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

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Message-ID: <CA+55aFwiVYfFfTjN5j-C6cwA1z3hkCaq7A+hqaW4dG4Yon4Saw@mail.gmail.com>
Date:	Tue, 28 Jun 2016 14:40:53 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Oleg Nesterov <oleg@...hat.com>, Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>, LKP <lkp@...org>,
	LKML <linux-kernel@...r.kernel.org>,
	kernel test robot <xiaolong.ye@...el.com>
Subject: Re: kthread_stop insanity (Re: [[DEBUG] force] 2642458962: BUG:
 unable to handle kernel paging request at ffffc90000997f18)

On Tue, Jun 28, 2016 at 2:35 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Tue, Jun 28, 2016 at 2:21 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
>> If so, that seems considerably more complicated than just adding a reference count.
>
> Fair enough.

Ahh, and if you put the reference count just in the task_struct (next
to the ->stack pointer), then I guess that's particularly trivial.

Then try_get_task_stack(tsk) becomes

     void *try_get_task_stack(struct task_struct *tsk)
     {
          void *stack = tsk->stack;
          if (!atomic_inc_not_zero(&tsk->stackref))
               stack = NULL;
          return stack;
     }

ok, color me convinced.

               Linus