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

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Date:   Mon, 10 Jan 2022 09:00:31 -0600
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        "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

David Laight <David.Laight@...LAB.COM> writes:

> 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.

Later in the patch series I change
module_put_and_exit -> module_put_and_kthread_exit
complete_and_exit -> complete_and_kthread_exit

The problem that I understand all was seeing was where people should
have been using complete_and_exit and were not.

Eric