RE: [PATCH 06/10] exit: Implement kthread_exit

David Laight
2022-01-08T22:44:39Z

From: Eric W. Biederman
> Sent: 08 January 2022 18:36
>
> Al Viro <viro@zeniv.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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