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: <20160627082823.GW30154@twins.programming.kicks-ass.net>
Date:	Mon, 27 Jun 2016 10:28:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, 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 Sun, Jun 26, 2016 at 10:22:32PM -0700, 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.

cute

> 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)

I suppose the easiest is to merge struct kthread into struct
kthread_create_info, and maybe union some bits if we're really worried
about size.

> and (ick!) hang it off ->vfork_done?

This... cleanup is going to be somewhat icky.