rcu: Prevent __call_rcu() from invoking RCU core on offline CPUs

The __call_rcu() function will invoke the RCU core, for example, if
it detects that the current CPU has too many callbacks.  However, this
can happen on an offline CPU that is on its way to the idle loop, in
which case it is an error to invoke the RCU core, and the excess callbacks
will be adopted in any case.  This commit therefore adds checks to
__call_rcu() for running on an offline CPU, refraining from invoking
the RCU core in this case.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

kernel/rcutree.c

@@ -1904,11 +1904,11 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
 	 * 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())
+	if (rcu_is_cpu_idle() && cpu_online(smp_processor_id()))
 		invoke_rcu_core();
 
-	/* If interrupts were disabled, don't dive into RCU core. */
-	if (irqs_disabled_flags(flags)) {
+	/* If interrupts were disabled or CPU offline, don't invoke RCU core. */
+	if (irqs_disabled_flags(flags) || cpu_is_offline(smp_processor_id())) {
 		local_irq_restore(flags);
 		return;
 	}