[PATCH -tip] irq: check chip->ack before calling (WAS: irq: remove unneeded desc->chip->ack check)

[Wang Chen <wangchen@cn.fujitsu.com>]

Ingo Molnar said the following on 2008-12-26 21:27:
> * Wang Chen <wangchen@cn.fujitsu.com> wrote:
> 
>> desc->chip->ack is initialized to ack_bad().
>> It will not be NULL.
> 
> hm, that is only true of no_irq_chip() - is it true of all irq_chip 
> definitions on all architectures?
> 

No, some arch's irq_chip doesn't have ack routine.
So I was wrong.
But this enlighten another thought that generic irq layer doesn't know
whether irq_chip has ack routine on some architectures.
Upon that, before calling chip->ack, should check it's not NULL.

Impact: fix theoretic NULL dereference

Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
---
diff --git a/kernel/irq/chip.c b/kernel/irq/chip.c
index f63c706..9a7fbb8 100644
--- a/kernel/irq/chip.c
+++ b/kernel/irq/chip.c
@@ -290,7 +290,8 @@ static inline void mask_ack_irq(struct irq_desc *desc, int irq)
 		desc->chip->mask_ack(irq);
 	else {
 		desc->chip->mask(irq);
-		desc->chip->ack(irq);
+		if (desc->chip->ack)
+			desc->chip->ack(irq);
 	}
 }
 
@@ -475,7 +476,8 @@ handle_edge_irq(unsigned int irq, struct irq_desc *desc)
 	kstat_incr_irqs_this_cpu(irq, desc);
 
 	/* Start handling the irq */
-	desc->chip->ack(irq);
+	if (desc->chip->ack)
+		desc->chip->ack(irq);
 	desc = irq_remap_to_desc(irq, desc);
 
 	/* Mark the IRQ currently in progress.*/