linux-kernel - Re: [PATCH 5.4 32/80] taskstats: Cleanup the use of task->exit

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Date:   Thu, 24 Feb 2022 18:23:13 -0600
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     "Dr. Thomas Orgis" <thomas.orgis@...-hamburg.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        <linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>
Subject: Re: [PATCH 5.4 32/80] taskstats: Cleanup the use of task->exit_code

"Dr. Thomas Orgis" <thomas.orgis@...-hamburg.de> writes:

> Am Tue, 22 Feb 2022 17:53:12 -0600
> schrieb "Eric W. Biederman" <ebiederm@...ssion.com>: 
>
>> How do you figure?
>
> I admit that I am struggling with understanding where exit codes come
> from in the non-usual cases. During my taskstats tests, I played with
> writing a multithreaded application that does call pthread_exit() in
> the main thread (pid==tgid), for example. I slowly had to learn just
> how messy this can be …
>
> Is it clearly defined what the exitcode of a task as part of a process
> is/should/can mean, as opposed to the process as a whole?

In the code it is clearly defined.  The decoding is exactly the same
as from an entire process and for a single threaded process there is no
difference.

Linux has a system 2 system calls "exit(2)" and "exit_group(2)" if a
thread exits by itself whatever is passed to exit(2) is the exit code.

What pthread_exit passes to exit(2) I don't know.  I have not been able
to trace glibc that far, and I have not instrumented up a kernel to see.

For threads that are alive when exit_group(2) is called they all get the
same final exit code.

>> For single-threaded processes ac_exitcode would always be reasonable,
>> and be what userspace passed to exit(3).
>
> Yes. That is the one case where we all know what we are dealing with;-)
>
>> For multi-threaded processes ac_exitcode before my change was set to
>> some completely arbitrary value for the thread whose tgid == tid.
>
> Isn't the only place where it really makes sense to set the exitcode
> when the last task of the process exits? I guess that was the intention
> of the earlier code — with the same wrong assumption that I fell victim
> to for quite some time: That the group leader (first task, tgid == pid)
> always exits last.
>
> I do not know in which cases group member threads have meaningful exit
> codes different from the last one (which is the one returned for the
> process in whole … ?). I'd love to see the exact reasoning on how
> multithreading got mapped into kernel tasks which used to track only
> single-threaded processes before.

The internal model in the kernel is there are tasks (which pthreads are
mapped to in a 1-1 fashion).  These tasks were the original process
abstraction.  In the case of CLONE_THREAD these tasks are glued together
into a POSIX process, with shared signal handling.

So from a kernel standpoint as it basically the original process
abstraction it is all well defined what happens when an individual task
exits.

>> With my change the value returned
>> is at least well defined.
>
> But defined to what?

See above.

>> Now maybe it would have been better to flag the bug fix with a version
>> number.  Unfortunately I did not even realize taskstats had a version
>> number.  I just know the code made no sense.
>
> Well, fixing a bug that has been there from the beginning (of adding
> multithreading, at least) is a significant change that one might want
> to know about. And I do think that it fits to thouroughly fix these
> issues that relate to identifying threads and processes (the shameless
> plug of my taskstats patch that I'm working on since 2018, and only got
> right in 2022, finally — I hope), while at that.

It looks like the bug was in commit f3cef7a99469 ("[PATCH] csa: basic
accounting over taskstats") in 2006 in 2.6.19-rc1 when taskstats were
added.  That is long after CLONE_THREAD support was added in the 2.5
development kernel.

I have been working to get a single place that code can look to find the
process exit status.  AKA so that the code can always set
SIGNAL_GROUP_EXIT, and look at signal->group_exit_code.  Fixing this was
just part of sorting out the misconceptions, and I didn't realize there
was anyone that paying attention and cared.

I will see if I can find some time to give your taskstats patch a
review.

Eric