Messages in this thread | | | Date | Mon, 28 Jan 2019 14:44:10 +0100 | From | Peter Zijlstra <> | Subject | Re: 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)
| |