rcu: Consolidate PREEMPT and !PREEMPT synchronize_rcu_expedited()

The CONFIG_PREEMPT=n and CONFIG_PREEMPT=y implementations of
synchronize_rcu_expedited() are quite similar, and with small
modifications to rcu_blocking_is_gp() can be made identical.  This commit
therefore makes this change in order to save a few lines of code and to
reduce the amount of duplicate code.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

kernel/rcu/tree_exp.h

@@ -643,6 +643,33 @@ static void _synchronize_rcu_expedited(void)
 	mutex_unlock(&rcu_state.exp_mutex);
 }
 
+/*
+ * During early boot, any blocking grace-period wait automatically
+ * implies a grace period.  Later on, this is never the case for PREEMPT.
+ *
+ * Howevr, because a context switch is a grace period for !PREEMPT, 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_rcu() or synchronize_rcu_expedited().  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.
+ */
+static int rcu_blocking_is_gp(void)
+{
+	int ret;
+
+	if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
+		return true;
+	if (IS_ENABLED(CONFIG_PREEMPT))
+		return false;
+	might_sleep();  /* Check for RCU read-side critical section. */
+	preempt_disable();
+	ret = num_online_cpus() <= 1;
+	preempt_enable();
+	return ret;
+}
+
 #ifdef CONFIG_PREEMPT_RCU
 
 /*
@@ -729,39 +756,6 @@ static void sync_sched_exp_online_cleanup(int cpu)
 {
 }
 
-/**
- * synchronize_rcu_expedited - Brute-force RCU grace period
- *
- * Wait for an RCU-preempt grace period, but expedite it.  The basic
- * idea is to IPI all non-idle non-nohz online CPUs.  The IPI handler
- * checks whether the CPU is in an RCU-preempt critical section, and
- * if so, it sets a flag that causes the outermost rcu_read_unlock()
- * to report the quiescent state.  On the other hand, if the CPU is
- * not in an RCU read-side critical section, the IPI handler reports
- * the quiescent state immediately.
- *
- * Although this is a greate improvement over previous expedited
- * implementations, it is still unfriendly to real-time workloads, so is
- * thus not recommended for any sort of common-case code.  In fact, if
- * you are using synchronize_rcu_expedited() in a loop, please restructure
- * your code to batch your updates, and then Use a single synchronize_rcu()
- * instead.
- *
- * This has the same semantics as (but is more brutal than) synchronize_rcu().
- */
-void synchronize_rcu_expedited(void)
-{
-	RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
-			 lock_is_held(&rcu_lock_map) ||
-			 lock_is_held(&rcu_sched_lock_map),
-			 "Illegal synchronize_rcu_expedited() in RCU read-side critical section");
-
-	if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
-		return;
-	_synchronize_rcu_expedited();
-}
-EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
-
 #else /* #ifdef CONFIG_PREEMPT_RCU */
 
 /* Invoked on each online non-idle CPU for expedited quiescent state. */
@@ -801,27 +795,28 @@ static void sync_sched_exp_online_cleanup(int cpu)
 	WARN_ON_ONCE(ret);
 }
 
-/*
- * Because a context switch is a grace period for !PREEMPT, 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_rcu() or synchronize_rcu_expedited().  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.
- */
-static int rcu_blocking_is_gp(void)
-{
-	int ret;
-
-	might_sleep();  /* Check for RCU read-side critical section. */
-	preempt_disable();
-	ret = num_online_cpus() <= 1;
-	preempt_enable();
-	return ret;
-}
+#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 
-/* PREEMPT=n implementation of synchronize_rcu_expedited(). */
+/**
+ * synchronize_rcu_expedited - Brute-force RCU grace period
+ *
+ * Wait for an RCU grace period, but expedite it.  The basic idea is to
+ * IPI all non-idle non-nohz online CPUs.  The IPI handler checks whether
+ * the CPU is in an RCU critical section, and if so, it sets a flag that
+ * causes the outermost rcu_read_unlock() to report the quiescent state
+ * for RCU-preempt or asks the scheduler for help for RCU-sched.  On the
+ * other hand, if the CPU is not in an RCU read-side critical section,
+ * the IPI handler reports the quiescent state immediately.
+ *
+ * Although this is a greate improvement over previous expedited
+ * implementations, it is still unfriendly to real-time workloads, so is
+ * thus not recommended for any sort of common-case code.  In fact, if
+ * you are using synchronize_rcu_expedited() in a loop, please restructure
+ * your code to batch your updates, and then Use a single synchronize_rcu()
+ * instead.
+ *
+ * This has the same semantics as (but is more brutal than) synchronize_rcu().
+ */
 void synchronize_rcu_expedited(void)
 {
 	RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
@@ -829,12 +824,10 @@ void synchronize_rcu_expedited(void)
 			 lock_is_held(&rcu_sched_lock_map),
 			 "Illegal synchronize_rcu_expedited() in RCU read-side critical section");
 
-	/* If only one CPU, this is automatically a grace period. */
+	/* Is the state is such that the call is a grace period? */
 	if (rcu_blocking_is_gp())
 		return;
 
 	_synchronize_rcu_expedited();
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
-
-#endif /* #else #ifdef CONFIG_PREEMPT_RCU */