rcu: add memory barriers and comments to rcu_check_callbacks()

	if (user ||

	    (idle_cpu(cpu) && !in_softirq() &&

				hardirq_count() <= (1 << HARDIRQ_SHIFT))) {

		/*

		 * Get here if this CPU took its interrupt from user

		 * mode or from the idle loop, and if this is not a

		 * nested interrupt.  In this case, the CPU is in

		 * a quiescent state, so count it.

		 *

		 * Also do a memory barrier.  This is needed to handle

		 * the case where writes from a preempt-disable section

		 * of code get reordered into schedule() by this CPU's

		 * write buffer.  The memory barrier makes sure that

		 * the rcu_qsctr_inc() and rcu_bh_qsctr_inc() are see

		 * by other CPUs to happen after any such write.

		 */

		smp_mb();  /* See above block comment. */

		rcu_qsctr_inc(cpu);

		rcu_bh_qsctr_inc(cpu);

	} else if (!in_softirq())

	} else if (!in_softirq()) {

		/*

		 * Get here if this CPU did not take its interrupt from

		 * softirq, in other words, if it is not interrupting

		 * a rcu_bh read-side critical section.  This is an _bh

		 * critical section, so count it.  The memory barrier

		 * is needed for the same reason as is the above one.

		 */

		smp_mb();  /* See above block comment. */

		rcu_bh_qsctr_inc(cpu);

	}

	raise_rcu_softirq();

}