linux-kernel - Re: [RFC/RFT PATCH v3] sched: automated per tty task groups

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Message-ID: <20101116150319.GA3475@redhat.com>
Date:	Tue, 16 Nov 2010 16:03:20 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Mike Galbraith <efault@....de>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC/RFT PATCH v3] sched: automated per tty task groups

On 11/16, Mike Galbraith wrote:
>
> On Tue, 2010-11-16 at 14:04 +0100, Oleg Nesterov wrote:
> > However, I must admit I dislike this check. Because, looking at this
> > code, it is not clear why do we check PF_EXITING. It looks as if it
> > is needed for correctness.
>
> Is _not_ needed I presume.
>
> I'll remove it, I'm not overly attached (a t t a..;) to it.

Argh!

I was wrong, it _is_ needed for correctness. Yes, it is always safe
to read the pointer, but

> > Yes, sure, rq->lock should ensure signal->autogroup can't go away.
> > (even if it can be changed under us). And it does, we are moving all
> > threads before kref_put().
>
> (yeah)

Exactly. And this means we can _only_ assume it can't go away if
autogroup_move_group() can see us on ->thread_group list.

Perhaps this deserve a commen (unless I missed something again).

Mike, sorry for confusion.

Oleg.

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