linux-kernel - Re: [PATCH 1/1] kthread: fix possible infinite wait for parking when kthread exits meanwhile

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Date:   Tue, 25 Oct 2016 17:16:40 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Roman Pen <roman.penyaev@...fitbricks.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] kthread: fix possible infinite wait for parking
        when kthread exits meanwhile

Well. I was going to ignore this patch, I will leave this to Thomas
anyway...

But can't resist, because I have to admit I dislike this one too ;)

On 10/25, Roman Pen wrote:
>
>  int kthread_park(struct task_struct *k)
>  {
> -	struct kthread *kthread = to_live_kthread_and_get(k);
> +	struct kthread *kthread;
>  	int ret = -ENOSYS;
>  
> -	if (kthread) {
> -		if (!test_bit(KTHREAD_IS_PARKED, &kthread->flags)) {
> +	/*
> +	 * Here we try to be careful and handle the case, when kthread
> +	 * is going to die and will never park.

But why?

IMO we should not complicate this code for the case when the kernel
thread crashes.

> +	do {
> +		kthread = to_live_kthread_and_get(k);
> +		if (!kthread)
> +			break;
> +		if (!__kthread_isparked(kthread)) {
>  			set_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
>  			if (k != current) {
>  				wake_up_process(k);
>  				wait_for_completion(&kthread->parked);
>  			}
>  		}
> +		if (k == current || __kthread_isparked(kthread))
> +			/* The way out */
> +			ret = 0;

And why do we need to restart if __kthread_isparked() is false? In this
case we know that we were woken up by the exiting kthread which won't
park anyway.

Oleg.