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SubjectRe: WARN_ON_ONCE(!new_owner) within wake_futex_pi() triggered
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
wake_futex_pi()
rt_mutex_next_owner() // whoops, no waiter
WARN

lock hb
unqueue_me_pi



So reverting that patch should cure things, because then there is no hb
lock break between queue/unqueue and futex_unlock_pi() cannot observe
this half-arsed state.

Now obviously reverting that makes RT unhappy; let me see what the
options are.

(concurrently tglx generated a trace that corroborates)

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