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

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Date:	Tue, 1 Sep 2009 18:00:44 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
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

On 09/01, Ingo Molnar wrote:
>
> * Oleg Nesterov <oleg@...hat.com> wrote:
>
> > But I must admit, now I don't understand what happens,
> >
> > 	The modification of that variable is protected by the BKL, but
> > 	the _ordering_ of the initial task (which becomes the idle
> > 	thread of CPU0) and the init task (which is spawned by the
> > 	initial task) is not synchronized.
> >
> > 	So we can occasionally end up init running sooner than
> > 	rest_init()
> >
> > How? rest_init() can't be preempted and it holds BKL. And
> > kernel_init() takes BKL before anything else. Confused...
>
> it cannot be preempted but it can schedule anywhere - and the BKL
> will be dropped silently.
>
> This is one of the biggest dangers of the BKL

Yes I see. But rest_init() runs under preempt_disable(). If it was
rescheduled, schedule_debug() should complain. No?

Oleg.

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