diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0ccdca441ddf..39e14cf6a9c0 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2396,6 +2396,7 @@ int rcutree_dead_cpu(unsigned int cpu)
 	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
 		return 0;
 
+	WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
 	/* Adjust any no-longer-needed kthreads. */
 	rcu_boost_kthread_setaffinity(rnp, -1);
 	/* Do any needed no-CB deferred wakeups from this CPU. */
@@ -3577,7 +3578,20 @@ static int rcu_blocking_is_gp(void)
 		return rcu_scheduler_active == RCU_SCHEDULER_INACTIVE;
 	might_sleep();  /* Check for RCU read-side critical section. */
 	preempt_disable();
-	ret = num_online_cpus() <= 1;
+	/*
+	 * If the rcu_state.n_online_cpus counter is equal to one,
+	 * there is only one CPU, and that CPU sees all prior accesses
+	 * made by any CPU that was online at the time of its access.
+	 * Furthermore, if this counter is equal to one, its value cannot
+	 * change until after the preempt_enable() below.
+	 *
+	 * Furthermore, if rcu_state.n_online_cpus is equal to one here,
+	 * all later CPUs (both this one and any that come online later
+	 * on) are guaranteed to see all accesses prior to this point
+	 * in the code, without the need for additional memory barriers.
+	 * Those memory barriers are provided by CPU-hotplug code.
+	 */
+	ret = READ_ONCE(rcu_state.n_online_cpus) <= 1;
 	preempt_enable();
 	return ret;
 }
@@ -3622,7 +3636,7 @@ void synchronize_rcu(void)
 			 lock_is_held(&rcu_sched_lock_map),
 			 "Illegal synchronize_rcu() in RCU read-side critical section");
 	if (rcu_blocking_is_gp())
-		return;
+		return;  // Context allows vacuous grace periods.
 	if (rcu_gp_is_expedited())
 		synchronize_rcu_expedited();
 	else
@@ -3962,6 +3976,7 @@ int rcutree_prepare_cpu(unsigned int cpu)
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	rcu_prepare_kthreads(cpu);
 	rcu_spawn_cpu_nocb_kthread(cpu);
+	WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus + 1);
 
 	return 0;
 }
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index e4f66b8f7c47..805c9eb6f7ae 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -298,6 +298,7 @@ struct rcu_state {
 						/* Hierarchy levels (+1 to */
 						/*  shut bogus gcc warning) */
 	int ncpus;				/* # CPUs seen so far. */
+	int n_online_cpus;			/* # CPUs online for RCU. */
 
 	/* The following fields are guarded by the root rcu_node's lock. */