linux-kernel - Re: [PATCH] kthreads: Fix startup synchronization boot crash

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Date:	Tue, 1 Sep 2009 15:14:40 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	arjan@...radead.org, jeremy@...p.org, mschmidt@...hat.com,
	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	tj@...nel.org, tglx@...utronix.de,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-tip-commits@...r.kernel.org
Subject: Re: [PATCH] kthreads: Fix startup synchronization boot crash


* Oleg Nesterov <oleg@...hat.com> wrote:

> 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.

yeah - was just a guess.

> >  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.

ok.

	Ingo
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