linux-kernel - Re: [PATCH 6/6] clone4: Introduce new CLONE_FD flag to get task exit notification via fd

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Date:	Sat, 14 Mar 2015 15:09:05 -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@...capital.net>,
	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>,
	Thiago Macieira <thiago.macieira@...el.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH 6/6] clone4: Introduce new CLONE_FD flag to get task exit
 notification via fd

On Sat, Mar 14, 2015 at 03:14:14PM +0100, Oleg Nesterov wrote:
> Again, again, I didn't read this series yet. But the proper solution (afaics)
> should move this "autoreap" check in release_task/__ptrace_detach(). If the
> task is traced. Debugger should check ->autoreap and skip another
> do_notify_parent().

As mentioned in the mail I just sent, I think I can just move the
autoreap handling *into* do_notify_parent, and treat it as though the
parent had SA_NOCLDWAIT set.

> Speaking of autoreap... If ->exit_signal is zero, then the exiting child
> doesn't send the notification to its parent, still it doesn't autoreap
> itself. To me this looks strange, and in fact it seems to me that this
> is only by mistake. I am wondering if we can treat ->exit_signal == 0
> as "autoreap" too. As usual, most probably the answer is "no, because it
> is too late to change the historical behaviour". But this is off-topic.

Historical behavior, and potentially sensible behavior; you might not
want notification, but you might still want to get the child's exit
status by calling wait, which means you need the process to stick around
as a zombie until you wait on it.

That'd be the main advantage of adding a CLONE_AUTOREAP flag: it allows
you to get the same autoreaping behavior you'd get if you had SIGCHLD
ignored, but without actually sending a signal and without caring how
the process-wide signal handling is set up.  So you'd pass a 0 signal,
and CLONE_AUTOREAP.  And then if you *want* the exit notification, you
can get it via the file descriptor.

- Josh Triplett
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