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

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Date:   Wed, 5 Sep 2018 13:33:19 +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 Tue, 4 Sep 2018, Neeraj Upadhyay wrote:
> If takedown_cpu() fails during _cpu_down(), st->state is reset,
> by calling cpuhp_reset_state(). This results in an additional
> increment of st->state, which results in CPUHP_AP_SMPBOOT_THREADS
> state being skipped during rollback. Fix this by not calling
> cpuhp_reset_state() and doing the state reset directly in
> _cpu_down().
> 
> Fixes: 4dddfb5faa61 ("smp/hotplug: Rewrite AP state machine core")
> Signed-off-by: Neeraj Upadhyay <neeraju@...eaurora.org>
> ---
>  kernel/cpu.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index aa7fe85..9f49edb 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -970,7 +970,14 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
>  	 */
>  	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.

Thanks,

	tglx