linux-kernel - Re: [PATCH] cpuhp: make target_store() a nop when target == state

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Date:   Tue, 24 May 2022 16:11:51 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Phil Auld <pauld@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] cpuhp: make target_store() a nop when target == state

On 23/05/22 10:47, Phil Auld wrote:
> writing the current state back into hotplug/target calls cpu_down()
> which will set cpu dying even when it isn't and then nothing will
> ever clear it. A stress test that reads values and writes them back
> for all cpu device files in sysfs will trigger the BUG() in
> select_fallback_rq once all cpus are marked as dying.
>
> kernel/cpu.c::target_store()
> 	...
>         if (st->state < target)
>                 ret = cpu_up(dev->id, target);
>         else
>                 ret = cpu_down(dev->id, target);
>
> cpu_down() -> cpu_set_state()
> 	 bool bringup = st->state < target;
> 	 ...
> 	 if (cpu_dying(cpu) != !bringup)
> 		set_cpu_dying(cpu, !bringup);
>
> Make this safe by catching the case where target == state
> and bailing early.
>
> Signed-off-by: Phil Auld <pauld@...hat.com>
> ---
>
> Yeah, I know... don't do that. But it's still messy.
>
> !< != > 
>
>  kernel/cpu.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index d0a9aa0b42e8..8a71b1149c60 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -2302,6 +2302,9 @@ static ssize_t target_store(struct device *dev, struct device_attribute *attr,
>  		return -EINVAL;
>  #endif
>  
> +	if (target == st->state)
> +		return count;
> +

The current checks are against static boundaries, this has to compare
against st->state - AFAICT this could race with another hotplug operation
to the same CPU, e.g.

  CPU42.cpuhp_state
    ->state  == CPUHP_AP_SCHED_STARTING
    ->target == CPUHP_ONLINE

  <write CPUHP_ONLINE via sysfs, OK because current state != CPUHP_ONLINE>

  CPU42.cpuhp_state == CPUHP_ONLINE
  
  <issues ensue>


_cpu_up() has:

	/*
	 * The caller of cpu_up() might have raced with another
	 * caller. Nothing to do.
	 */
	if (st->state >= target)
		goto out;

Looks like we want an equivalent in _cpu_down(), what do you think?

>  	ret = lock_device_hotplug_sysfs();
>  	if (ret)
>  		return ret;
> -- 
> 2.18.0