Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG: unable to handle kernel paging request at ffffc90000997f18)

[Oleg Nesterov <oleg@redhat.com>]

On 06/26, Andy Lutomirski wrote:
>
> kthread_stop is *sick*.
>
>     struct kthread self;
>
> ...
>
>     current->vfork_done = &self.exited;
>
> ...
>
>     do_exit(ret);
>
> And then some other thread goes and waits for the completion, which is
> *on the stack*, which, in any sane world (e.g. with my series
> applied), is long gone by then.

Yes, I forgot this when we discussed the problems with ti->flags/etc...

> But this is broken even without any changes: since when is gcc
> guaranteed to preserve the stack contents when a function ends with a
> sibling call, let alone with a __noreturn call?

I don't know if gcc can actually drop the stack frame in this case,
but even if it can this looks fixeable.

> Is there seriously no way to directly wait for a struct task_struct to
> exit?  Could we, say, kmalloc the completion (or maybe even the whole
> struct kthread) and (ick!) hang it off ->vfork_done?

Sure we can... And yes, I think we need to alloc the whole struct kthread.
Just another (unfortunate) complication, the current code is simple.

And probably kthread/kthread_stop should switch to task_work_exit().

Oleg.