Re: [PATCH] kthreads: Fix startup synchronization boot crash

From: Oleg Nesterov
Date: Tue Sep 01 2009 - 12:06:44 EST

On 09/01, Ingo Molnar wrote:
> * Oleg Nesterov <oleg@xxxxxxxxxx> 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?


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