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.

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.