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From:   Valentin Schneider <vschneid@redhat.com>
To:     Phil Auld <pauld@redhat.com>, linux-kernel@vger.kernel.org
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2 1/2] cpuhp: make target_store() a nop when target ==
 state
In-Reply-To: <20220526160615.7976-2-pauld@redhat.com>
References: <20220526160615.7976-1-pauld@redhat.com>
 <20220526160615.7976-2-pauld@redhat.com>
Date:   Fri, 27 May 2022 10:38:24 +0100
Message-ID: <xhsmhy1ynl3hr.mognet@vschneid.remote.csb>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On 26/05/22 12:06, 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.
>

To go back on my data race paranoia: writes to both cpu$x/online and
cpu$x/hotplug/target are serialized by device_hotplug_lock, and so are the
exported kernel hotplug functions ({add, remove}_cpu()).

That's not cpu_add_remove_lock as I was looking for, but that's still all
under one lock, so I think we're good. Sorry for that!

> Signed-off-by: Phil Auld <pauld@redhat.com>

Reviewed-by: Valentin Schneider <vschneid@redhat.com>

> ---
>  kernel/cpu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index d0a9aa0b42e8..cdb6ac10ad94 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -2315,7 +2315,7 @@ 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);
>  out:
>  	unlock_device_hotplug();
> -- 
> 2.18.0