Updates for CPU hotplug:

- Initialize the per CPU structures during early boot so that the state
     is consistent from the very beginning.
 
   - Make the virtualization hotplug state handling more robust and let the
     core bringup CPUs which timed out in an earlier attempt again.
 
   - Make the x86/XEN CPU state tracking consistent on a failed online
     attempt, so a consecutive bringup does not fall over the inconsistent
     state.
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Merge tag 'smp-core-2022-05-23' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull CPU hotplug updates from Thomas Gleixner:

 - Initialize the per-CPU structures during early boot so that the state
   is consistent from the very beginning.

 - Make the virtualization hotplug state handling more robust and let
   the core bringup CPUs which timed out in an earlier attempt again.

 - Make the x86/xen CPU state tracking consistent on a failed online
   attempt, so a consecutive bringup does not fall over the inconsistent
   state.

* tag 'smp-core-2022-05-23' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  cpu/hotplug: Initialise all cpuhp_cpu_state structs earlier
  cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to be brought up again.
  x86/xen: Allow to retry if cpu_initialize_context() failed.

arch/x86/xen/smp_pv.c

@@ -260,8 +260,11 @@ cpu_initialize_context(unsigned int cpu, struct task_struct *idle)
 		return 0;
 
 	ctxt = kzalloc(sizeof(*ctxt), GFP_KERNEL);
-	if (ctxt == NULL)
+	if (ctxt == NULL) {
+		cpumask_clear_cpu(cpu, xen_cpu_initialized_map);
+		cpumask_clear_cpu(cpu, cpu_callout_mask);
 		return -ENOMEM;
+	}
 
 	gdt = get_cpu_gdt_rw(cpu);
 

kernel/cpu.c

@@ -716,14 +716,6 @@ static int cpuhp_up_callbacks(unsigned int cpu, struct cpuhp_cpu_state *st,
 /*
  * The cpu hotplug threads manage the bringup and teardown of the cpus
  */
-static void cpuhp_create(unsigned int cpu)
-{
-	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
-
-	init_completion(&st->done_up);
-	init_completion(&st->done_down);
-}
-
 static int cpuhp_should_run(unsigned int cpu)
 {
 	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
@@ -883,15 +875,27 @@ static int cpuhp_kick_ap_work(unsigned int cpu)
 
 static struct smp_hotplug_thread cpuhp_threads = {
 	.store			= &cpuhp_state.thread,
-	.create			= &cpuhp_create,
 	.thread_should_run	= cpuhp_should_run,
 	.thread_fn		= cpuhp_thread_fun,
 	.thread_comm		= "cpuhp/%u",
 	.selfparking		= true,
 };
 
+static __init void cpuhp_init_state(void)
+{
+	struct cpuhp_cpu_state *st;
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		st = per_cpu_ptr(&cpuhp_state, cpu);
+		init_completion(&st->done_up);
+		init_completion(&st->done_down);
+	}
+}
+
 void __init cpuhp_threads_init(void)
 {
+	cpuhp_init_state();
 	BUG_ON(smpboot_register_percpu_thread(&cpuhp_threads));
 	kthread_unpark(this_cpu_read(cpuhp_state.thread));
 }

kernel/smpboot.c

@@ -392,6 +392,13 @@ int cpu_check_up_prepare(int cpu)
 		 */
 		return -EAGAIN;
 
+	case CPU_UP_PREPARE:
+		/*
+		 * Timeout while waiting for the CPU to show up. Allow to try
+		 * again later.
+		 */
+		return 0;
+
 	default:
 
 		/* Should not happen.  Famous last words. */