Date: Mon, 24 Oct 2016 18:40:43 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Roman Pen <roman.penyaev@profitbricks.com>
Cc: Andy Lutomirski <luto@kernel.org>, Oleg Nesterov <oleg@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Tejun Heo <tj@kernel.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/2] workqueue: ignore dead tasks in a workqueue sleep
 hook
Message-ID: <20161024164043.GS3102@twins.programming.kicks-ass.net>
References: <20161024160814.3126-1-roman.penyaev@profitbricks.com>
 <20161024160814.3126-2-roman.penyaev@profitbricks.com>
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On Mon, Oct 24, 2016 at 06:08:14PM +0200, Roman Pen wrote:

> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -875,9 +875,31 @@ void wq_worker_waking_up(struct task_struct *task, int cpu)
>   */
>  struct task_struct *wq_worker_sleeping(struct task_struct *task)
>  {
> -	struct worker *worker = kthread_data(task), *to_wakeup = NULL;
> +	struct worker *worker, *to_wakeup = NULL;
>  	struct worker_pool *pool;
>  
> +
> +	if (task->state == TASK_DEAD)
> +		/* Here we try to catch the following path before
> +		 * accessing NULL kthread->vfork_done ptr thru
> +		 * kthread_data():
> +		 *
> +		 *    oops_end()
> +		 *    do_exit()
> +		 *    schedule()
> +		 *
> +		 * If panic_on_oops is not set and oops happens on
> +		 * a workqueue execution path, thread will be killed.
> +		 * That is definitly sad, but not to make the situation
> +		 * even worse we have to ignore dead tasks in order not
> +		 * to step on zeroed out members (e.g. t->vfork_done is
> +		 * already NULL on that path, since we were called by
> +		 * do_exit())).
> +		 */
> +		return NULL;

https://lkml.kernel.org/r/CA+55aFyQYJerovMsSoSKS7PessZBr4vNp-3QUUwhqk4A4_jcbg@mail.gmail.com

Also, that misses { }.

> +
> +	worker = kthread_data(task);
> +
>  	/*
>  	 * Rescuers, which may not have all the fields set up like normal
>  	 * workers, also reach here, let's not access anything before
> -- 
> 2.9.3
>