Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1759373AbcJYPSS (ORCPT <rfc822;w@1wt.eu>);
        Tue, 25 Oct 2016 11:18:18 -0400
Received: from mx1.redhat.com ([209.132.183.28]:60288 "EHLO mx1.redhat.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1759329AbcJYPSO (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 25 Oct 2016 11:18:14 -0400
Date: Tue, 25 Oct 2016 17:16:40 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Roman Pen <roman.penyaev@profitbricks.com>
Cc: Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Tejun Heo <tj@kernel.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] kthread: fix possible infinite wait for parking
        when kthread exits meanwhile
Message-ID: <20161025151639.GE4326@redhat.com>
References: <20161025110525.9100-1-roman.penyaev@profitbricks.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161025110525.9100-1-roman.penyaev@profitbricks.com>
User-Agent: Mutt/1.5.18 (2008-05-17)
X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Tue, 25 Oct 2016 15:18:14 +0000 (UTC)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 1178
Lines: 44

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.