Re: [RFC][PATCH 4/4] futex: Rewrite FUTEX_UNLOCK_PI

From: Thomas Gleixner
Date: Sat Oct 08 2016 - 13:08:50 EST

On Sat, 8 Oct 2016, Peter Zijlstra wrote:
> > So in this case we tell the caller on CPU 1 that the futex is in
> > inconsistent state, because pistate->owner still points to the unlocking
> > task while the user space value alread shows the new owner. So this sanity
> > check triggers and we simply fail while we should not. It's [10] in the
> > state matrix above attach_to_pi_state().
> Urgh, yes. I think I can cure that, by taking
> pi_state->pi_mutex.wait_lock in attach_to_pi_state(), but blergh.
> > I suspect that there are more issues like this, especially since I did not
> > look at requeue_pi yet, but by now my brain is completely fried.
> Yes, I know about fried brains :-( This stuff has far too many moving
> parts. I've been staring at this stuff far too long.
> Also, we need better tools to stress this stuff.

I tried to come up with something which forces all corner cases of this
years ago and failed. The state space seems to be infinite.