Re: [PATCH 3/4] kthreads: rework kthread_stop()

[Oleg Nesterov <oleg@redhat.com>]

On 02/02, Eric W. Biederman wrote:
>
> Oleg on that note we should not need a barrier at all. We should be
> able to simply say:
>
> cmplp = k->vfork_done;
> if (cmplp){
> 	/* if vfork_done is NULL we have passed mm_release */
> 	kthread = container_of(cmplp, struct kthread, exited);
> 	kthread->should_stop = 1;
> 	wake_up_process(k);
> 	wait_for_completion(&kthread->exited);
> }

Yes, but the compiler can read ->vfork_done twice, and turn this code
into

	cmplp = k->vfork_done;
	if (cmplp){
		kthread = container_of(k->vfork_done, struct kthread, exited);
		...

and when we read k->vfork_done again it can be already NULL.
Probably we could use ACCESS_ONCE() instead.

Perhaps this barrier() is not needed in practice, but just to be safe.
And in fact I saw the bug report with this code:

	ac.ac_tty = current->signal->tty ?
		old_encode_dev(tty_devnum(current->signal->tty)) : 0;

this code is wrong anyway, but ->tty was read twice. I specially
asked for .s file because I wasn't able to believe the bug manifests
itself this way.

> Thinking of it I wish we had someplace we could store a pointer
> that would not be cleared so we could remove that whole confusing
> conditional.  I just looked through task_struct and there doesn't
> appear to be anything promising.
>
> Perhaps we could rename vfork_done mm_done and not clear it in
> mm_release.

Yes, in that case we don't need the barrier().

I was thinking about changing mm_release() too, but we should clear
->vfork_done (or whatever) in exec_mmap() anyway.

Oleg.