From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: irq_work: Allow irq_work_sync() to sleep if irq_work() no IRQ support.
Date: Wed, 06 Oct 2021 13:18:50 +0200

irq_work() triggers instantly an interrupt if supported by the
architecture. Otherwise the work will be processed on the next timer
tick. In worst case irq_work_sync() could spin up to a jiffy.

irq_work_sync() is usually used in tear down context which is fully
preemptible. Based on review irq_work_sync() is invoked from preemptible
context and there is one waiter at a time. This qualifies it to use
rcuwait for synchronisation.

Let irq_work_sync() synchronize with rcuwait if the architecture
processes irqwork via the timer tick.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20211006111852.1514359-3-bigeasy@linutronix.de
---
 include/linux/irq_work.h |    3 +++
 kernel/irq_work.c        |   10 ++++++++++
 2 files changed, 13 insertions(+)

@ include/linux/irq_work.h:6 @
 #define _LINUX_IRQ_WORK_H
 
 #include <linux/smp_types.h>
+#include <linux/rcuwait.h>
 
 /*
  * An entry can be in one of four states:
@ include/linux/irq_work.h:20 @
 struct irq_work {
 	struct __call_single_node node;
 	void (*func)(struct irq_work *);
+	struct rcuwait irqwait;
 };
 
 #define __IRQ_WORK_INIT(_func, _flags) (struct irq_work){	\
 	.node = { .u_flags = (_flags), },			\
 	.func = (_func),					\
+	.irqwait = __RCUWAIT_INITIALIZER(irqwait),		\
 }
 
 #define IRQ_WORK_INIT(_func) __IRQ_WORK_INIT(_func, 0)
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@ include/linux/irq_work.h:163 @ void irq_work_single(void *arg)
 	 * else claimed it meanwhile.
 	 */
 	(void)atomic_cmpxchg(&work->node.a_flags, flags, flags & ~IRQ_WORK_BUSY);
+
+	if (!arch_irq_work_has_interrupt())
+		rcuwait_wake_up(&work->irqwait);
 }
 
 static void irq_work_run_list(struct llist_head *list)
@ include/linux/irq_work.h:210 @ void irq_work_tick(void)
 void irq_work_sync(struct irq_work *work)
 {
 	lockdep_assert_irqs_enabled();
+	might_sleep();
+
+	if (!arch_irq_work_has_interrupt()) {
+		rcuwait_wait_event(&work->irqwait, !irq_work_is_busy(work),
+				   TASK_UNINTERRUPTIBLE);
+		return;
+	}
 
 	while (irq_work_is_busy(work))
 		cpu_relax();