Re: WARN_ON_ONCE(!new_owner) within wake_futex_pi() triggered

[Peter Zijlstra]

On Mon, Jan 28, 2019 at 02:44:10PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 29, 2018 at 12:23:21PM +0100, Heiko Carstens wrote:
> 
> > And indeed, if I run only this test case in an endless loop and do
> > some parallel work (like kernel compile) it currently seems to be
> > possible to reproduce the warning:
> > 
> > while true; do time ./testrun.sh nptl/tst-robustpi8 --direct ; done
> > 
> > within the build directory of glibc (2.28).
> 
> Right; so that reproduces for me.
> 
> After staring at all that for a while; trying to remember how it all
> worked (or supposed to work rather), I became suspiscous of commit:
> 
>   56222b212e8e ("futex: Drop hb->lock before enqueueing on the rtmutex")
> 
> And indeed, when I revert that; the above reproducer no longer works (as
> in, it no longer triggers in minutes and has -- so far -- held up for an
> hour+ or so).
> 
> That patch in particular allows futex_unlock_pi() to 'start' early:
> 
> 
> futex_lock_pi()			futex_unlock_pi()
>   lock hb
>   queue
>   lock wait_lock
>   unlock hb
> 					lock hb
> 					futex_top_waiter
> 					get_pi_state
> 					lock wait_lock
>   rt_mutex_proxy_start // fail
>   unlock wait_lock
> 					// acquired wait_lock
					unlock hb
> 					wake_futex_pi()
> 					rt_mutex_next_owner() // whoops, no waiter
> 					WARN

and simply removing that WARN, would allow futex_unlock_pi() to spin on
retry until the futex_lock_pi() CPU comes around to doing the lock hb
below:

>   lock hb
>   unqueue_me_pi

Which seems undesirable from a determinsm POV.