Re: fs: uninterruptible hang in handle_userfault

[Andrea Arcangeli]

On Wed, Mar 02, 2016 at 09:03:01AM -0800, Linus Torvalds wrote:
> It's not just "exit_futex()" (what is that? I assume you mean

That come from a cleanup (appended below but not very well tested) I
did initially to consolidate the futex exit code before attempting to
relocate its call location.

> exit_robust_list()) that triggers the problem, it's also the
> 
>         put_user(0, tsk->clear_child_tid);
> 
> in mm_release().

>From the stack trace it didn't appear to refault there and it was
still stuck in exit_futex, but this could end up in the same problem
and your fix already took care of this one as well.

> So it's not just about futexes.
> 
> The might be other final user space accesses lurking too that I didn't
> even think about.
> 
> Anyway, I committed (a) as the safest version with the least side
> effects. If people think some more about this and come up with
> solutions how to avoid these kinds of "very late user space accesses"
> cleanly, I think that would be great.

Agreed.

Thanks,
Andrea