace-period kthread and from the
CPU on which the task_struct structure's task is running.  This commit
therefore marks the accesses appropriately.
Reported-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.com>

kernel/rcu/tasks.h

@@ -975,7 +975,7 @@ static void check_holdout_task(struct task_struct *t,
 	    t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
 	    !rcu_tasks_is_holdout(t) ||
 	    (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
-	     !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) {
+	     !is_idle_task(t) && READ_ONCE(t->rcu_tasks_idle_cpu) >= 0)) {
 		WRITE_ONCE(t->rcu_tasks_holdout, false);
 		list_del_init(&t->rcu_tasks_holdout_list);
 		put_task_struct(t);
@@ -993,7 +993,7 @@ static void check_holdout_task(struct task_struct *t,
 		 t, ".I"[is_idle_task(t)],
 		 "N."[cpu < 0 || !tick_nohz_full_cpu(cpu)],
 		 t->rcu_tasks_nvcsw, t->nvcsw, t->rcu_tasks_holdout,
-		 t->rcu_tasks_idle_cpu, cpu);
+		 data_race(t->rcu_tasks_idle_cpu), cpu);
 	sched_show_task(t);
 }