linux-kernel - RE: [PATCH 06/10] exit: Implement kthread

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Date:   Sat, 8 Jan 2022 22:44:33 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Eric W. Biederman'" <ebiederm@...ssion.com>,
        Al Viro <viro@...iv.linux.org.uk>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexey Gladkov <legion@...nel.org>,
        "Kyle Huey" <me@...ehuey.com>, Oleg Nesterov <oleg@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        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>
Subject: RE: [PATCH 06/10] exit: Implement kthread_exit

From: Eric W. Biederman
> Sent: 08 January 2022 18:36
> 
> Al Viro <viro@...iv.linux.org.uk> writes:
> 
> > IMO the right way to handle that would be
> > 	1) turn these two do_exit() into do_exit(0), to reduce
> > confusion
> > 	2) deal with all do_exit() in kthread payloads.  Your
> > name for the primitive is fine, IMO.
> > 	3) make that primitive pass the return value by way of
> > a field in struct kthread, adjusting kthread_stop() accordingly
> > and passing 0 to do_exit() in kthread_exit() itself.
> >
> > (2) is not as trivial as you seem to hope, though.  Your patches
> > in drivers/staging/rt*/ had papered over the problem in there,
> > but hadn't really solved it.
> >
> > thread_exit() should've been shot, all right, but it really ought
> > to have been complete_and_exit() there.  The thing is, complete()
> > + return does *not* guarantee that driver won't get unloaded before
> > the thread terminates.  Possibly freeing its .code and leaving
> > a thread to resume running in there as soon as it regains CPU.
> >
> > The point of complete_and_exit() is that it's noreturn *and* in
> > core kernel.  So it can be safely used in a modular kthread,
> > if paired with wait_for_completion() in or before module_exit.
> > complete() + do_exit() (or complete + return as you've gotten
> > there) doesn't give such guarantees at all.
> 
> 
> I think we are mostly in agreement here.
> 
> There are kernel threads started by modules that do:
> 	complete(...);
>         return 0;
> 
> That should be at a minimum calling complete_and_exit.  Possibly should
> be restructured to use kthread_stop().

There is also module_put_and_exit(0);
Which must have an implied THIS_MODULE.

	David

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