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

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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.

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kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG: unable to  handle kernel paging request at ffffc90000997f18) Andy Lutomirski <luto@kernel.org> - 2016-06-27 07:30 +0200
  Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Peter Zijlstra <peterz@infradead.org> - 2016-06-27 10:30 +0200
  Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Oleg Nesterov <oleg@redhat.com> - 2016-06-27 17:00 +0200
    Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Andy Lutomirski <luto@amacapital.net> - 2016-06-27 17:50 +0200
      Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Oleg Nesterov <oleg@redhat.com> - 2016-06-27 19:00 +0200
        Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Oleg Nesterov <oleg@redhat.com> - 2016-06-28 21:00 +0200
          Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Andy Lutomirski <luto@amacapital.net> - 2016-06-28 21:20 +0200
            Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Oleg Nesterov <oleg@redhat.com> - 2016-06-28 22:20 +0200
              Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Andy Lutomirski <luto@amacapital.net> - 2016-06-28 23:00 +0200
                Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Linus Torvalds <torvalds@linux-foundation.org> - 2016-06-28 23:20 +0200
                Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Linus Torvalds <torvalds@linux-foundation.org> - 2016-06-28 23:20 +0200
                Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Andy Lutomirski <luto@amacapital.net> - 2016-06-28 23:30 +0200
                Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Linus Torvalds <torvalds@linux-foundation.org> - 2016-06-28 23:40 +0200
                Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Linus Torvalds <torvalds@linux-foundation.org> - 2016-06-28 23:50 +0200
                Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Oleg Nesterov <oleg@redhat.com> - 2016-06-29 01:00 +0200
                Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Oleg Nesterov <oleg@redhat.com> - 2016-06-29 01:10 +0200
                Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Andy Lutomirski <luto@amacapital.net> - 2016-06-29 17:40 +0200
                [PATCH] kthread: to_live_kthread() needs try_get_task_stack() Oleg Nesterov <oleg@redhat.com> - 2016-06-29 20:10 +0200
                Re: [PATCH] kthread: to_live_kthread() needs try_get_task_stack() kbuild test robot <lkp@intel.com> - 2016-06-29 20:40 +0200
                Re: [PATCH] kthread: to_live_kthread() needs try_get_task_stack() Oleg Nesterov <oleg@redhat.com> - 2016-06-29 20:50 +0200
                Re: [PATCH] kthread: to_live_kthread() needs try_get_task_stack() kbuild test robot <lkp@intel.com> - 2016-06-29 21:00 +0200
                Re: [PATCH] kthread: to_live_kthread() needs try_get_task_stack() Andy Lutomirski <luto@amacapital.net> - 2016-06-30 01:10 +0200
          Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Andy Lutomirski <luto@amacapital.net> - 2016-06-30 01:40 +0200
  Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:  unable to handle kernel paging request at ffffc90000997f18) Linus Torvalds <torvalds@linux-foundation.org> - 2016-06-27 19:20 +0200

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