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

[Andy Lutomirski]

On Tue, Jun 28, 2016 at 11:58 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 06/27, Oleg Nesterov wrote:
>>
>> On 06/27, Andy Lutomirski wrote:
>> >
>> > Want to send a patch?  I could do it, but you understand this code
>> > much better than I do.
>>
>> Well, I'll try to do this tomorrow unless you do it.
>
> I have cloned luto/linux.git to see if kthread_stop() can pin ->stack
> somehow, but it seems this is not possible, finish_task_switch() does
> free_thread_stack() unconditionally.
>
> Then how (say) proc_pid_stack() can work? If it hits the task which is
> alreay dead we are (probably) fine, valid_stack_ptr() should fail iiuc.
>

I changed save_stack_trace_tsk() to use try_get_task_stack().  I think
that's sufficient to fix this.

--Andy