linux-kernel - Re: [RFC][PATCH] exec: Freeze the other threads during a multi-threaded exec

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Date:   Tue, 28 Jul 2020 11:17:02 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>, Pavel Machek <pavel@....cz>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [RFC][PATCH] exec: Freeze the other threads during a
 multi-threaded exec

On Tue, Jul 28, 2020 at 6:23 AM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>
> For exec all I care about are user space threads.  So it appears the
> freezer infrastructure adds very little.

Yeah. 99% of the freezer stuff is for just adding the magic notations
for kernel threads, and for any user space threads it seems the wrong
interface.

> Now to see if I can find another way to divert a task into a slow path
> as it wakes up, so I don't need to manually wrap all of the sleeping
> calls.  Something that plays nice with the scheduler.

The thing is, how many places really care?

Because I think there are like five of them. And they are all marked
by taking cred_guard_mutex, or the file table lock.

So it seems really excessive to then create some whole new "let's
serialize every thread", when you actually don't care about any of it,
except for a couple of very very special cases.

If you care about "thread count stable", you care about exit() and
clone().  You don't care about threads that are happily running - or
sleeping - doing their own thing.

So trying to catch those threads and freezing them really feels like
entirely the wrong interface.

             Linus