diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 0c7db5e..b5a5a1e 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -10,6 +10,7 @@
 #include <linux/kernel.h>
 #include <linux/threads.h>
 #include <linux/bitmap.h>
+#include <linux/atomic.h>
 #include <linux/bug.h>
 
 /* Don't assign or return these: may not be this big! */
@@ -95,8 +96,21 @@ extern struct cpumask __cpu_active_mask;
 #define cpu_present_mask  ((const struct cpumask *)&__cpu_present_mask)
 #define cpu_active_mask   ((const struct cpumask *)&__cpu_active_mask)
 
+extern atomic_t __num_online_cpus;
+
 #if NR_CPUS > 1
-#define num_online_cpus()	cpumask_weight(cpu_online_mask)
+/**
+ * num_online_cpus() - Read the number of online CPUs
+ *
+ * Despite the fact that __num_online_cpus is of type atomic_t, this
+ * interface gives only a momentary snapshot and is not protected against
+ * concurrent CPU hotplug operations unless invoked from a cpuhp_lock held
+ * region.
+ */
+static inline unsigned int num_online_cpus(void)
+{
+	return atomic_read(&__num_online_cpus);
+}
 #define num_possible_cpus()	cpumask_weight(cpu_possible_mask)
 #define num_present_cpus()	cpumask_weight(cpu_present_mask)
 #define num_active_cpus()	cpumask_weight(cpu_active_mask)
@@ -821,14 +835,7 @@ set_cpu_present(unsigned int cpu, bool present)
 		cpumask_clear_cpu(cpu, &__cpu_present_mask);
 }
 
-static inline void
-set_cpu_online(unsigned int cpu, bool online)
-{
-	if (online)
-		cpumask_set_cpu(cpu, &__cpu_online_mask);
-	else
-		cpumask_clear_cpu(cpu, &__cpu_online_mask);
-}
+void set_cpu_online(unsigned int cpu, bool online);
 
 static inline void
 set_cpu_active(unsigned int cpu, bool active)
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 05778e3..e1967e9 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2298,6 +2298,9 @@ EXPORT_SYMBOL(__cpu_present_mask);
 struct cpumask __cpu_active_mask __read_mostly;
 EXPORT_SYMBOL(__cpu_active_mask);
 
+atomic_t __num_online_cpus __read_mostly;
+EXPORT_SYMBOL(__num_online_cpus);
+
 void init_cpu_present(const struct cpumask *src)
 {
 	cpumask_copy(&__cpu_present_mask, src);
@@ -2313,6 +2316,27 @@ void init_cpu_online(const struct cpumask *src)
 	cpumask_copy(&__cpu_online_mask, src);
 }
 
+void set_cpu_online(unsigned int cpu, bool online)
+{
+	/*
+	 * atomic_inc/dec() is required to handle the horrid abuse of this
+	 * function by the reboot and kexec code which invoke it from
+	 * IPI/NMI broadcasts when shutting down CPUs. Invocation from
+	 * regular CPU hotplug is properly serialized.
+	 *
+	 * Note, that the fact that __num_online_cpus is of type atomic_t
+	 * does not protect readers which are not serialized against
+	 * concurrent hotplug operations.
+	 */
+	if (online) {
+		if (!cpumask_test_and_set_cpu(cpu, &__cpu_online_mask))
+			atomic_inc(&__num_online_cpus);
+	} else {
+		if (cpumask_test_and_clear_cpu(cpu, &__cpu_online_mask))
+			atomic_dec(&__num_online_cpus);
+	}
+}
+
 /*
  * Activate the first processor.
  */