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

Eric W. Biederman
2022-01-08T18:36:08Z

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().

Some of those users of the now removed thread_exit() in staging are
among the offenders.

However thread_exit() was implemented as:
#define thread_exit() complete_and_exit(NULL, 0)

Which does nothing with a completion, it was just a really funny way to
spell "do_exit(0)".

While I agree digging through all of the kernel threads and finding the
ones that should be calling complete_and_exit is a fine idea. It is
a concern independent of these patches.

> I'm (re)crawling through that zoo right now, will post when
> I get more details.

Eric