linux-kernel - Re: [PATCH 3/4] kthreads: rework kthread

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Message-Id: <200902051133.59409.rusty@rustcorp.com.au>
Date:	Thu, 5 Feb 2009 11:33:58 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	paulmck@...ux.vnet.ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>, Ingo Molnar <mingo@...e.hu>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Vitaliy Gusev <vgusev@...nvz.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] kthreads: rework kthread_stop()

On Thursday 05 February 2009 02:29:35 Eric W. Biederman wrote:
> Rusty Russell <rusty@...tcorp.com.au> writes:
> > Clever? Sure.  Neat? Yes.
> >
> > But you are using a definition of obvious with which I was not previously
> > familiar :)
...
> Now Rusty I don't know about you but after I learned to do
> addition and subtraction it has always been obvious to me that
> one is the opposite of the other.

It is *not* obvious that the offset must be constant across all kthreads.  On
all architectures, and always will be.  That noone will *ever* put a
variable-size object on the stack in this code path.

I *think* it's true, but I've been surprised before.

> I am slightly concerned that using task_stack_page(tsk) may be
> overly clever, but compared to ACCESS_ONCE(), memory barriers,
> or not letting kthread_stop be called on a thread that may exit
> I think I am ahead of the game.

Absolutely agreed.  Just humor me please and put a BUG_ON in there :)

Thanks,
Rusty.
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