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

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Date:	Wed, 2 Sep 2009 15:06:44 +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:
> >
> > * 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?

hm, either something is broken, or some other codepath learned to do 
preempt_enable() in early init ... [which i'd call broken too]

Weird.

	Ingo
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