rcu: Disable preemption in rcu_blocking_is_gp()

 * 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)

{

	int ret;

	might_sleep();  /* Check for RCU read-side critical section. */

	return num_online_cpus() <= 1;

	preempt_disable();

	ret = num_online_cpus() <= 1;

	preempt_enable();

	return ret;

}

/**