genirq: prevent wakeup of freed irq thread

free_irq() can remove an irqaction while the corresponding interrupt
is in progress, but free_irq() sets action->thread to NULL
unconditionally, which might lead to a NULL pointer dereference in
handle_IRQ_event() when the hard interrupt context tries to wake up
the handler thread.

Prevent this by moving the thread stop after synchronize_irq(). No
need to set action->thread to NULL either as action is going to be
freed anyway.

This fixes a boot crash reported against preempt-rt which uses the
mainline irq threads code to implement full irq threading.

[ tglx: removed local irqthread variable ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/irq/manage.c

@@ -761,7 +761,6 @@ static struct irqaction *__free_irq(unsigned int irq, void *dev_id)
 {
 	struct irq_desc *desc = irq_to_desc(irq);
 	struct irqaction *action, **action_ptr;
-	struct task_struct *irqthread;
 	unsigned long flags;
 
 	WARN(in_interrupt(), "Trying to free IRQ %d from IRQ context!\n", irq);
@@ -809,9 +808,6 @@ static struct irqaction *__free_irq(unsigned int irq, void *dev_id)
 			desc->chip->disable(irq);
 	}
 
-	irqthread = action->thread;
-	action->thread = NULL;
-
 	spin_unlock_irqrestore(&desc->lock, flags);
 
 	unregister_handler_proc(irq, action);
@@ -819,12 +815,6 @@ static struct irqaction *__free_irq(unsigned int irq, void *dev_id)
 	/* Make sure it's not being used on another CPU: */
 	synchronize_irq(irq);
 
-	if (irqthread) {
-		if (!test_bit(IRQTF_DIED, &action->thread_flags))
-			kthread_stop(irqthread);
-		put_task_struct(irqthread);
-	}
-
 #ifdef CONFIG_DEBUG_SHIRQ
 	/*
 	 * It's a shared IRQ -- the driver ought to be prepared for an IRQ
@@ -840,6 +830,13 @@ static struct irqaction *__free_irq(unsigned int irq, void *dev_id)
 		local_irq_restore(flags);
 	}
 #endif
+
+	if (action->thread) {
+		if (!test_bit(IRQTF_DIED, &action->thread_flags))
+			kthread_stop(action->thread);
+		put_task_struct(action->thread);
+	}
+
 	return action;
 }