From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: irq_work: Also rcuwait for !IRQ_WORK_HARD_IRQ on PREEMPT_RT
Date: Wed, 06 Oct 2021 13:18:52 +0200

On PREEMPT_RT most items are processed as LAZY via softirq context.
Avoid to spin-wait for them because irq_work_sync() could have higher
priority and not allow the irq-work to be completed.

Wait additionally for !IRQ_WORK_HARD_IRQ irq_work items on PREEMPT_RT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20211006111852.1514359-5-bigeasy@linutronix.de
---
 include/linux/irq_work.h |    5 +++++
 kernel/irq_work.c        |    6 ++++--
 2 files changed, 9 insertions(+), 2 deletions(-)

--- a/include/linux/irq_work.h
+++ b/include/linux/irq_work.h
@@ -49,6 +49,11 @@ static inline bool irq_work_is_busy(stru
 	return atomic_read(&work->node.a_flags) & IRQ_WORK_BUSY;
 }
 
+static inline bool irq_work_is_hard(struct irq_work *work)
+{
+	return atomic_read(&work->node.a_flags) & IRQ_WORK_HARD_IRQ;
+}
+
 bool irq_work_queue(struct irq_work *work);
 bool irq_work_queue_on(struct irq_work *work, int cpu);
 
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -217,7 +217,8 @@ void irq_work_single(void *arg)
 	 */
 	(void)atomic_cmpxchg(&work->node.a_flags, flags, flags & ~IRQ_WORK_BUSY);
 
-	if (!arch_irq_work_has_interrupt())
+	if ((IS_ENABLED(CONFIG_PREEMPT_RT) && !irq_work_is_hard(work)) ||
+	    !arch_irq_work_has_interrupt())
 		rcuwait_wake_up(&work->irqwait);
 }
 
@@ -277,7 +278,8 @@ void irq_work_sync(struct irq_work *work
 	lockdep_assert_irqs_enabled();
 	might_sleep();
 
-	if (!arch_irq_work_has_interrupt()) {
+	if ((IS_ENABLED(CONFIG_PREEMPT_RT) && !irq_work_is_hard(work)) ||
+	    !arch_irq_work_has_interrupt()) {
 		rcuwait_wait_event(&work->irqwait, !irq_work_is_busy(work),
 				   TASK_UNINTERRUPTIBLE);
 		return;