rcu: Add *_ONCE() to rcu_node ->boost_kthread_status

The rcu_node structure's ->boost_kthread_status field is accessed
locklessly, so this commit causes all updates to use WRITE_ONCE() and
all reads to use READ_ONCE().

This data race was reported by KCSAN.  Not appropriate for backporting
due to failure being unlikely.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree_plugin.h

@@ -1032,18 +1032,18 @@ static int rcu_boost_kthread(void *arg)
 
 	trace_rcu_utilization(TPS("Start boost kthread@init"));
 	for (;;) {
-		rnp->boost_kthread_status = RCU_KTHREAD_WAITING;
+		WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_WAITING);
 		trace_rcu_utilization(TPS("End boost kthread@rcu_wait"));
 		rcu_wait(rnp->boost_tasks || rnp->exp_tasks);
 		trace_rcu_utilization(TPS("Start boost kthread@rcu_wait"));
-		rnp->boost_kthread_status = RCU_KTHREAD_RUNNING;
+		WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_RUNNING);
 		more2boost = rcu_boost(rnp);
 		if (more2boost)
 			spincnt++;
 		else
 			spincnt = 0;
 		if (spincnt > 10) {
-			rnp->boost_kthread_status = RCU_KTHREAD_YIELDING;
+			WRITE_ONCE(rnp->boost_kthread_status, RCU_KTHREAD_YIELDING);
 			trace_rcu_utilization(TPS("End boost kthread@rcu_yield"));
 			schedule_timeout_interruptible(2);
 			trace_rcu_utilization(TPS("Start boost kthread@rcu_yield"));
@@ -1082,7 +1082,7 @@ static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
 			rnp->boost_tasks = rnp->gp_tasks;
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 		rcu_wake_cond(rnp->boost_kthread_task,
-			      rnp->boost_kthread_status);
+			      READ_ONCE(rnp->boost_kthread_status));
 	} else {
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	}