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 21:20:59 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	Thiago Macieira <thiago.macieira@...el.com>,
	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>,
	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 03/14, Josh Triplett wrote:
>
> On Sat, Mar 14, 2015 at 08:18:36PM +0100, Oleg Nesterov wrote:
>
> Is there any information somewhere on how this state machine of doom is
> *supposed* to work? :)

This looks as if you think that other parts of this kernel differ ;)

> Why would "p->task_state == EXIT_DEAD" mean
> something different in wait_consider_task?

different? but once again, EXIT_DEAD never meant "autoreap". It always
means "already reaped". OK, more correctly it means "release_task() will
be called soon, please ignore me".

> Pulling the EXIT_DEAD tasks out of those lists completely does sound
> like a good simplification.  However, that doesn't seem to be the
> current expectation in wait_consider_task, which just returns if
> p->task_state == EXIT_DEAD to skip considering that task.

See above. And another email I sent.

> And an autoreaping task isn't necessarily dead yet; it just shouldn't be
> waited on.

Yes, sure, but please do not confuse this with EXIT_DEAD. In fact, please
do not confuse this with ->task_state.

> > However, currently this is TODO. The main problem is the locking in
> > wait_task_zombie(), we can set EXIT_DEAD and remove the task from list
                        ^^^^^^
I mean, "we can't".

> > under read_lock().
>
> That appears to be only reachable for zombies, which an autoreaping task
> should never become.

Sure. I meant that other paths which can set EXIT_DEAD are simpler wrt
"kill the EXIT_DEAD and EXIT_TRACE state" patch we actually want. But lets
not discuss this here, this is offtopic.

Oleg.

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