[PATCH] x86/x2apic: fix x2apic_cluster_probe null pointer

[Huaitong Han]

percpu cpus_in_cluster is a pointer with CONFIG_CPUMASK_OFFSTACK, cpu_in_cluster
is not alloced memory before x2apic_prepare_cpu is invoked, so cpumask_set_cpu
would get a NULL pointer bug:
 BUG: unable to handle kernel NULL pointer dereference at (null)
 IP: [<ffffffff8105ac25>] x2apic_cluster_probe+0x35/0x70

Signed-off-by: Huaitong Han <huaitong.han@xxxxxxxxx>
---
 arch/x86/kernel/apic/x2apic_cluster.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/apic/x2apic_cluster.c b/arch/x86/kernel/apic/x2apic_cluster.c
index 6368fa6..6acb055 100644
--- a/arch/x86/kernel/apic/x2apic_cluster.c
+++ b/arch/x86/kernel/apic/x2apic_cluster.c
@@ -190,9 +190,10 @@ static int x2apic_cluster_probe(void)
 	if (!x2apic_mode)
 		return 0;
 
-	cpumask_set_cpu(cpu, per_cpu(cpus_in_cluster, cpu));
 	cpuhp_setup_state(CPUHP_X2APIC_PREPARE, "X2APIC_PREPARE",
 			  x2apic_prepare_cpu, x2apic_dead_cpu);
+	cpumask_set_cpu(cpu, per_cpu(cpus_in_cluster, cpu));
+
 	return 1;
 }
 
-- 
1.8.3.1