linux-kernel - Re: [PATCH] cpu/hotplug: Fix rollback during error-out in takedown

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Date:   Thu, 6 Sep 2018 10:18:40 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Neeraj Upadhyay <neeraju@...eaurora.org>
cc:     josh@...htriplett.org, peterz@...radead.org, mingo@...nel.org,
        jiangshanlai@...il.com, dzickus@...hat.com,
        brendan.jackman@....com, malat@...ian.org,
        linux-kernel@...r.kernel.org, sramana@...eaurora.org,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] cpu/hotplug: Fix rollback during error-out in
 takedown_cpu()

On Thu, 6 Sep 2018, Neeraj Upadhyay wrote:
> On 09/05/2018 06:47 PM, Thomas Gleixner wrote:
> > On Wed, 5 Sep 2018, Neeraj Upadhyay wrote:
> > > On 09/05/2018 05:53 PM, Thomas Gleixner wrote:
> > > > 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.
> > > > 
> > > As per my understanding, there are 2 problems here; one is fixed with your
> > > patch, and other is cpuhp_reset_state() is used during rollback from
> > > non-AP to
> > > AP state, which seem to result in 2 increments of st->state (one increment
> > > done by cpuhp_reset_state() and another by cpu_thread_fun()) .
> > And how did your hack fix that up magically? I'll have a look later today.
> > 
> > Thanks,
> > 
> > 	tglx
> 
> The hack fixes it by not calling cpuhp_reset_state() and doing rollback state
> reset inline in  _cpu_down().

And how is that any different from the proper patch I sent? It ends up in
the same state. So I have a hard time to understand your blurb above where
you claim that my patch just solves one of two problems?

Thanks,

	tglx