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)
