Re: [PATCH RFC 06/10] pidfs: allow to retrieve exit information

[Oleg Nesterov <oleg@redhat.com>]

Christian,

I am already sleeping. I'll try to reply right now, but quite possibly
I will need to correct myself tomorrow ;)

On 03/02, Christian Brauner wrote:
>
> Ok, so:
>
> release_task()
> -> __exit_signal()
>    -> detach_pid()
>       -> __change_pid()
>
> That sounds good. So could we do something like:

Yes, this is what I meant, except...

> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -127,8 +127,10 @@ static void __unhash_process(struct task_struct *p, bool group_dead)
>  {
>         nr_threads--;
>         detach_pid(p, PIDTYPE_PID);
> +       pidfs_exit(p); // record exit information for individual thread

To me it would be better to do this in the caller, release_task().
But this is minor and I won't insist. Please see below.

>         if (group_dead) {
>                 detach_pid(p, PIDTYPE_TGID);
> +               pidfs_exit(p); // record exit information for thread-group leader

This looks pointless, task_pid(p) is the same.

> I know, as written this won't work but I'm just trying to get the idea
> across of recording exit information for both the individual thread and
> the thread-group leader in __unhash_process().
>
> That should tackle both problems, i.e., recording exit information for
> both thread and thread-group leader as well as exec?

This will fix the problem with mt-exec, but this won't help to discriminate
the leader-exit and the-whole-group-exit cases...

With this this (or something like this) change pidfd_info() can only report
the exit code of the already reaped thread/process, leader or not.

I mean... If the leader L exits using sys_exit() and it has the live sub-
threads, release_task(L) / __unhash_process(L) will be only called when
the last sub-thread exits and it (or debugger) does "goto repeat;" in
release_task() to finally reap the leader.

IOW. If someone does sys_pidfd_create(group-leader-pid, PIDFD_THREAD),
pidfd_info() won't report PIDFD_INFO_EXIT if the leader has exited using
sys_exit() before other threads.

But perhaps this is fine?

Let me repeat, I have no idea how and why people use pidfd ;)

Oleg.