diff --git a/[refs] b/[refs]
index 4d4970d067a0..0abc9ed94451 100644
--- a/[refs]
+++ b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 8532159f5521ba24e697f0d25970ae89ff62a1f2
+refs/heads/master: 82c3c03a4096badd026c6e337f3c5dde020e9ec6
diff --git a/trunk/arch/i386/kernel/smpboot.c b/trunk/arch/i386/kernel/smpboot.c
index eba7f53f8b4a..7007e1783797 100644
--- a/trunk/arch/i386/kernel/smpboot.c
+++ b/trunk/arch/i386/kernel/smpboot.c
@@ -1029,6 +1029,16 @@ int __devinit smp_prepare_cpu(int cpu)
 	int	apicid, ret;
 
 	lock_cpu_hotplug();
+
+	/*
+	 * On x86, CPU0 is never offlined.  Trying to bring up an
+	 * already-booted CPU will hang.  So check for that case.
+	 */
+	if (cpu_online(cpu)) {
+		ret = -EINVAL;
+		goto exit;
+	}
+
 	apicid = x86_cpu_to_apicid[cpu];
 	if (apicid == BAD_APICID) {
 		ret = -ENODEV;