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: <20160628185853.GA3998@redhat.com>
Date:	Tue, 28 Jun 2016 20:58:54 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
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 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.

But what if we race with the last schedule() ? "addr = *stack" can read
the already vfree'ed memory, no?

Looks like print_context_stack/etc need probe_kernel_address or I missed
something.

Oleg.