Re: Question on task_blocks_on_rt_mutex()

[Davidlohr Bueso]

On Tue, 01 Sep 2020, Paul E. McKenney wrote:

> And it appears that a default-niced CPU-bound SCHED_OTHER process is
> not preempted by a newly awakened MAX_NICE SCHED_OTHER process.  OK,
> OK, I never waited for more than 10 minutes, but on my 2.2GHz that is
> close enough to a hang for most people.
>
> Which means that the patch below prevents the hangs.  And maybe does
> other things as well, firing rcutorture up on it to check.
>
> But is this indefinite delay expected behavior?
>
> This reproduces for me on current mainline as follows:
>
> tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --torture lock --duration 3 --configs LOCK05
>
> This hangs within a minute of boot on my setup.  Here "hangs" is defined
> as stopping the per-15-second console output of:
> 	Writes:  Total: 569906696 Max/Min: 81495031/63736508   Fail: 0

Ok this doesn't seem to be related to lockless wake_qs then. fyi there have
been missed wakeups in the past where wake_q_add() fails the cmpxchg because
the task is already pending a wakeup leading to the actual wakeup ocurring
before its corresponding wake_up_q(). This is why we have wake_q_add_safe().
But for rtmutexes, because there is no lock stealing only top-waiter is awoken
as well as try_to_take_rt_mutex() is done under the lock->wait_lock I was not
seeing an actual race here.

Thanks,
Davidlohr