linux-kernel - Re: [PATCH 0/2] kthread: kill task_get_live

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Message-ID: <20130312170424.GA12747@redhat.com>
Date:	Tue, 12 Mar 2013 18:04:24 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Namhyung Kim <namhyung@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] kthread: kill task_get_live_kthread()

Hi Thomas,

On 03/11, Thomas Gleixner wrote:
>
> On Mon, 11 Mar 2013, Oleg Nesterov wrote:
> >
> > But the actual reason for this cleanup is that I do not understand
> > why park/unpark abuse kthread.c.
>
> It's not abusing it :)

Yes, yes, I didn't mean the code looks bad or something like this.

Just I thought that, perhaps, it would be more clean to hide this
park/unpark logic in kernel/smpboot.c and do not add the "special"
new members into "struct kthread".

But let me repeat, mostly I simply wanted to ask the question. I
just noticed this new code and I was curious if this park/unpark
logic should be applied to every kthread (in future) or it is only
for smpboot_register_percpu_thread/etc.

> > Thomas, can't we move kthread->parked/cpu to smpboot_thread_data
> > and move all this code into kernel/smpboot.c? Just for example,
> > why kthread() does __kthread_parkme() ? smpboot_thread_fn() can do
> > this at the start.
>
> No objection. When I implemented this, I thought this would be the
> correct place and I followed the conventions of kthread.c ...

OK, I'll try to think again if this change is actually possible _and_
it can really make the things more clean/simple.

> What's the issue with that, other than some superflous task_get/put
> calls ?

Do you mean this particular cleanup?

No issues, this is only cleanup. But every cleanup is subjective, so
please tell me if you disagree.

Firstly, to_kthread() + barrier() + "vfork_done != NULL" doesn't look
very clear (cough, yes, this was written by me). And after 1/2

	static struct kthread *task_get_live_kthread(struct task_struct *k)
	{
		get_task_struct(k);
		return to_live_kthread(k);
	}

looks confusing too because it mixes 2 different things and because
its usage is not clear. I mean, it is not clear why the caller needs
get_task_struct() and why it is safe if we do not have a reference.

Oleg.

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