x86, MCE: Robustify mcheck_init_device

BorisO reports that misc_register() fails often on xen. The current code
unregisters the CPU hotplug notifier in that case. If then a CPU is
offlined and onlined back again, we end up with a second timer running
on that CPU, leading to soft lockups and system hangs.

So let's leave the hotcpu notifier always registered - even if
mce_device_create failed for some cores and never unreg it so that we
can deal with the timer handling accordingly.
Reported-and-Tested-by: NBoris Ostrovsky <boris.ostrovsky@oracle.com>
Link: http://lkml.kernel.org/r/1403274493-1371-1-git-send-email-boris.ostrovsky@oracle.comSigned-off-by: NBorislav Petkov <bp@suse.de>

arch/x86/kernel/cpu/mcheck/mce.c

@@ -2450,6 +2450,12 @@ static __init int mcheck_init_device(void)
 	for_each_online_cpu(i) {
 		err = mce_device_create(i);
 		if (err) {
+			/*
+			 * Register notifier anyway (and do not unreg it) so
+			 * that we don't leave undeleted timers, see notifier
+			 * callback above.
+			 */
+			__register_hotcpu_notifier(&mce_cpu_notifier);
 			cpu_notifier_register_done();
 			goto err_device_create;
 		}
@@ -2470,10 +2476,6 @@ static __init int mcheck_init_device(void)
 err_register:
 	unregister_syscore_ops(&mce_syscore_ops);
 
-	cpu_notifier_register_begin();
-	__unregister_hotcpu_notifier(&mce_cpu_notifier);
-	cpu_notifier_register_done();
-
 err_device_create:
 	/*
 	 * We didn't keep track of which devices were created above, but