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

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Date:	Wed, 29 Jun 2016 08:34:01 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andy Lutomirski <luto@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.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 3:59 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 06/28, Andy Lutomirski wrote:
>>
>> On Tue, Jun 28, 2016 at 1:12 PM, Oleg Nesterov <oleg@...hat.com> wrote:
>> >
>> > So please forget unless you see another reason for this change.
>> >
>>
>> But I might need to that anyway for procfs to read the the stack,
>> right?  Do you see another way to handle that case?
>
> Well, we could use probe_kernel_text() and recheck tsk->stack != NULL
> after this.
>
> But,
>
>> I'm thinking of adding:
>>
>> void *try_get_task_stack(struct task_struct *tsk);
>> void put_task_stack(struct task_struct *tsk);
>
> Yes, agreed, this looks better.
>
> Oleg.
>

I pushed that change to my tree (seems to work well enough to boot
without warnings as long as I don't unmount XFS, but not particularly
well tested).  Want to refresh your patch on top?

I'll probably have to split the patch to introduce no-op
try_get_task_stack / put_task_stack first so I can avoid breaking
bisection, but the interface shouldn't change unless something's wrong
with it.

--Andy