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

[Oleg Nesterov <oleg@redhat.com>]

(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.