Index: head-2006-04-24/arch/i386/kernel/io_apic-xen.c
===================================================================
--- head-2006-04-24.orig/arch/i386/kernel/io_apic-xen.c	2006-04-25 12:14:36.000000000 +0200
+++ head-2006-04-24/arch/i386/kernel/io_apic-xen.c	2006-04-25 13:11:31.000000000 +0200
@@ -1207,7 +1207,7 @@ int assign_irq_vector(int irq)
 {
 	physdev_op_t op;
 
-	BUG_ON(irq >= NR_IRQ_VECTORS);
+	BUG_ON(irq != AUTO_ASSIGN && (unsigned)irq >= NR_IRQ_VECTORS);
 	if (irq != AUTO_ASSIGN && IO_APIC_VECTOR(irq) > 0)
 		return IO_APIC_VECTOR(irq);
 
@@ -1217,8 +1217,11 @@ int assign_irq_vector(int irq)
 		return -ENOSPC;
 
 	vector_irq[op.u.irq_op.vector] = irq;
-	if (irq != AUTO_ASSIGN)
-		IO_APIC_VECTOR(irq) = op.u.irq_op.vector;
+	if (irq != AUTO_ASSIGN) {
+		u8 vector = cmpxchg(&IO_APIC_VECTOR(irq), 0, op.u.irq_op.vector);
+
+		BUG_ON(vector && vector != op.u.irq_op.vector);
+	}
 
 	return op.u.irq_op.vector;
 }
Index: head-2006-04-24/arch/x86_64/kernel/io_apic-xen.c
===================================================================
--- head-2006-04-24.orig/arch/x86_64/kernel/io_apic-xen.c	2006-04-25 12:14:36.000000000 +0200
+++ head-2006-04-24/arch/x86_64/kernel/io_apic-xen.c	2006-04-25 13:11:31.000000000 +0200
@@ -881,14 +881,17 @@ int assign_irq_vector(int irq)
 		return -ENOSPC;
 
 	vector_irq[op.u.irq_op.vector] = irq;
-	if (irq != AUTO_ASSIGN)
-		IO_APIC_VECTOR(irq) = op.u.irq_op.vector;
+	if (irq != AUTO_ASSIGN) {
+		u8 vector = cmpxchg(&IO_APIC_VECTOR(irq), 0, op.u.irq_op.vector);
+
+		BUG_ON(vector && vector != op.u.irq_op.vector);
+	}
 
 	return op.u.irq_op.vector;
 }
 
-extern void (*interrupt[NR_IRQS])(void);
 #ifndef CONFIG_XEN
+extern void (*interrupt[NR_IRQS])(void);
 static struct hw_interrupt_type ioapic_level_type;
 static struct hw_interrupt_type ioapic_edge_type;