linux-kernel - Re: [PATCH v3 1/2] cpuhp: make target_store() a nop when target == state

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Message-ID: <xhsmhk0896ujo.mognet@vschneid.remote.csb>
Date:   Tue, 19 Jul 2022 15:34:03 +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 v3 1/2] cpuhp: make target_store() a nop when target ==
 state

On 11/07/22 17:16, Phil Auld wrote:
> writing the current state back in 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);
>
> Fix this by letting state==target fall through in the target_store()
> conditional. Also make sure st->target == target in that case.
>
> Signed-off-by: Phil Auld <pauld@...hat.com>

One nit below, otherwise:
Reviewed-by: Valentin Schneider <vschneid@...hat.com>

> ---
>  kernel/cpu.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index bbad5e375d3b..305694a2ca26 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -2326,8 +2326,10 @@ static ssize_t target_store(struct device *dev, struct device_attribute *attr,
>
>       if (st->state < target)
>               ret = cpu_up(dev->id, target);
> -	else
> +	else if (st->state > target)
>               ret = cpu_down(dev->id, target);
> +	else if (st->target != target)

Should we make this:

        else if (WARN(st->target != target))

> +		st->target = target;
>  out:
>       unlock_device_hotplug();
>       return ret ? ret : count;
> --
> 2.31.1