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

On Sat, Mar 14, 2015 at 08:18:36PM +0100, Oleg Nesterov wrote:

On 03/14, Josh Triplett wrote:

quoted

On Sat, Mar 14, 2015 at 11:38:29AM -0700, Thiago Macieira wrote:

quoted

On Saturday 14 March 2015 15:32:35 Oleg Nesterov wrote:

quoted

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

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.

quoted

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".

quoted

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.