linux-kernel - Re: [PATCH RFC] kthread: Unify kernel_thread() and user_mode

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Message-ID: <CAAhV-H79bUF396+dWaafanzcMq41VtcOsfa_3SYSUftyNDOyvw@mail.gmail.com>
Date:   Sat, 13 May 2023 11:18:37 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Huacai Chen <chenhuacai@...ngson.cn>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] kthread: Unify kernel_thread() and user_mode_thread()

Hi, Eric,

On Wed, May 10, 2023 at 11:45 PM Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
> Huacai Chen <chenhuacai@...ngson.cn> writes:
>
> > Commit 343f4c49f2438d8 ("kthread: Don't allocate kthread_struct for init
> > and umh") introduces a new function user_mode_thread() for init and umh.
> > But the name is a bit confusing because init and umh are indeed kernel
> > threads at creation time, the real difference is "they will become user
> > processes".
>
> No they are not "kernel threads" at creation time.  At creation time
> init and umh are threads running in the kernel.
>
> It is a very important distinction and you are loosing it.
>
> Because they don't have a kthread_struct such tasks in the kernel
> are not allowed to depend on anything that is ``kthread''.
Hmm, traditionally, we call a "task" without userland address space
(i.e., the task_struct has no mm, it shares kernel's address space) as
a kernel thread, so init and umh are kernel threads until they call
kernel_execve(). Of course in your patch a kernel thread should have a
"kthread" struct (I can't grep "kthread_struct" so I suppose you are
saying "kthread"), but I think the traditional definition is more
natural for most people?

Huacai

>
> Having this a separate function highlights the distinction.
> Highlighting should hopefully cause people to ask why there is a
> distinction, and what is going on.
>
> > So let's unify the kernel_thread() and user_mode_thread() to
> > kernel_thread() again, and add a new 'user' parameter for init and
> > umh
>
> Now that is confusing.
>
> Eric