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

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Sudeep,

Thanks for the CC!

On Wed, Sep 5, 2018 at 4:54 PM Sudeep Holla <sudeep.holla@arm.com> wrote:
> 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.

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

Thomas' patch fixes the issue for me:
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>


Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds