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

On 03/14, 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 ;)

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.

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().

Let me clarify in case I confused you.

The EXIT_DEAD check in do_wait() paths doesn't mean "autoreap". It means
that this thread/process (depending on ptrace) was already reaped. It was
reaped by our sub-thread, or it was reaped because we ignore SIGCHLD, or
other reasons. This doesn't matter.

In short, EXIT_DEAD means: we have to keep this thread on lists until the
task which set this state calls release_task().

Oleg.