[patch 2/5] genirq: Add optional hardware synchronization for shutdown

[Thomas Gleixner <tglx@linutronix.de>]

free_irq() ensures that no hardware interrupt handler is executing on a
different CPU before actually releasing resources and deactivating the
interrupt completely in a domain hierarchy.

But that does not catch the case where the interrupt is on flight at the
hardware level but not yet serviced by the target CPU. That creates an
interesing race condition:

   CPU 0                  CPU 1               IRQ CHIP

                                              interrupt is raised
                                              sent to CPU1
			  Unable to handle
			  immediately
			  (interrupts off,
			   deep idle delay)
   mask()
   ...
   free()
     shutdown()
     synchronize_irq()
     release_resources()
                          do_IRQ()
                            -> resources are not available

That might be harmless and just trigger a spurious interrupt warning, but
some interrupt chips might get into a wedged state.

Provide infrastructure for interrupt chips to provide an optional
irq_inflight() callback and use it for the synchronization in free_irq().

synchronize_[hard]irq() are not using this mechanism as it might actually
deadlock unter certain conditions.

Fixes: 464d12309e1b ("x86/vector: Switch IOAPIC to global reservation mode")
Reported-by: Robert Hodaszi <Robert.Hodaszi@digi.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 include/linux/irq.h |    2 ++
 kernel/irq/manage.c |   29 ++++++++++++++++++++++++-----
 2 files changed, 26 insertions(+), 5 deletions(-)

--- a/include/linux/irq.h
+++ b/include/linux/irq.h
@@ -418,6 +418,7 @@ static inline irq_hw_number_t irqd_to_hw
  *			required. This is used for CPU hotplug where the
  *			target CPU is not yet set in the cpu_online_mask.
  * @irq_retrigger:	resend an IRQ to the CPU
+ * @irq_inflight:	chip level detection of interrupts in flight (optional)
  * @irq_set_type:	set the flow type (IRQ_TYPE_LEVEL/etc.) of an IRQ
  * @irq_set_wake:	enable/disable power-management wake-on of an IRQ
  * @irq_bus_lock:	function to lock access to slow bus (i2c) chips
@@ -462,6 +463,7 @@ struct irq_chip {
 
 	int		(*irq_set_affinity)(struct irq_data *data, const struct cpumask *dest, bool force);
 	int		(*irq_retrigger)(struct irq_data *data);
+	int		(*irq_inflight)(struct irq_data *data);
 	int		(*irq_set_type)(struct irq_data *data, unsigned int flow_type);
 	int		(*irq_set_wake)(struct irq_data *data, unsigned int on);
 
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -35,8 +35,10 @@ static int __init setup_forced_irqthread
 early_param("threadirqs", setup_forced_irqthreads);
 #endif
 
-static void __synchronize_hardirq(struct irq_desc *desc)
+static void __synchronize_hardirq(struct irq_desc *desc, bool sync_chip)
 {
+	struct irq_data *irqd = irq_desc_get_irq_data(desc);
+	struct irq_chip *chip = irq_data_get_irq_chip(irqd);
 	bool inprogress;
 
 	do {
@@ -52,6 +54,13 @@ static void __synchronize_hardirq(struct
 		/* Ok, that indicated we're done: double-check carefully. */
 		raw_spin_lock_irqsave(&desc->lock, flags);
 		inprogress = irqd_irq_inprogress(&desc->irq_data);
+
+		/*
+		 * If requested and supported, check at the chip whether it
+		 * is in flight at the hardware level:
+		 */
+		if (!inprogress && sync_chip && chip && chip->irq_inflight)
+			inprogress = chip->irq_inflight(irqd);
 		raw_spin_unlock_irqrestore(&desc->lock, flags);
 
 		/* Oops, that failed? */
@@ -74,13 +83,16 @@ static void __synchronize_hardirq(struct
  *	Returns: false if a threaded handler is active.
  *
  *	This function may be called - with care - from IRQ context.
+ *
+ *	It does not check whether there is an interrupt on flight at the
+ *	hardware level, but not serviced yet, as this might deadlock.
  */
 bool synchronize_hardirq(unsigned int irq)
 {
 	struct irq_desc *desc = irq_to_desc(irq);
 
 	if (desc) {
-		__synchronize_hardirq(desc);
+		__synchronize_hardirq(desc, false);
 		return !atomic_read(&desc->threads_active);
 	}
 
@@ -97,13 +109,16 @@ EXPORT_SYMBOL(synchronize_hardirq);
  *	holding a resource the IRQ handler may need you will deadlock.
  *
  *	This function may be called - with care - from IRQ context.
+ *
+ *	It does not check whether there is an interrupt on flight at the
+ *	hardware level, but not serviced yet, as this might deadlock.
  */
 void synchronize_irq(unsigned int irq)
 {
 	struct irq_desc *desc = irq_to_desc(irq);
 
 	if (desc) {
-		__synchronize_hardirq(desc);
+		__synchronize_hardirq(desc, false);
 		/*
 		 * We made sure that no hardirq handler is
 		 * running. Now verify that no threaded handlers are
@@ -1729,8 +1744,12 @@ static struct irqaction *__free_irq(stru
 
 	unregister_handler_proc(irq, action);
 
-	/* Make sure it's not being used on another CPU: */
-	synchronize_hardirq(irq);
+	/*
+	 * Make sure it's not being used on another CPU and if the chip
+	 * supports it also make sure that there is no (not yet serviced)
+	 * interrupt on flight at the hardware level.
+	 */
+	__synchronize_hardirq(desc, true);
 
 #ifdef CONFIG_DEBUG_SHIRQ
 	/*