linux-kernel - Re: [PATCH] init: Fix race between init and kthreadd -v2

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Date:	Mon, 28 Jun 2010 16:19:27 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ilya Loginov <isloginov@...il.com>, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] init: Fix race between init and kthreadd -v2


* Peter Zijlstra <peterz@...radead.org> wrote:

> +static __initdata DEFINE_MUTEX(kthreadd_lock);

> +	/*
> +	 * We need to spawn init first so that it obtains pid-1, however
> +	 * the init task will end up wanting to create kthreads, which
> +	 * if we schedule it before we create kthreadd, will OOPS.
> +	 */
> +	mutex_lock(&kthreadd_lock);
>  	kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);
>  	numa_default_policy();
>  	pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
>  	rcu_read_lock();
>  	kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
>  	rcu_read_unlock();
> +	mutex_unlock(&kthreadd_lock);
>  	unlock_kernel();
>  
>  	/*
> @@ -847,6 +856,13 @@ static noinline int init_post(void)
>  
>  static int __init kernel_init(void * unused)
>  {
> +	/*
> +	 * We spawned this thread while holding this lock, ensure the
> +	 * locked section in rest_init() is complete before proceeding.
> +	 */
> +	mutex_lock(&kthreadd_lock);
> +	mutex_unlock(&kthreadd_lock);

I think you may be using a mutex as a completion in essence. Why not use 
completions instead?

	Ingo
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