linux-kernel - Re: [PATCH 09/10] kthread: Ensure struct kthread is present for all kthreads

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Date:   Wed, 22 Dec 2021 21:34:39 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Alexey Gladkov <legion@...nel.org>,
        Kyle Huey <me@...ehuey.com>, Oleg Nesterov <oleg@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Mike Christie <michael.christie@...cle.com>,
        Mike Christie <michael.christie@...cle.com>,
        virtualization@...ts.linux-foundation.org,
        christian.brauner@...ntu.com, axboe@...nel.dk
Subject: Re: [PATCH 09/10] kthread: Ensure struct kthread is present for all kthreads


Added a couple of people from the vhost thread.

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Wed, Dec 22, 2021 at 3:25 PM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>
>> Solve this by skipping the put_user for all kthreads.
>
> Ugh.
>
> While this fixes the problem, could we please just not mis-use that
> 'set_child_tid' as that kthread pointer any more?
>
> It was always kind of hacky. I think a new pointer with the proper
> 'struct kthread *' type would be an improvement.
>
> One of the "arguments" in the comment for re-using that set_child_tid
> pointer was that 'fork()' used to not wrongly copy it, but your patch
> literally now does that "allocate new kthread struct" at fork-time, so
> that argument is actually bogus now.

I agree.  I think I saw in the recent vhost patches that were
generalizing create_io_thread that the pf_io_worker field of
struct task_struct was being generalized as well.

If so I think it makes sense just to take that approach.

Just build some basic infrastructure that can be used for io_workers,
vhost_workers, and kthreads.

Eric