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SubjectRe: WARN_ON_ONCE(!new_owner) within wake_futex_pi() triggered
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.


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