linux-kernel - Re: [PATCH v3] CPU hotplug: active_writer not woken up in some cases

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Date:	Wed, 10 Dec 2014 01:12:39 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, heiko.carstens@...ibm.com,
	borntraeger@...ibm.com, rafael.j.wysocki@...el.com,
	paulmck@...ux.vnet.ibm.com, peterz@...radead.org, bp@...e.de,
	jkosina@...e.cz
Subject: Re: [PATCH v3] CPU hotplug: active_writer not woken up in some
	cases - deadlock

(sorry if this was already discussed, I ignored most of my emails
 I got this week)

On 12/09, David Hildenbrand wrote:
>
> @@ -116,7 +118,13 @@ void put_online_cpus(void)
>  	if (cpu_hotplug.active_writer == current)
>  		return;
>  	if (!mutex_trylock(&cpu_hotplug.lock)) {
> +		/* inc before testing for active_writer to not lose wake ups */
>  		atomic_inc(&cpu_hotplug.puts_pending);
> +		spin_lock(&cpu_hotplug.awr_lock);
> +		/* we might be the last one */
> +		if (unlikely(cpu_hotplug.active_writer))
> +			wake_up_process(cpu_hotplug.active_writer);
> +		spin_unlock(&cpu_hotplug.awr_lock);

Not sure I understand. awr_lock can only ensure that active_writer
can't go away.

Why active_writer should see .puts_pending != 0 if this is called
right after cpu_hotplug_begin() takes cpu_hotplug.lock but before
it sets TASK_UNINTERRUPTIBLE?

IOW,

>  void cpu_hotplug_begin(void)
>  {
> +	spin_lock(&cpu_hotplug.awr_lock);
>  	cpu_hotplug.active_writer = current;
> +	spin_unlock(&cpu_hotplug.awr_lock);
>  
>  	cpuhp_lock_acquire();
>  	for (;;) {
>  		mutex_lock(&cpu_hotplug.lock);
> +		__set_current_state(TASK_UNINTERRUPTIBLE);

don't we need set_current_state() here ?

Oleg.

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