[RFC] rcu/tree: Try to invoke_rcu_core() if in_irq() during unlock

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

When we're in hard interrupt context in rcu_read_unlock_special(), we
can still benefit from invoke_rcu_core() doing wake ups of rcuc
threads when the !use_softirq parameter is passed.  This is safe
to do so because:

1. We avoid the scheduler deadlock issues thanks to the deferred_qs bit
introduced in commit 23634ebc1d94 ("rcu: Check for wakeup-safe
conditions in rcu_read_unlock_special()") by checking for the same in
this patch.

2. in_irq() implies in_interrupt() which implies raising softirq will
not do any wake ups.

The rcuc thread which is awakened will run when the interrupt returns.

We also honor 25102de (“rcu: Only do rcu_read_unlock_special() wakeups
if expedited”) thus doing the rcuc awakening only when none of the
following are true:
  1. Critical section is blocking an expedited GP.
  2. A nohz_full CPU.
If neither of these cases are true (exp == false), then the "else" block
will run to do the irq_work stuff.

This commit is based on a partial revert of d143b3d1cd89 ("rcu: Simplify
rcu_read_unlock_special() deferred wakeups") with an additional in_irq()
check added.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

---
 kernel/rcu/tree_plugin.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 2defc7fe74c3..f4b3055026dc 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -621,6 +621,11 @@ static void rcu_read_unlock_special(struct task_struct *t)
 			// Using softirq, safe to awaken, and we get
 			// no help from enabling irqs, unlike bh/preempt.
 			raise_softirq_irqoff(RCU_SOFTIRQ);
+		} else if (exp && in_irq() && !use_softirq &&
+			   !t->rcu_read_unlock_special.b.deferred_qs) {
+			// Safe to awaken rcuc thread which will be
+                       // scheduled in from the interrupt return path.
+			invoke_rcu_core();
 		} else {
 			// Enabling BH or preempt does reschedule, so...
 			// Also if no expediting or NO_HZ_FULL, slow is OK.
-- 
2.23.0.rc1.153.gdeed80330f-goog