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

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Date:	Tue, 1 Sep 2009 17:54:27 +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:
> >
> > > On 09/01, Ingo Molnar wrote:
> > > >
> > > > * Oleg Nesterov <oleg@...hat.com> wrote:
> > > >
> > > > > 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.
> > >
> > > Just in case, the patch is ready. [...]
> >
> > yes - that's roughly the cleanup i referred to in the commit log.
> >
> > way too late for -rc8 though - the minimal fix i did _might_ be
> > eligible.
> >
> > agreed?
> 
> Agreed. Then I will sent the patch on top of this change.
> 
> But. May be your minimal patch needs a small tweak ?
> 
> rest_init()->complete(&kthreadd_task_init_done) assumes that exactly
> _one_ caller of kthread_create() can race with kernel_thread(kthreadd).
> Perhap we need complete_all() ?
> 
> 
> 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 - rescheduling 
_somewhere_ in a huge codepath might change timings and 'breaks up 
the critical path' - breaking ancient assumptions.

	Ingo
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