Merge branch 'smp-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull cpu hotplug fixes from Thomas Gleixner:
 "Two fixes for the hotplug state machine code:

   - Move the misplaces smb() in the hotplug thread function to the
     proper place, otherwise a half update control struct could be
     observed

   - Prevent state corruption on error rollback, which causes the state
     to advance by one and as a consequence skip it in the bringup
     sequence"

* 'smp-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  cpu/hotplug: Prevent state corruption on error rollback
  cpu/hotplug: Adjust misplaced smb() in cpuhp_thread_fun()

kernel/cpu.c

@@ -607,15 +607,15 @@ static void cpuhp_thread_fun(unsigned int cpu)
 	bool bringup = st->bringup;
 	enum cpuhp_state state;
 
+	if (WARN_ON_ONCE(!st->should_run))
+		return;
+
 	/*
 	 * ACQUIRE for the cpuhp_should_run() load of ->should_run. Ensures
 	 * that if we see ->should_run we also see the rest of the state.
 	 */
 	smp_mb();
 
-	if (WARN_ON_ONCE(!st->should_run))
-		return;
-
 	cpuhp_lock_acquire(bringup);
 
 	if (st->single) {
@@ -916,7 +916,8 @@ static int cpuhp_down_callbacks(unsigned int cpu, struct cpuhp_cpu_state *st,
 		ret = cpuhp_invoke_callback(cpu, st->state, false, NULL, NULL);
 		if (ret) {
 			st->target = prev_state;
-			undo_cpu_down(cpu, st);
+			if (st->state < prev_state)
+				undo_cpu_down(cpu, st);
 			break;
 		}
 	}
@@ -969,7 +970,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
 	 * to do the further cleanups.
 	 */
 	ret = cpuhp_down_callbacks(cpu, st, target);
-	if (ret && st->state > CPUHP_TEARDOWN_CPU && st->state < prev_state) {
+	if (ret && st->state == CPUHP_TEARDOWN_CPU && st->state < prev_state) {
 		cpuhp_reset_state(st, prev_state);
 		__cpuhp_kick_ap(st);
 	}