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

[Sudeep Holla <sudeep.holla@arm.com>]

On Wed, Sep 05, 2018 at 02:23:46PM +0200, Thomas Gleixner wrote:
> 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.

This patch fixes the issue reported @[1]. Lorenzo did some debugging and
I wanted to have a look at it at some point but this discussion drew my
attention and sounded very similar[2]. So I did a quick test with this
patch and it fixes the issue.

--
Regards,
Sudeep

[1] https://lore.kernel.org/lkml/CAMuHMdVg868LgL5xTg5Dp5rReKxoo+8fRy+ETJiMxGWZCp+hWw@mail.gmail.com/
[2] https://lore.kernel.org/lkml/20180823131505.GA31558@red-moon/