linux-kernel - Re: [PATCH v2 5/7] clone4: Add a CLONE_AUTOREAP flag to automatically reap the child process

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Date:	Sun, 15 Mar 2015 16:34:40 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>, Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Thiago Macieira <thiago.macieira@...el.com>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v2 5/7] clone4: Add a CLONE_AUTOREAP flag to
 automatically reap the child process

On Sun, Mar 15, 2015 at 08:55:06PM +0100, Oleg Nesterov wrote:
> On 03/15, Josh Triplett wrote:
> > On Sun, Mar 15, 2015 at 03:52:23PM +0100, Oleg Nesterov wrote:
> > > On 03/15, Josh Triplett wrote:
> > > > Add a CLONE_AUTOREAP flag to request this behavior unconditionally,
> > >
> > > Yes, CLONE_AUTOREAP is much better. And I agree (mostly) with that
> > > we should rely on do_notify_parent().
> > >
> > > Howver the patch still doesn't look right. First of all, ->autoreap
> > > should be per-process, not per-thread.
> >
> > Ah, you're thinking of the case where the parent process launches a
> > ...
> 
> Not really, although we probably need more sanity checks.
> 
> It should be per-process simply because this "autoreap" affects the whole
> process. And the sub-threads are already "autoreap". And these 2 autoreap's
> semantics differ, we should not confuse them.

Will the approach I suggested, of having clones with CLONE_THREAD
inherit the autoreap value rather than setting it from CLONE_AUTOREAP,
implement the semantics you're looking for?

Also, are you suggesting that CLONE_AUTOREAP with CLONE_THREAD should
produce -EINVAL, or just that it should be ignored?

> > (As an aside, what *is* the use case for CLONE_PARENT without
> > CLONE_THREAD?)
> 
> To me CLONE_PARENT is another historical mistake and the source of misc
> problems ;)

I kinda figured. :)

> > > And there are ptrace/mt issues,
> > > it seems. Just for example, we should avoid EXIT_TRACE if autoreap in
> > > wait_task_zombie() even if we are going to re-notify parent.
> >
> > I don't see how EXIT_TRACE can happen in wait_task_zombie if autoreap is
> > set.  wait_task_zombie does a cmpxchg with exit_state and doesn't
> > proceed unless exit_state was EXIT_ZOMBIE, and I don't see how we can
> > ever reach the EXIT_ZOMBIE state if autoreap.
> 
> Because you again forgot about ptrace ;)
> 
> Josh. Let me try to summarise this later when I have time. Again, I am
> not sure, perhaps this is even simpler than I currently think. And let
> me apologize in advance, most probably I will be busy tomorrow.

I look forward to your later review and feedback.

- Josh Triplett
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