2 files changed, 32 insertions, 7 deletions

diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index e8ee5dd0854c..b06363055ef8 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -98,13 +98,6 @@ extern void rcu_force_quiescent_state(void);
 extern void rcu_bh_force_quiescent_state(void);
 extern void rcu_sched_force_quiescent_state(void);
 
-/* A context switch is a grace period for RCU-sched and RCU-bh. */
-static inline int rcu_blocking_is_gp(void)
-{
-        might_sleep();  /* Check for RCU read-side critical section. */
-        return num_online_cpus() == 1;
-}
-
 extern void rcu_scheduler_starting(void);
 extern int rcu_scheduler_active __read_mostly;
 
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 780acf8e15e9..8f6a344306e6 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1894,6 +1894,38 @@ void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
 }
 EXPORT_SYMBOL_GPL(call_rcu_bh);
 
+/*
+ * Because a context switch is a grace period for RCU-sched and RCU-bh,
+ * any blocking grace-period wait automatically implies a grace period
+ * if there is only one CPU online at any point time during execution
+ * of either synchronize_sched() or synchronize_rcu_bh().  It is OK to
+ * occasionally incorrectly indicate that there are multiple CPUs online
+ * when there was in fact only one the whole time, as this just adds
+ * some overhead: RCU still operates correctly.
+ *
+ * Of course, sampling num_online_cpus() with preemption enabled can
+ * give erroneous results if there are concurrent CPU-hotplug operations.
+ * For example, given a demonic sequence of preemptions in num_online_cpus()
+ * and CPU-hotplug operations, there could be two or more CPUs online at
+ * all times, but num_online_cpus() might well return one (or even zero).
+ *
+ * However, all such demonic sequences require at least one CPU-offline
+ * operation.  Furthermore, rcu_blocking_is_gp() giving the wrong answer
+ * is only a problem if there is an RCU read-side critical section executing
+ * throughout.  But RCU-sched and RCU-bh read-side critical sections
+ * disable either preemption or bh, which prevents a CPU from going offline.
+ * Therefore, the only way that rcu_blocking_is_gp() can incorrectly return
+ * that there is only one CPU when in fact there was more than one throughout
+ * is when there were no RCU readers in the system.  If there are no
+ * RCU readers, the grace period by definition can be of zero length,
+ * regardless of the number of online CPUs.
+ */
+static inline int rcu_blocking_is_gp(void)
+{
+        might_sleep();  /* Check for RCU read-side critical section. */
+        return num_online_cpus() <= 1;
+}
+
 /**
  * synchronize_sched - wait until an rcu-sched grace period has elapsed.
  *