rcu: Make rcu_read_unlock_special() checks match raise_softirq_irqoff()

Threaded interrupts provide additional interesting interactions between
RCU and raise_softirq() that can result in self-deadlocks in v5.0-2 of
the Linux kernel.  These self-deadlocks can be provoked in susceptible
kernels within a few minutes using the following rcutorture command on
an 8-CPU system:

tools/testing/selftests/rcutorture/bin/kvm.sh --duration 5 --configs "TREE03" --bootargs "threadirqs"

Although post-v5.2 RCU commits have at least greatly reduced the
probability of these self-deadlocks, this was entirely by accident.
Although this sort of accident should be rowdily celebrated on those
rare occasions when it does occur, such celebrations should be quickly
followed by a principled patch, which is what this patch purports to be.

The key point behind this patch is that when in_interrupt() returns
true, __raise_softirq_irqoff() will never attempt a wakeup.  Therefore,
if in_interrupt(), calls to raise_softirq*() are both safe and
extremely cheap.

This commit therefore replaces the in_irq() calls in the "if" statement
in rcu_read_unlock_special() with in_interrupt() and simplifies the
"if" condition to the following:

	if (irqs_were_disabled && use_softirq &&
	    (in_interrupt() ||
	     (exp && !t->rcu_read_unlock_special.b.deferred_qs))) {
		raise_softirq_irqoff(RCU_SOFTIRQ);
	} else {
		/* Appeal to the scheduler. */
	}

The rationale behind the "if" condition is as follows:

1.	irqs_were_disabled:  If interrupts are enabled, we should
	instead appeal to the scheduler so as to let the upcoming
	irq_enable()/local_bh_enable() do the rescheduling for us.
2.	use_softirq: If this kernel isn't using softirq, then
	raise_softirq_irqoff() will be unhelpful.
3.	a.	in_interrupt(): If this returns true, the subsequent
		call to raise_softirq_irqoff() is guaranteed not to
		do a wakeup, so that call will be both very cheap and
		quite safe.
	b.	Otherwise, if !in_interrupt() the raise_softirq_irqoff()
		might do a wakeup, which is expensive and, in some
		contexts, unsafe.
		i.	The "exp" (an expedited RCU grace period is being
			blocked) says that the wakeup is worthwhile, and:
		ii.	The !.deferred_qs says that scheduler locks
			cannot be held, so the wakeup will be safe.

Backporting this requires considerable care, so no auto-backport, please!

Fixes: 05f41571 ("rcu: Speed up expedited GPs when interrupting RCU reader")
Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

kernel/rcu/tree_plugin.h

@@ -626,8 +626,9 @@ static void rcu_read_unlock_special(struct task_struct *t)
 		      (rdp->grpmask & rnp->expmask) ||
 		      tick_nohz_full_cpu(rdp->cpu);
 		// Need to defer quiescent state until everything is enabled.
-		if ((exp || in_irq()) && irqs_were_disabled && use_softirq &&
-		    (in_irq() || !t->rcu_read_unlock_special.b.deferred_qs)) {
+		if (irqs_were_disabled && use_softirq &&
+		    (in_interrupt() ||
+		     (exp && !t->rcu_read_unlock_special.b.deferred_qs))) {
 			// Using softirq, safe to awaken, and we get
 			// no help from enabling irqs, unlike bh/preempt.
 			raise_softirq_irqoff(RCU_SOFTIRQ);