Re: [PATCH] cpu/hotplug: Fix rollback during error-out in takedown_cpu()

[Thomas Gleixner]

On Wed, 5 Sep 2018, Thomas Gleixner wrote:
> On Tue, 4 Sep 2018, Neeraj Upadhyay wrote:
> >  	ret = cpuhp_down_callbacks(cpu, st, target);
> >  	if (ret && st->state > CPUHP_TEARDOWN_CPU && st->state < prev_state) {
> > -		cpuhp_reset_state(st, prev_state);
> > +		/*
> > +		 * As st->last is not set, cpuhp_reset_state() increments
> > +		 * st->state, which results in CPUHP_AP_SMPBOOT_THREADS being
> > +		 * skipped during rollback. So, don't use it here.
> > +		 */
> > +		st->rollback = true;
> > +		st->target = prev_state;
> > +		st->bringup = !st->bringup;
> 
> No, this is just papering over the actual problem.
> 
> The state inconsistency happens in take_cpu_down() when it returns with a
> failure from __cpu_disable() because that returns with state = TEARDOWN_CPU
> and st->state is then incremented in undo_cpu_down().
> 
> That's the real issue and we need to analyze the whole cpu_down rollback
> logic first.

And looking closer this is a general issue. Just that the TEARDOWN state
makes it simple to observe. It's universaly broken, when the first teardown
callback fails because, st->state is only decremented _AFTER_ the callback
returns success, but undo_cpu_down() increments unconditionally.

Patch below.

Thanks,

	tglx
----
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -916,7 +916,8 @@ static int cpuhp_down_callbacks(unsigned
 		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
 	 * 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);
 	}