linux-kernel - Re: [PATCH 2/4] cpu/hotplug: CPUHP_BRINGUP

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Date:   Wed, 20 Jan 2021 13:58:35 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     vincent.donnefort@....com
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        valentin.schneider@....com
Subject: Re: [PATCH 2/4] cpu/hotplug: CPUHP_BRINGUP_CPU exception in fail
 injection

On Mon, Jan 11, 2021 at 05:10:45PM +0000, vincent.donnefort@....com wrote:
> From: Vincent Donnefort <vincent.donnefort@....com>
> 
> The atomic states (between CPUHP_AP_IDLE_DEAD and CPUHP_AP_ONLINE) are
> triggered by the CPUHP_BRINGUP_CPU step. If the latter doesn't run, none
> of the atomic can. Hence, rollback is not possible after a hotunplug
> CPUHP_BRINGUP_CPU step failure and the "fail" interface shouldn't allow
> it. Moreover, the current CPUHP_BRINGUP_CPU teardown callback
> (finish_cpu()) cannot fail anyway.
> 
> Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
> ---
>  kernel/cpu.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 9121edf..bcd7b2a 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -2216,9 +2216,14 @@ static ssize_t write_cpuhp_fail(struct device *dev,
>  		return -EINVAL;
>  
>  	/*
> -	 * Cannot fail STARTING/DYING callbacks.
> +	 * Cannot fail STARTING/DYING callbacks. Also, those callbacks are
> +	 * triggered by BRINGUP_CPU bringup callback. Therefore, the latter
> +	 * can't fail during hotunplug, as it would mean we have no way of
> +	 * rolling back the atomic states that have been previously teared
> +	 * down.
>  	 */
> -	if (cpuhp_is_atomic_state(fail))
> +	if (cpuhp_is_atomic_state(fail) ||
> +	    (fail == CPUHP_BRINGUP_CPU && st->state > CPUHP_BRINGUP_CPU))
>  		return -EINVAL;

Should we instead disallow failing any state that has .cant_stop ?