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 20:55:06 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Josh Triplett <josh@...htriplett.org>
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 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.

> (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 ;)

> > 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.

> > EXCEPT: do we really want SIGCHLD from the exiting child? I think we
> > do not. I won't really argue though, but this should be discussed and
> > documented. IIUC, with your patch it is still sent.
>
> I think we do, yes.  The caller of clone can already specify what signal
> they want, including no signal at all.  If they specify a signal
> (SIGCHLD or otherwise) along with CLONE_AUTOREAP, we can send that
> signal.

OK. Agreed.

Oleg.

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