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

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Message-ID: <m1r62ew5ug.fsf@fess.ebiederm.org>
Date:	Wed, 04 Feb 2009 07:59:35 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Rusty Russell <rusty@...tcorp.com.au>
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()

Rusty Russell <rusty@...tcorp.com.au> writes:

> On Wednesday 04 February 2009 15:40:06 Eric W. Biederman wrote:
>> static struct kthread *to_kthread(struct task_struct *tsk)
>> {
>> 	void *stack = task_stack_page(tsk);
>> 	return (struct kthread *)(stack + kthread_offset);
>>         
>> }
> ...
>> It would remove the test and be simple and obviously correct.
>
> Clever? Sure.  Neat? Yes.
>
> But you are using a definition of obvious with which I was not previously
> familiar :)

Well the way you compute kthread_offset is:

	struct kthread kthread;
	void *stack = task_stack_page(current);
        kthread_offset = (void *)&kthread - stack;

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.

Further I think the rest of that code becomes a lot clearer if
we can remove that stupid, unnecessary conditional.  As worrying
if the process has exited implies we care about a lot of things
that we really don't and seem to make the code generally less
comprehensible.

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.

Eric
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