linux-kernel - Re: [PATCH] cpu/hotplug: handle unbalanced hotplug enable/disable

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Date:	Thu, 5 May 2016 14:13:06 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Lianwei Wang <lianwei.wang@...il.com>
cc:	Peter Zijlstra <peterz@...radead.org>, oleg@...hat.com,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [PATCH] cpu/hotplug: handle unbalanced hotplug enable/disable

On Wed, 4 May 2016, Lianwei Wang wrote:
> In this example, the unbalanced count is caused by the
> cpu_hotplug_pm_callback pm notifier callback function.

I doubt that.

> We can add a variable to avoid the unbalanced call of cpu_hotplug_enable
> ,e.g.

> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 3e3f6e49eabb..aa6694f0e9d3 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1140,16 +1140,21 @@ static int
>  cpu_hotplug_pm_callback(struct notifier_block *nb,
>                         unsigned long action, void *ptr)
>  {
> +       static int disabled;
> +
>         switch (action) {
> 
>         case PM_SUSPEND_PREPARE:
>         case PM_HIBERNATION_PREPARE:
>                 cpu_hotplug_disable();
> +               disabled = 1;
>                 break;
> 
>         case PM_POST_SUSPEND:
>         case PM_POST_HIBERNATION:
> -               cpu_hotplug_enable();
> +               if (disabled)
> +                       cpu_hotplug_enable();
> +               disabled = 0;
>                 break;
> 
>         default:
> 
> Please let me know if you like to fix it in this way.

So you are moving the work around one step down w/o providing any reasonable
explanation how this asymetric call of that callback can happen.

Can you eventually come up with a coherent explanation of the problem down to
the root cause or are we going to play this "move the workaround one step
down" game for another 10 rounds?
 
> +static void _cpu_hotplug_enable(void)
> +{
> +       if (WARN(!cpu_hotplug_disabled, "Unbalanced cpu hotplug enable\n"))
> +               return;
> +
> +       cpu_hotplug_disabled--;
> +}
> 
> I like to fix it in the cpu_hotplug_enable because it is a public

You CANNOT fix it there. The problem is the call site and NOT
cpu_hotplug_enable(). Can you finally accept this?

> kernel API and fix in it can prevent any other unbalanced calling. I

It cannot prevent any unbalanced calls. It mitigates the issue, but that's a
different problem.

We can discuss that seperately after fixing the offending call site.

Thanks,

	tglx