linux-kernel - Re: [PATCH 3/4] cpu/hotplug: Add cpuhp_invoke_callback

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Date:   Thu, 21 Jan 2021 12:18:33 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Donnefort <vincent.donnefort@....com>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        valentin.schneider@....com
Subject: Re: [PATCH 3/4] cpu/hotplug: Add cpuhp_invoke_callback_range()

On Thu, Jan 21, 2021 at 10:57:57AM +0000, Vincent Donnefort wrote:
> On Wed, Jan 20, 2021 at 06:53:33PM +0100, Peter Zijlstra wrote:
> > On Wed, Jan 20, 2021 at 06:45:16PM +0100, Peter Zijlstra wrote:
> > > On Mon, Jan 11, 2021 at 05:10:46PM +0000, vincent.donnefort@....com wrote:
> > > > @@ -475,6 +478,11 @@ cpuhp_set_state(struct cpuhp_cpu_state *st, enum cpuhp_state target)
> > > >  static inline void
> > > >  cpuhp_reset_state(struct cpuhp_cpu_state *st, enum cpuhp_state prev_state)
> > > >  {
> > > > +	st->target = prev_state;
> > > > +
> > > > +	if (st->rollback)
> > > > +		return;
> > > 
> > > I'm thinking that if we call rollback while already rollback we're hosed
> > > something fierce, no?
> > > 
> > > That like going up, failing, going back down again, also failing, giving
> > > up in a fiery death.
> > 
> > Ooh, is this a hack for _cpu_down():
> > 
> > 	ret = cpuhp_down_callbacks(cpu, st, target);
> > 	if (ret && st->state == CPUHP_TEARDOWN_CPU && st->state < prev_state) {
> > 		cpuhp_reset_state(st, prev_state);
> > 		__cpuhp_kick_ap(st);
> > 	}
> > 
> > Where cpuhp_down_callbacks() can already have called cpuhp_reset_state() ?
> 
> Yes, it is now possible that this function will be called twice during the
> rollback. Shall I avoid this and treat the case above differently ? i.e. "if we
> are here, state has already been reset, and we should only set st->target".

Not sure, but a comment would be useful :-)