xen/spinlock: Check against default value of -1 for IRQ line.

The default (uninitialized) value of the IRQ line is -1.
Check if we already have allocated an spinlock interrupt line
and if somebody is trying to do it again. Also set it to -1
when we offline the CPU.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/spinlock.c

@@ -364,6 +364,9 @@ void __cpuinit xen_init_lock_cpu(int cpu)
 	int irq;
 	const char *name;
 
+	WARN(per_cpu(lock_kicker_irq, cpu) > 0, "spinlock on CPU%d exists on IRQ%d!\n",
+	     cpu, per_cpu(lock_kicker_irq, cpu));
+
 	name = kasprintf(GFP_KERNEL, "spinlock%d", cpu);
 	irq = bind_ipi_to_irqhandler(XEN_SPIN_UNLOCK_VECTOR,
 				     cpu,
@@ -383,6 +386,7 @@ void __cpuinit xen_init_lock_cpu(int cpu)
 void xen_uninit_lock_cpu(int cpu)
 {
 	unbind_from_irqhandler(per_cpu(lock_kicker_irq, cpu), NULL);
+	per_cpu(lock_kicker_irq, cpu) = -1;
 }
 
 void __init xen_init_spinlocks(void)