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Date: Tue, 1 Sep 2009 15:04:36 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: arjan@infradead.org, jeremy@goop.org, mschmidt@redhat.com,
       mingo@redhat.com, hpa@zytor.com, linux-kernel@vger.kernel.org,
       tj@kernel.org, tglx@linutronix.de,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       linux-tip-commits@vger.kernel.org
Subject: Re: [PATCH] kthreads: Fix startup synchronization boot crash
Message-ID: <20090901130436.GA22514@redhat.com>
References: <20090829182718.10f566b1@leela> <tip-23386d63bbb3199cf247313ec088878d72debcfd@git.kernel.org> <20090901100351.GA3361@elte.hu> <20090901113914.GA23578@elte.hu>
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On 09/01, Ingo Molnar wrote:
>
> In fact i dont see any proper serialization here: there appears to
> be a race between the initial task and the init task (which are not
> one and the same). The race is possibly timing dependent as well,
> hence the (in hindsight, false) dependency on the stackprotector
> commit.

Yes, this looks racy, and I think this was always racy.

> I think the bug was introduced
> via:
>
>   cdd140b: kthreads: simplify the startup synchronization

Cough ;) No, I don't think this patch introduced this bug. With or without
this patch, kthread_create() assumes kthreadd_task != NULL, otherwise
wake_up_process(kthreadd_task) is obviously can crash.

>  static DEFINE_SPINLOCK(kthread_create_lock);
>  static LIST_HEAD(kthread_create_list);
> +
>  struct task_struct *kthreadd_task;
> +DECLARE_COMPLETION(kthreadd_task_init_done);
>
>  struct kthread_create_info
>  {
> @@ -129,6 +131,9 @@ struct task_struct *kthread_create(int (*threadfn)(void *data),
>  	list_add_tail(&create.list, &kthread_create_list);
>  	spin_unlock(&kthread_create_lock);
>
> +	if (unlikely(!kthreadd_task))
> +		wait_for_completion(&kthreadd_task_init_done);
> +

Yes, this should work. But I _think_ we can make the better fix...

I'll try to make the patch soon. Afaics we don't need kthreadd_task_init_done.

Oleg.