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

From: Thomas Gleixner
Date: Tue Jan 29 2019 - 04:45:56 EST

On Tue, 29 Jan 2019, Heiko Carstens wrote:
> On Mon, Jan 28, 2019 at 04:53:19PM +0100, Thomas Gleixner wrote:
> > Patch below cures that.
> With your patch the kernel warning doesn't occur anymore. So if this
> is supposed to be the fix feel free to add:

Yes, it's supposed to be the fix.
> However now I see every now and then the following failure from the
> same test case:
> tst-robustpi8: ../nptl/pthread_mutex_lock.c:425: __pthread_mutex_lock_full: Assertion `INTERNAL_SYSCALL_ERRNO (e, __err) != ESRCH || !robust' failed.
> /* ESRCH can happen only for non-robust PI mutexes where
> the owner of the lock died. */
> assert (INTERNAL_SYSCALL_ERRNO (e, __err) != ESRCH || !robust);
> I just verified that this happened also without your patch, I just
> didn't see it since I started my tests with panic_on_warn=1 and the
> warning triggered always earlier.
> So, this seems to be something different.

Moo. I ran the test loop all night (simply because I forgot to stop it) and
of course this does not trigger here. Could you try to gather a bit more
information with lightweight tracing?

Something like the below should give us at least a clue. Stop the tst loop
when the assert triggers and then extract the trace.





# Substitute with your ld comm string if it starts differently

echo 'comm ~ "$C*"' >/sys/kernel/debug/tracing/events/syscalls/sys_enter_futex/filter
echo 'comm ~ "$C*"' >/sys/kernel/debug/tracing/events/syscalls/sys_exit_futex/filter
echo 'comm ~ "$C*"' >/sys/kernel/debug/tracing/events/sched/sched_process_exit/filter
echo 'prev_comm ~ "$C*" || next_comm ~ "$C*"' >/sys/kernel/debug/tracing/events/sched/sched_switch/filter

echo 1 > /sys/kernel/debug/tracing/events/syscalls/sys_enter_futex/enable
echo 1 > /sys/kernel/debug/tracing/events/syscalls/sys_exit_futex/enable
echo 1 > /sys/kernel/debug/tracing/events/sched/sched_process_exit/enable
echo 1 > /sys/kernel/debug/tracing/events/sched/sched_switch/enable