Re: [PATCH v2 0/5] pid: add pidfd_open()

[Linus Torvalds]

On Mon, Apr 1, 2019 at 9:22 AM Daniel Colascione <dancol@xxxxxxxxxx> wrote:
>
> There's a subtlety: suppose I'm a library and I want to create a
> private subprocess. I use the new clone facility, whatever it is, and
> get a pidfd back. I need to be able to read the child's exit status
> even if the child exits before clone returns in the parent. Doesn't
> this requirement imply that the pidfd, kernel-side, contain something
> a bit more than a struct pid?

Note that clone() has always supported this, but it has basically
never been used.

One of the early thinkings behind clone() was that it could be used
for basically "AIO in user space", by having exactly these kinds of
library-private internal subthreads that are hidden as real threads.

It's why we have that special CSIGNAL mask for setting the exit
signal. It doesn't *just* set the signal to be sent when the thread
exits - setting the signal to something else than SIGCHLD actually
changes behavior in other ways too.

In particular a non-SIGCHLD thread won't be found by a regular
"waitpid()". You have to explicitly wait for it with __WCLONE (which
in turn is supposed to be used with the explicit pid to be waited
for).

Now, none of this was ever really used. The people who wanted AIO
wanted the brain-damaged POSIX kind of AIO, not something cool and
clever. So I suspect the whole exit-signal thing was just used for
random small per-project things.

              Linus