rcu: Make __call_rcu() handle invocation from idle

Although __call_rcu() is handled correctly when called from a momentary
non-idle period, if it is called on a CPU that RCU believes to be idle
on RCU_FAST_NO_HZ kernels, the callback might be indefinitely postponed.
This commit therefore ensures that RCU is aware of the new callback and
has a chance to force the CPU out of dyntick-idle mode when a new callback
is posted.

Reported-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/rcutree.c

@@ -585,8 +585,6 @@ void rcu_nmi_exit(void)
 	WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);
 }
 
-#ifdef CONFIG_PROVE_RCU
-
 /**
  * rcu_is_cpu_idle - see if RCU thinks that the current CPU is idle
  *
@@ -604,7 +602,7 @@ int rcu_is_cpu_idle(void)
 }
 EXPORT_SYMBOL(rcu_is_cpu_idle);
 
-#ifdef CONFIG_HOTPLUG_CPU
+#if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU)
 
 /*
  * Is the current CPU online?  Disable preemption to avoid false positives
@@ -645,9 +643,7 @@ bool rcu_lockdep_current_cpu_online(void)
 }
 EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online);
 
-#endif /* #ifdef CONFIG_HOTPLUG_CPU */
-
-#endif /* #ifdef CONFIG_PROVE_RCU */
+#endif /* #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU) */
 
 /**
  * rcu_is_cpu_rrupt_from_idle - see if idle or immediately interrupted from idle
@@ -1904,6 +1900,13 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
 	else
 		trace_rcu_callback(rsp->name, head, rdp->qlen_lazy, rdp->qlen);
 
+	/*
+	 * If called from an extended quiescent state, invoke the RCU
+	 * core in order to force a re-evaluation of RCU's idleness.
+	 */
+	if (rcu_is_cpu_idle())
+		invoke_rcu_core();
+
 	/* If interrupts were disabled, don't dive into RCU core. */
 	if (irqs_disabled_flags(flags)) {
 		local_irq_restore(flags);