linux-kernel - Re: [RFC, PATCH 1/3] introduce SYS_CLONE

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Date:	Mon, 9 Apr 2007 13:06:43 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Davide Libenzi <davidel@...ilserver.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Robin Holt <holt@....com>, Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [RFC, PATCH 1/3] introduce SYS_CLONE_MASK

On 04/08, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...sign.ru> writes:
> 
> > Perhaps it is better to add reparent_kthread() (next patch) to kthread()
> > and forget about CLONE_KERNEL_THREAD.
> 
> Please. 

OK, will do tomorrow.

> > Anyway, re-parenting to swapper breaks pstree, it doesn't show kernel
> > threads. And if ->parent == /sbin/init, we can't remove us from ->children
> > (unless we forbid sub-thread-of-init exec). So the only safe change is
> > set ->exit_state = -1.
> 
> Yes.  We certainly need ->exit_state = -1.
> Earlier I had forgotten about second the use of ->children to update
> the parent pointer of processes when their parent exits.
> 
> There is a practical question how much we care about pstree being
> confused (I assume it doesn't crash).  If this is just a confusion
> issue then I say go for it.  PPID == 0 is a very legitimate way to say
> the kernel is the parent process.

No, it doesn't crash. It just doesn't show kernel threads (ps ax is OK).
I didn't look into the sources, but I guess the reason is that pstree
assumes that the "root" of the tree is "pid == 1" process.

I personally think this is acceptable (and Roland seems to think the same).
Still, to be safe, I'll break this into 2 patches, the first one sets
->exit_state, the second re-parents to swapper.

In fact, we can do some odd things to make pstree happy. We need ->parent
only because /proc needs some ->parent fields. But I'd prefer to avoid
these hacks.

Still, it is sad that we can't have additional flags for kernel_thread().
However, I agree with your and Roland's objections.

Oleg.

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