[PATCH RFC v2 tip/core/rcu 4/4] rcu: Allow rcu_read_unlock_special() to raise_softirq() if in_irq()

["Paul E. McKenney" <paulmck@linux.ibm.com>]

When running in an interrupt handler, raise_softirq() and
raise_softirq_irqoff() have extremely low overhead: They simply set a
bit in a per-CPU mask, which is checked upon exit from that interrupt
handler.  Therefore, if rcu_read_unlock_special() is invoked within an
interrupt handler and RCU_SOFTIRQ is in use, this commit make use of
raise_softirq_irqoff() even if there is no expedited grace period in
flight and even if this is not a nohz_full CPU.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
 kernel/rcu/tree_plugin.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index d15cdab6aeb4..e1005f5e8094 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -615,7 +615,7 @@ 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 && irqs_were_disabled && use_softirq &&
+		if ((exp || in_irq()) && irqs_were_disabled && use_softirq &&
 		    (in_irq() || !t->rcu_read_unlock_special.b.deferred_qs)) {
 			// Using softirq, safe to awaken, and we get
 			// no help from enabling irqs, unlike bh/preempt.
-- 
2.17.1