rcu: Inform RCU of irq_exit() activity

This is a port to TINY_RCU of Peter Zijlstra's commit #ec433f0c

The rcu_read_unlock_special() function relies on in_irq() to exclude
scheduler activity from interrupt level.  This fails because exit_irq()
can invoke the scheduler after clearing the preempt_count() bits that
in_irq() uses to determine that it is at interrupt level.  This situation
can result in failures as follows:

     $task			IRQ		SoftIRQ

     rcu_read_lock()

     /* do stuff */

     <preempt> |= UNLOCK_BLOCKED

     rcu_read_unlock()
       --t->rcu_read_lock_nesting

    			irq_enter();
    			/* do stuff, don't use RCU */
    			irq_exit();
    			  sub_preempt_count(IRQ_EXIT_OFFSET);
    			  invoke_softirq()

    					ttwu();
    					  spin_lock_irq(&pi->lock)
    					  rcu_read_lock();
    					  /* do stuff */
    					  rcu_read_unlock();
    					    rcu_read_unlock_special()
    					      rcu_report_exp_rnp()
    					        ttwu()
    					          spin_lock_irq(&pi->lock) /* deadlock */

       rcu_read_unlock_special(t);

This can be triggered 'easily' because invoke_softirq() immediately does
a ttwu() of ksoftirqd/# instead of doing the in-place softirq stuff first,
but even without that the above happens.

Cure this by also excluding softirqs from the rcu_read_unlock_special()
handler and ensuring the force_irqthreads ksoftirqd/# wakeup is done
from full softirq context.

It is also necessary to delay the ->rcu_read_lock_nesting decrement until
after rcu_read_unlock_special().  This delay is handled by the commit
"Protect __rcu_read_unlock() against scheduler-using irq handlers".
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcutiny_plugin.h

@@ -570,7 +570,7 @@ static noinline void rcu_read_unlock_special(struct task_struct *t)
 		rcu_preempt_cpu_qs();
 
 	/* Hardware IRQ handlers cannot block. */
-	if (in_irq()) {
+	if (in_irq() || in_serving_softirq()) {
 		local_irq_restore(flags);
 		return;
 	}