Re: [PATCH 1/3] rcu: Use static initializer for krc.lock

[Mike Galbraith <efault@gmx.de>]

On Sun, 2020-04-19 at 21:17 -0400, Joel Fernandes wrote:
>
> > > Can we do even better by moving the work-scheduling out from under the
> > > spinlock?  This of course means that it is necessary to handle the
> > > occasional spurious call to the work handler, but that should be rare
> > > and should be in the noise compared to the reduction in contention.
> >
> > Yes I think that will be required since -rt will sleep on workqueue locks as
> > well :-(. I'm looking into it right now.
> >
> >         /*
> >          * If @work was previously on a different pool, it might still be
> >          * running there, in which case the work needs to be queued on that
> >          * pool to guarantee non-reentrancy.
> >          */
> >         last_pool = get_work_pool(work);
> >         if (last_pool && last_pool != pwq->pool) {
> >                 struct worker *worker;
> >
> >                 spin_lock(&last_pool->lock);
>
> Hmm, I think moving schedule_delayed_work() outside lock will work. Just took
> a good look and that's not an issue. However calling schedule_delayed_work()
> itself is an issue if the caller of kfree_rcu() is !preemptible() on
> PREEMPT_RT. Because the schedule_delayed_work() calls spin_lock on pool->lock
> which can sleep on PREEMPT_RT :-(.

As of 4.19-rt, workqueue locks are converted to raw_spinlock_t.

	-Mike