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From: Josh Triplett <josh@joshtriplett.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Thiago Macieira <thiago.macieira@intel.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Ingo Molnar <mingo@redhat.com>, Kees Cook <keescook@chromium.org>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Rik van Riel <riel@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH 6/6] clone4: Introduce new CLONE_FD flag to get task exit notification via fd
Date: Sat, 14 Mar 2015 13:03:18 -0700	[thread overview]
Message-ID: <20150314200318.GG22130@thin> (raw)
In-Reply-To: <20150314191836.GA8416@redhat.com>

On Sat, Mar 14, 2015 at 08:18:36PM +0100, Oleg Nesterov wrote:
> On 03/14, Josh Triplett wrote:
> >
> > On Sat, Mar 14, 2015 at 11:38:29AM -0700, Thiago Macieira wrote:
> > > On Saturday 14 March 2015 15:32:35 Oleg Nesterov wrote:
> > > > It is not clear to me what do_wait() should do with ->autoreap child, even
> > > > ignoring ptrace.
> > > >
> > > > Just suppose that real_parent has a single "autoreap" child. Should
> > > > wait(NULL) hanf then?
> > >
> > > It should ignore the child that is set to autoreap. wait(NULL) should return -
> > > ECHILD, indicating there are no children waiting to be reaped.
> >
> > Right.  And I don't think the current code does this.  I think we need
> > to change wait_consider_task to early-return for ->autoreap just as it
> > does for task_state == EXIT_DEAD.
> 
> No. This EXIT_DEAD is absolutely different. And this is another indication
> that you might use it wrongly ;)

Is there any information somewhere on how this state machine of doom is
*supposed* to work? :)  Why would "p->task_state == EXIT_DEAD" mean
something different in wait_consider_task?

> What we actually want is BUG_ON(task_state == EXIT_DEAD) here. We do not
> want the EXIT_DEAD tasks in ->children/ptraced lists. These EXIT_DEAD tasks
> complicate the exit/wait/reparent paths.

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.

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

> 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
> under read_lock().

That appears to be only reachable for zombies, which an autoreaping task
should never become.

> And please see another email from me. So far  I disagree that wait(NULL)
> should return ECHILD unconditionally. At least unless this is discussed
> separately.

I'll respond in that separate thread, but one issue there: waiting for
any child process cannot safely return an autoreaping child process,
because that would introduce a race condition.  The PID the parent gets
back can disappear at any time, so there's nothing useful the parent can
do with it.

- Josh Triplett

  parent reply	other threads:[~2015-03-14 20:03 UTC|newest]

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-13  1:40 [PATCH 0/6] CLONE_FD: Task exit notification via file descriptor Josh Triplett
2015-03-13  1:40 ` [PATCH 1/6] clone: Support passing tls argument via C rather than pt_regs magic Josh Triplett
2015-03-13  1:40 ` [PATCH 2/6] x86: Opt into HAVE_COPY_THREAD_TLS, for both 32-bit and 64-bit Josh Triplett
2015-03-13 22:01   ` Andy Lutomirski
2015-03-13 22:31     ` josh
2015-03-13 22:38       ` Andy Lutomirski
2015-03-13 22:43         ` josh
2015-03-13 22:45           ` Andy Lutomirski
2015-03-13 23:01             ` josh
2015-03-13  1:40 ` [PATCH 3/6] Introduce a new clone4 syscall with more flag bits and extensible arguments Josh Triplett
2015-03-13  1:40 ` [PATCH 4/6] signal: Factor out a helper function to process task_struct exit_code Josh Triplett
2015-03-13  1:40 ` [PATCH 5/6] fs: Make alloc_fd non-private Josh Triplett
2015-03-13  1:41 ` [PATCH 6/6] clone4: Introduce new CLONE_FD flag to get task exit notification via fd Josh Triplett
2015-03-13 16:21   ` Oleg Nesterov
2015-03-13 19:57     ` josh
2015-03-13 21:34       ` Andy Lutomirski
2015-03-13 22:20         ` josh
2015-03-13 22:28           ` Andy Lutomirski
2015-03-13 22:34             ` josh
2015-03-13 22:38               ` Andy Lutomirski
2015-03-14 14:14       ` Oleg Nesterov
2015-03-14 14:32         ` Oleg Nesterov
2015-03-14 18:38           ` Thiago Macieira
2015-03-14 18:54             ` Oleg Nesterov
2015-03-14 22:03               ` Josh Triplett
2015-03-14 22:26                 ` Thiago Macieira
2015-03-14 19:01             ` Josh Triplett
2015-03-14 19:18               ` Oleg Nesterov
2015-03-14 19:47                 ` Oleg Nesterov
2015-03-14 20:14                   ` Josh Triplett
2015-03-14 20:30                     ` Oleg Nesterov
2015-03-14 22:14                       ` Josh Triplett
2015-03-14 20:03                 ` Josh Triplett [this message]
2015-03-14 20:20                   ` Oleg Nesterov
2015-03-14 22:09         ` Josh Triplett
2015-03-14 14:35   ` Oleg Nesterov
2015-03-14 19:15     ` Josh Triplett
2015-03-14 19:24       ` Oleg Nesterov
2015-03-14 19:48         ` Josh Triplett
2015-03-13  1:41 ` [PATCH] clone4.2: New manpage documenting clone4(2) Josh Triplett
2015-03-13  2:07 ` [PATCH 0/6] CLONE_FD: Task exit notification via file descriptor Thiago Macieira
2015-03-13 16:05 ` David Drysdale
2015-03-13 19:42   ` Josh Triplett
2015-03-13 21:16     ` Thiago Macieira
2015-03-13 21:44       ` josh
2015-03-13 21:33     ` Andy Lutomirski
2015-03-13 21:45       ` josh
2015-03-13 21:51         ` Andy Lutomirski
2015-03-14  1:11           ` Thiago Macieira
2015-03-14 19:03             ` Thiago Macieira
2015-03-14 19:29               ` Josh Triplett
2015-03-15 10:18                 ` David Drysdale
2015-03-15 10:59                   ` Josh Triplett
2015-03-15  8:55     ` David Drysdale

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