[PATCH] synchronize_irq needs a barrier

synchronize_irq needs at the very least a compiler barrier and a
read barrier on SMP, but there are enough cases around where a
write barrier is also needed and it's not a hot path so I prefer
using a full smp_mb() here.

It will degrade to a compiler barrier on !SMP.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---

Index: linux-work/kernel/irq/manage.c
===================================================================

--- linux-work.orig/kernel/irq/manage.c	2007-10-18 11:22:16.000000000 +1000
+++ linux-work/kernel/irq/manage.c	2007-10-18 11:22:20.000000000 +1000

@@ -33,6 +33,7 @@ void synchronize_irq(unsigned int irq)
 	if (irq >= NR_IRQS)
 		return;
 
+	smp_mb();
 	while (desc->status & IRQ_INPROGRESS)
 		cpu_relax();
 }