Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs

[Paul E. McKenney]

On Fri, Jun 28, 2019 at 04:15:22PM +0200, Peter Zijlstra wrote:
> On Thu, Jun 27, 2019 at 01:36:12PM -0700, Paul E. McKenney wrote:
> > On Thu, Jun 27, 2019 at 03:17:27PM -0500, Scott Wood wrote:
> > > On Thu, 2019-06-27 at 11:41 -0700, Paul E. McKenney wrote:
> > > > On Thu, Jun 27, 2019 at 02:16:38PM -0400, Joel Fernandes wrote:
> > > > > 
> > > > > I think the fix should be to prevent the wake-up not based on whether we
> > > > > are
> > > > > in hard/soft-interrupt mode but that we are doing the rcu_read_unlock()
> > > > > from
> > > > > a scheduler path (if we can detect that)
> > > > 
> > > > Or just don't do the wakeup at all, if it comes to that.  I don't know
> > > > of any way to determine whether rcu_read_unlock() is being called from
> > > > the scheduler, but it has been some time since I asked Peter Zijlstra
> > > > about that.
> > > > 
> > > > Of course, unconditionally refusing to do the wakeup might not be happy
> > > > thing for NO_HZ_FULL kernels that don't implement IRQ work.
> > > 
> > > Couldn't smp_send_reschedule() be used instead?
> > 
> > Good point.  If current -rcu doesn't fix things for Sebastian's case,
> > that would be well worth looking at.  But there must be some reason
> > why Peter Zijlstra didn't suggest it when he instead suggested using
> > the IRQ work approach.
> > 
> > Peter, thoughts?
> 
> I've not exactly kept up with the thread; but irq_work allows you to run
> some actual code on the remote CPU which is often useful and it is only
> a little more expensive than smp_send_reschedule().
> 
> Also, just smp_send_reschedule() doesn't really do anything without
> first poking TIF_NEED_RESCHED (or other scheduler state) and if you want
> to do both, there's other helpers you should use, like resched_cpu().

Thank you!  Plus it looks like scheduler_ipi() takes an early exit if
->wake_list is empty, regardless of TIF_NEED_RESCHED, right?

							Thanx, Paul